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• Getting to grips with the stack in nasm

  - by MarkPearl

  Today I spent a good part of my day getting to grips with the stack and nasm. After looking at my notes on nasm I think this is one area for the course I am doing they could focus more on… So here are some snippets I have put together that have helped me understand a little bit about the stack… Simplest example of the stack You will probably see examples like the following in circulation… these demonstrate the simplest use of the stack… org 0x100 bits 16 jmp main main: push 42h push 43h push 44h mov ah,2h ;set to display characters pop dx    ;get the first value int 21h   ;and display it pop dx    ;get 2nd value int 21h   ;and display it pop dx    ;get 3rd value int 21h   ;and display it int 20h The output from above code would be… DCB Decoupling code using “call” and “ret” This is great, but it oversimplifies what I want to use the stack for… I do not know if this goes against the grain of assembly programmers or not, but I want to write loosely coupled assembly code – and I want to use the stack as a mechanism for passing values into my decoupled code. In nasm we have the call and return instructions, which provides a mechanism for decoupling code, for example the following could be done… org 0x100 bits 16 jmp main ;---------------------------------------- displayChar: mov ah,2h mov dx,41h int 21h ret ;---------------------------------------- main: call displayChar int 20h   This would output the following to the console A So, it would seem that call and ret allow us to jump to segments of our code and then return back to the calling position – a form of segmenting the code into what we would called in higher order languages “functions” or “methods”. The only issue is, in higher order languages there is a way to pass parameters into the functions and return results. Because of the primitive nature of the call and ret instructions, this does not seem to be obvious. We could of course use the registers to pass values into the subroutine and set values coming out, but the problem with this is we… Have a limited number of registers Are threading our code with tight coupling (it would be hard to migrate methods outside of their intended use in a particular program to another one) With that in mind, I turn to the stack to provide a loosely coupled way of calling subroutines… First attempt with the Stack Initially I thought this would be simple… we could use code that looks as follows to achieve what I want… org 0x100 bits 16 jmp main ;---------------------------------------- displayChar: mov ah,2h pop dx int 21h ret ;---------------------------------------- main: push 41h call displayChar int 20h   However running this application does not give the desired result, I want an ‘A’ to be returned, and I am getting something totally different (you will to). Reading up on the call and ret instructions a discovery is made… they are pushing and popping things onto and off the stack as well… When the call instruction is executed, the current value of IP (the address of the instruction to follow) is pushed onto the stack, when ret is called, the last value on the stack is popped off into the IP register. In effect what the above code is doing is as follows with the stack… push 41h push current value of ip pop current value of ip to dx pop 41h to ip This is not what I want, I need to access the 41h that I pushed onto the stack, but the call value (which is necessary) is putting something in my way. So, what to do? Remember we have other registers we can use as well as a thing called indirect addressing… So, after some reading around, I came up with the following approach using indirect addressing… org 0x100 bits 16 jmp main ;---------------------------------------- displayChar: mov bp,sp mov ah,2h mov dx,[bp+2] int 21h ret ;---------------------------------------- main: push 41h call displayChar int 20h In essence, what I have done here is used a trick with the stack pointer… it goes as follows… Push 41 onto the stack Make the call to the function, which will push the IP register onto the stack and then jump to the displayChar label Move the value in the stack point to the bp register (sp currently points at IP register) Move the at the location of bp minus 2 bytes to dx (this is now the value 41h) display it, execute the ret instruction, which pops the ip value off the stack and goes back to the calling point This approach is still very raw, some further reading around shows that I should be pushing the value of bp onto the stack before replacing it with sp, but it is the starting thread to getting loosely coupled subroutines. Let’s see if you get what the following output would be? org 0x100 bits 16 jmp main ;---------------------------------------- displayChar: mov bp,sp mov ah,2h mov dx,[bp+4] int 21h mov dx,[bp+2] int 21h ret ;---------------------------------------- main: push 41h push 42h call displayChar int 20h The output is… AB Where to from here? If by any luck some assembly programmer comes along and see this code and notices that I have made some fundamental flaw in my logic… I would like to know, so please leave a comment… appreciate any feedback!

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• Organizations &amp; Architecture UNISA Studies &ndash; Chap 7

  - by MarkPearl

  Learning Outcomes Name different device categories Discuss the functions and structure of I/.O modules Describe the principles of Programmed I/O Describe the principles of Interrupt-driven I/O Describe the principles of DMA Discuss the evolution characteristic of I/O channels Describe different types of I/O interface Explain the principles of point-to-point and multipoint configurations Discuss the way in which a FireWire serial bus functions Discuss the principles of InfiniBand architecture External Devices An external device attaches to the computer by a link to an I/O module. The link is used to exchange control, status, and data between the I/O module and the external device. External devices can be classified into 3 categories… Human readable – e.g. video display Machine readable – e.g. magnetic disk Communications – e.g. wifi card I/O Modules An I/O module has two major functions… Interface to the processor and memory via the system bus or central switch Interface to one or more peripheral devices by tailored data links Module Functions The major functions or requirements for an I/O module fall into the following categories… Control and timing Processor communication Device communication Data buffering Error detection I/O function includes a control and timing requirement, to coordinate the flow of traffic between internal resources and external devices. Processor communication involves the following… Command decoding Data Status reporting Address recognition The I/O device must be able to perform device communication. This communication involves commands, status information, and data. An essential task of an I/O module is data buffering due to the relative slow speeds of most external devices. An I/O module is often responsible for error detection and for subsequently reporting errors to the processor. I/O Module Structure An I/O module functions to allow the processor to view a wide range of devices in a simple minded way. The I/O module may hide the details of timing, formats, and the electro mechanics of an external device so that the processor can function in terms of simple reads and write commands. An I/O channel/processor is an I/O module that takes on most of the detailed processing burden, presenting a high-level interface to the processor. There are 3 techniques are possible for I/O operations Programmed I/O Interrupt[t I/O DMA Access Programmed I/O When a processor is executing a program and encounters an instruction relating to I/O it executes that instruction by issuing a command to the appropriate I/O module. With programmed I/O, the I/O module will perform the requested action and then set the appropriate bits in the I/O status register. The I/O module takes no further actions to alert the processor. I/O Commands To execute an I/O related instruction, the processor issues an address, specifying the particular I/O module and external device, and an I/O command. There are four types of I/O commands that an I/O module may receive when it is addressed by a processor… Control – used to activate a peripheral and tell it what to do Test – Used to test various status conditions associated with an I/O module and its peripherals Read – Causes the I/O module to obtain an item of data from the peripheral and place it in an internal buffer Write – Causes the I/O module to take an item of data form the data bus and subsequently transmit that data item to the peripheral The main disadvantage of this technique is it is a time consuming process that keeps the processor busy needlessly I/O Instructions With programmed I/O there is a close correspondence between the I/O related instructions that the processor fetches from memory and the I/O commands that the processor issues to an I/O module to execute the instructions. Typically there will be many I/O devices connected through I/O modules to the system – each device is given a unique identifier or address – when the processor issues an I/O command, the command contains the address of the address of the desired device, thus each I/O module must interpret the address lines to determine if the command is for itself. When the processor, main memory and I/O share a common bus, two modes of addressing are possible… Memory mapped I/O Isolated I/O (for a detailed explanation read page 245 of book) The advantage of memory mapped I/O over isolated I/O is that it has a large repertoire of instructions that can be used, allowing more efficient programming. The disadvantage of memory mapped I/O over isolated I/O is that valuable memory address space is sued up. Interrupts driven I/O Interrupt driven I/O works as follows… The processor issues an I/O command to a module and then goes on to do some other useful work The I/O module will then interrupts the processor to request service when is is ready to exchange data with the processor The processor then executes the data transfer and then resumes its former processing Interrupt Processing The occurrence of an interrupt triggers a number of events, both in the processor hardware and in software. When an I/O device completes an I/O operations the following sequence of hardware events occurs… The device issues an interrupt signal to the processor The processor finishes execution of the current instruction before responding to the interrupt The processor tests for an interrupt – determines that there is one – and sends an acknowledgement signal to the device that issues the interrupt. The acknowledgement allows the device to remove its interrupt signal The processor now needs to prepare to transfer control to the interrupt routine. To begin, it needs to save information needed to resume the current program at the point of interrupt. The minimum information required is the status of the processor and the location of the next instruction to be executed. The processor now loads the program counter with the entry location of the interrupt-handling program that will respond to this interrupt. It also saves the values of the process registers because the Interrupt operation may modify these The interrupt handler processes the interrupt – this includes examination of status information relating to the I/O operation or other event that caused an interrupt When interrupt processing is complete, the saved register values are retrieved from the stack and restored to the registers Finally, the PSW and program counter values from the stack are restored. Design Issues Two design issues arise in implementing interrupt I/O Because there will be multiple I/O modules, how does the processor determine which device issued the interrupt? If multiple interrupts have occurred, how does the processor decide which one to process? Addressing device recognition, 4 general categories of techniques are in common use… Multiple interrupt lines Software poll Daisy chain Bus arbitration For a detailed explanation of these approaches read page 250 of the textbook. Interrupt driven I/O while more efficient than simple programmed I/O still requires the active intervention of the processor to transfer data between memory and an I/O module, and any data transfer must traverse a path through the processor. Thus is suffers from two inherent drawbacks… The I/O transfer rate is limited by the speed with which the processor can test and service a device The processor is tied up in managing an I/O transfer; a number of instructions must be executed for each I/O transfer Direct Memory Access When large volumes of data are to be moved, an efficient technique is direct memory access (DMA) DMA Function DMA involves an additional module on the system bus. The DMA module is capable of mimicking the processor and taking over control of the system from the processor. It needs to do this to transfer data to and from memory over the system bus. DMA must the bus only when the processor does not need it, or it must force the processor to suspend operation temporarily (most common – referred to as cycle stealing). When the processor wishes to read or write a block of data, it issues a command to the DMA module by sending to the DMA module the following information… Whether a read or write is requested using the read or write control line between the processor and the DMA module The address of the I/O device involved, communicated on the data lines The starting location in memory to read from or write to, communicated on the data lines and stored by the DMA module in its address register The number of words to be read or written, communicated via the data lines and stored in the data count register The processor then continues with other work, it delegates the I/O operation to the DMA module which transfers the entire block of data, one word at a time, directly to or from memory without going through the processor. When the transfer is complete, the DMA module sends an interrupt signal to the processor, this the processor is involved only at the beginning and end of the transfer. I/O Channels and Processors Characteristics of I/O Channels As one proceeds along the evolutionary path, more and more of the I/O function is performed without CPU involvement. The I/O channel represents an extension of the DMA concept. An I/O channel ahs the ability to execute I/O instructions, which gives it complete control over I/O operations. In a computer system with such devices, the CPU does not execute I/O instructions – such instructions are stored in main memory to be executed by a special purpose processor in the I/O channel itself. Two types of I/O channels are common A selector channel controls multiple high-speed devices. A multiplexor channel can handle I/O with multiple characters as fast as possible to multiple devices. The external interface: FireWire and InfiniBand Types of Interfaces One major characteristic of the interface is whether it is serial or parallel parallel interface – there are multiple lines connecting the I/O module and the peripheral, and multiple bits are transferred simultaneously serial interface – there is only one line used to transmit data, and bits must be transmitted one at a time With new generation serial interfaces, parallel interfaces are becoming less common. In either case, the I/O module must engage in a dialogue with the peripheral. In general terms the dialog may look as follows… The I/O module sends a control signal requesting permission to send data The peripheral acknowledges the request The I/O module transfers data The peripheral acknowledges receipt of data For a detailed explanation of FireWire and InfiniBand technology read page 264 – 270 of the textbook

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• xcopy file, suppress &ldquo;Does xxx specify a file name&hellip;&rdquo; message

  - by MarkPearl

  Today we had an interesting problem with file copying. We wanted to use xcopy to copy a file from one location to another and rename the copied file but do this impersonating another user. Getting the impersonation to work was fairly simple, however we then had the challenge of getting xcopy to work. The problem was that xcopy kept prompting us with a prompt similar to the following… Does file.xxx specify a file name or directory name on the target (F = file, D = directory)? At which point we needed to press ‘Y’. This seems to be a fairly common challenge with xcopy, as illustrated by the following stack overflow link… One of the solutions was to do the following… echo f | xcopy /f /y srcfile destfile This is fine if you are running from the command prompt, but if you are triggering this from c# how could we daisy chain a bunch of commands…. The solution was fairly simple, we eventually ended up with the following method… public void Copy(string initialFile, string targetFile) { string xcopyExe = @"C:\windows\system32\xcopy.exe"; string cmdExe = @"C:\windows\system32\cmd.exe"; ProcessStartInfo p = new ProcessStartInfo(); p.FileName = cmdExe; p.Arguments = string.Format(@"/c echo f | {2} {0} {1} /Y", initialFile, targetFile, xcopyExe); Process.Start(p); } Where we wrapped the commands we wanted to chain as arguments and instead of calling xcopy directly, we called cmd.exe passing xcopy as an argument.

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• Simple Mouse Move Event in F# with Winforms

  - by MarkPearl

  This evening I had the pleasure of reading one of ThomasP’s blog posts on first class events. It was an excellent read, and I thought I would make a brief derivative of his post to explore some of the basics. In Thomas’s post he has a form with an ellipse on it that when he clicks on the ellipse it pops up a message box with the button clicked… awesome. Something that got me on the post though was the code similar to the one below… // React to Mouse Move events on the form let evtMessages = frm.MouseMove |> Event.map (fun mi -> mi.Location.ToString()) |> Event.map (sprintf "Hey, you clicked on the ellipse.\nUsing: %s") |> Event.add (MessageBox.Show >> ignore) The MessageBox is a function with a string passed into it. What if I wanted to rather change a mutable value holder instead, how would the syntax go for that? Immediately the thought came to me of anonymous functions. I’ve used them before to do something like this… let HelloPerson personName = "Hello " + personName |> fun(x) -> Console.WriteLine(x) So using the same approach I adapted the event code to instead of showing a Message Box with a string passed in to it, to rather change the forms header. |> Event.map (sprintf "Your mouse position is %s") |> Event.add(fun(x) -> frm.Text <- x) Okay… it looks a bit weird with the –> x <- syntax, but makes sense and works… The next thing I wanted to do was change Thomas’s code sample from having an ellipse, and reacting to the position of the mouse and click, to rather trigger the event whenever the mouse moved. This simple involved removing some filtering code. Finally I wanted the code to work as a FSharp Project without having to run through the F# interactive. To achieve this I just needed to find out how to trigger the window event loop. This can be achieved with the code below… // Program eventloop while frm.Created do Application.DoEvents()   So lets look at the complete code sample… #light open System open System.Drawing open System.Windows.Forms // Create the main form let frm = new Form(ClientSize=Size(600,400)) // React to Mouse Move events on the form let evtMessages = frm.MouseMove |> Event.map (fun mi -> mi.Location.ToString()) |> Event.map (sprintf "Your mouse position is %s") |> Event.add(fun(x) -> frm.Text <- x) // Show the form frm.Show() // Program eventloop while frm.Created do Application.DoEvents()

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• Getting Started with NASM

  - by MarkPearl

  Today I got to play with NASM. This is an assembler and disassembler that can be used to write 16-bit, 32-bit & 64-bit programs. Let me say upfront that the last time I looked at assembly code at any depth was when I was studying Computer Science in Pietermaritzburg – ten years ago – and we never ever got to touch any real assembly code so a lot of what I am looking at today is very new to me. The first thing I did was download NASM compiler. This turned out to be a bit more complicated than I thought. Originally I went to http://www.nasm.us/ and downloaded the nasm-2.09.04.zip file which I thought had all I needed. No luck! It seemed to just have the uncompiled code, and from what I could tell I would need to recompile and build it – possibly in c++? Well, I wasn’t going to waste my time with that, so a bit more searching and I found the Win32 (http://www.nasm.us/pub/nasm/releasebuilds/2.09.04/win32/) folder Nasm.exe which I downloaded. Choosing an IDE So, I have NASM compiler but to compile anything you need to pass a string of special characters in the command prompt. That’s fine if I was going to just do one program once every couple of years, but since I am aiming to do quite a bit more exploration of NASM I began searching for an IDE. There were a few options, even apparently Visual Studio with a bit of tweeking could do the job, but from past experience I wanted to avoid the VS route as it can sometimes get confusing. I eventually settled on TextPad which I had used a few years ago for a similar project and it had been simple enough yet powerful enough to do the job. A bit of searching and I found a syntax file for NASM and everything seemed hunky dory. Configuring TextPad to run the NASM Compiler Next was to get TextPad to run the NASM compiler. TextPad has this external tools option that allows one to configure special commands. To simplify the process I first created a bat file in the NASM directory that allowed me to simply compile asm files. The bat file was called as.bat and had just one line of code… nasm -f bin %1.asm -o %1.com -l %1.lst Once I had created as.bat I just needed to go into TextPad and create a tool. I have made a quick video of that just showing you where the various settings are which is viewable below. The 64Bit Problem So I now have an ‘IDE’ linked to my NASM compiler so everything should be fine right? No! Whenever I tried to compile an asm program it compiles fine, but when I try and run it I get an error – “This version of the file is not compatible with the version Windows you’re running. Check your computer’s system information to see whether you need an x86 (32-bit) or x64 (64-bit) version of the program, and then contact the software publisher." Well.. it turns out there are a few complications with having a 64 bit OS! So after searching google and coming to any real solution that I could find other than perhaps attempting to build the code for nasm, I eventually resorted to running a VM with Windows XP on it and putting NASM there… My first hello world program So I attempt my first hello world program as per an example I found… the code was quite simple and is shown below… bits16 org 0x100 jmp main message: db 'Hello World',0ah,0dh,'$' main: mov dx,message mov ah,09 int 21h int 20h Running the build tool from TextPad and everything compiles fine and I now have a console app with helllo world shown. Conclusion It’s very early days with NASM. I have been spoilt with Visual Studio and high order languages so I assume it will be a painful ride getting into the basics of assembly programming but I am hoping that at the end of it, I will at least have a bit more exposure to a language closer to the metal.

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• Getting App.config to be configuration specific in VS2010

  - by MarkPearl

  I recently wanted to have a console application that had configuration specific settings. For instance, if I had two configurations “Debug” and “Release”, depending on the currently selected configuration I wanted it to use a specific configuration file (either debug or config). If you are wanting to do something similar, here is a potential solution that worked for me. Setting up a demo app to illustrate the point First, let’s set up an application that will demonstrate the most basic concept. using System; using System.Configuration; namespace ConsoleSpecificConfiguration { class Program { static void Main(string[] args) { Console.WriteLine("Config"); Console.WriteLine(ConfigurationManager.AppSettings["Example Config"]); Console.ReadLine(); } } }   This does a really simple thing. Display a config when run. To do this, you also need a config file set up. My default looks as follows… <?xml version="1.0" encoding="utf-8" ?> <configuration> <appSettings> <add key="Example Config" value="Default"/> </appSettings> </configuration>   Your entire solution will look as follows… Running the project you will get the following amazing output…   Let’s now say instead of having one config file we want depending on whether we are running in “Debug” or “Release” for the solution configuration we want different config settings to be propagated across you can do the following… Step 1 – Create alternate config Files First add additional config files to your solution. You should have some form of naming convention for these config files, I have decided to follow a similar convention to the one used for web.config, so in my instance I am going to add a App.Debug.config and a App.Release.config file BUT you can follow any naming convention you want provided you wire up the rest of the approach to use this convention. My files look as follows.. App.Debug.config <?xml version="1.0" encoding="utf-8" ?> <configuration> <appSettings> <add key="Example Config" value="Debug"/> </appSettings> </configuration>   App.Release.config <?xml version="1.0" encoding="utf-8" ?> <configuration> <appSettings> <add key="Example Config" value="Release"/> </appSettings> </configuration>   Your solution will now look as follows… Step 2 – Create a bat file that will overwrite files The next step is to create a bat file that will overwrite one file with another. If you right click on the solution in the solution explorer there will be a menu option to add new items to the solution. Create a text file called “copyifnewer.bat” which will be our copy script. It’s contents should look as follows… @echo off echo Comparing two files: %1 with %2 if not exist %1 goto File1NotFound if not exist %2 goto File2NotFound fc %1 %2 /A if %ERRORLEVEL%==0 GOTO NoCopy echo Files are not the same. Copying %1 over %2 copy %1 %2 /y & goto END :NoCopy echo Files are the same. Did nothing goto END :File1NotFound echo %1 not found. goto END :File2NotFound copy %1 %2 /y goto END :END echo Done. Your solution should now look as follows…   Step 3 – Customize the Post Build event command line We now need to wire up everything – which we will do using the post build event command line in VS2010. Right click on your project and go to it’s properties We are now going to wire up the script so that when we build our project it will overwrite the default App.config with whatever file we want. The syntax goes as follows… call "$(SolutionDir)copyifnewer.bat" "$(ProjectDir)App.$(ConfigurationName).config" "$(ProjectDir)$(OutDir)\$(TargetFileName).config" Testing it If I now change my project configuration to Release   And then run my application I get the following output… Toggling between Release and Debug mode will show that the config file is changing each time. And that is it!

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• Moving from Tortoise to TFS

  - by MarkPearl

  The Past A few years ago my small software company made the jump from storing code on a shared folder to source code control. At the time we had evaluated a few of the options and settled on Tortoise SVN. The main motivation for going the SVN route was that we found a great plugin for Visual Studio that allowed us to avoid the command prompt for uploading changes (like I said we are windows programmers… command prompt bad!! ) and it was free. Up to now we have been pretty happy with SVN as it removed many of the worries that I had about how safe my code was on a shared folder and also gave us the opportunity to safely have several developers work on the same project at the same time. The only times when we have been unhappy has been when we have had SVN hell days – which pretty much occur when you are doing something out of the norm and suddenly SVN just won’t resolve conflicts or something along those lines. This happens once every 4 or 5 months and is not necessarily a problem caused directly by SVN – but a problem augmented by SVN. When you have SVN hell days you want to curse SVN! With that in mind I recently have been relooking at our source code control. I have explored using GIT and was very impressed by it and have also looked at TFS. From a source code control perspective I don’t want to get into a heated discussion on which one is better – but I do want to mention that I wear two hats in my organization – software developer & manager, and with the manager hat on I tend to sway the TFS route. So when I was given a coupon to test DiscountASP.Net Team Foundation Server Service for a year, I thought it was the perfect opportunity to try TFS in a distributed environment and also make the first step towards having an integrated development management system. Some of the things that appeal to me about DiscountASP’s offering are the following… Basic management / planning facilities like to do lists inside Visual Studio Daily backup of data on the server – we are developers, not IT managers and so the more of this I could outsource the better Distributed solution – all of us work remotely and so this was a big one as well. Registering and Setting Up with DiscountASP.NET The whole registration process was simple and intuitive. The web interface is not the most visually impressive one, but it is functional and a few seconds after I clicked the last submit button a email was sitting in my inbox giving me my control panel username and suggesting that I read the “Getting Started” article. The getting started article was easy to read and understand so no complaints there either. Next to set my dev environment to work. With a few references to the getting started article I had completed the whole setup process in a matter of minutes. Ten minutes after initiating the whole thing I was logged into VS2010 and creating my first TFS project. With the service that I signed up for, I have access for 5 users – which is sufficient for my internal needs. So from what I can tell, to set the rest of us up on the system I just need to supply them with their user credentials and url. My Concerns Resolved 1) Security So, a few concerns I had about the service. First and foremost – is it secure? I would hate for someone to get access to our code and the whole idea of putting it up on the internet is a concern for me. Turning to the Knowledge Base on the DiscountASP website this is one of the first question I can see answered. According to them it is secure. I have extracted their comment below regarding this. Our TFS hosting service is secure. We only accept HTTPS connections ensuring that any client-server data transmission is encrypted. At the network level, all of our systems are protected by multiple Juniper firewalls, Tipping Point's Intrusion Detection System (see Tipping Point's case study of our use here), and we also employ DDoS mitigation to add extra layers of security. Additionally, physical access to the servers is tightly restricted. Please see the security section of this Knowledge Base article for further details. 2) Web Portal Access The other big concern I have is regarding web portal access. In the ideal world I would like to be able to give my end users access to a web portal for reporting bugs etc. When I initially read through the FAQ of the site it mentioned that there was web portal access – but from what I can see this is just for “users”. Since I am limited to 5 users for the account, it would not be practical to set up external users that we could get feedback from on bugs etc. I would be interested if this is possible – and if so if someone could post it in the comments it would be much appreciated. If this isn’t possible, it is a slight let down as we rely heavily on end user feedback to get feedback and it would have been ideal to have gotten this within the service. Other than those two items, I didn’t have any real concerns that were unresolved. So where do I go from here? So time passed by from the initial writing of this post and as work whirred in and out of my inbox I have still not had a proper opportunity to give the service a test run. Recently though things have began to slow down and then surprise surprise I had another SVN Hell day. With that experience I had a new found resolve to get our team on TFS and so today we are going to start to use the service as a team. I am hoping that I do not have TFS hell days – but if I do, I will be sure to write about them. In short - the verdict is still out on whether this service is going to be invaluable to my business or whether it will create more headaches than it is worth BUT I am hopping it will be an invaluable service. I will only really be able to determine that in a few months… till then!

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• Organization &amp; Architecture UNISA Studies &ndash; Chap 5

  - by MarkPearl

  Learning Outcomes Describe the operation of a memory cell Explain the difference between DRAM and SRAM Discuss the different types of ROM Explain the concepts of a hard failure and a soft error respectively Describe SDRAM organization Semiconductor Main Memory The two traditional forms of RAM used in computers are DRAM and SRAM DRAM (Dynamic RAM) Divided into two technologies… Dynamic Static Dynamic RAM is made with cells that store data as charge on capacitors. The presence or absence of charge in a capacitor is interpreted as a binary 1 or 0. Because capacitors have natural tendency to discharge, dynamic RAM requires periodic charge refreshing to maintain data storage. The term dynamic refers to the tendency of the stored charge to leak away, even with power continuously applied. Although the DRAM cell is used to store a single bit (0 or 1), it is essentially an analogue device. The capacitor can store any charge value within a range, a threshold value determines whether the charge is interpreted as a 1 or 0. SRAM (Static RAM) SRAM is a digital device that uses the same logic elements used in the processor. In SRAM, binary values are stored using traditional flip flop logic configurations. SRAM will hold its data as along as power is supplied to it. Unlike DRAM, no refresh is required to retain data. SRAM vs. DRAM DRAM is simpler and smaller than SRAM. Thus it is more dense and less expensive than SRAM. The cost of the refreshing circuitry for DRAM needs to be considered, but if the machine requires a large amount of memory, DRAM turns out to be cheaper than SRAM. SRAMS are somewhat faster than DRAM, thus SRAM is generally used for cache memory and DRAM is used for main memory. Types of ROM Read Only Memory (ROM) contains a permanent pattern of data that cannot be changed. ROM is non volatile meaning no power source is required to maintain the bit values in memory. While it is possible to read a ROM, it is not possible to write new data into it. An important application of ROM is microprogramming, other applications include library subroutines for frequently wanted functions, System programs, Function tables. A ROM is created like any other integrated circuit chip, with the data actually wired into the chip as part of the fabrication process. To reduce costs of fabrication, we have PROMS. PROMS are… Written only once Non-volatile Written after fabrication Another variation of ROM is the read-mostly memory, which is useful for applications in which read operations are far more frequent than write operations, but for which non volatile storage is required. There are three common forms of read-mostly memory, namely… EPROM EEPROM Flash memory Error Correction Semiconductor memory is subject to errors, which can be classed into two categories… Hard failure – Permanent physical defect so that the memory cell or cells cannot reliably store data Soft failure – Random error that alters the contents of one or more memory cells without damaging the memory (common cause includes power supply issues, etc.) Most modern main memory systems include logic for both detecting and correcting errors. Error detection works as follows… When data is to be read into memory, a calculation is performed on the data to produce a code Both the code and the data are stored When the previously stored word is read out, the code is used to detect and possibly correct errors The error checking provides one of 3 possible results… No errors are detected – the fetched data bits are sent out An error is detected, and it is possible to correct the error. The data bits plus error correction bits are fed into a corrector, which produces a corrected set of bits to be sent out An error is detected, but it is not possible to correct it. This condition is reported Hamming Code See wiki for detailed explanation. We will probably need to know how to do a hemming code – refer to the textbook (pg. 188 – 189) Advanced DRAM organization One of the most critical system bottlenecks when using high-performance processors is the interface to main memory. This interface is the most important pathway in the entire computer system. The basic building block of main memory remains the DRAM chip. In recent years a number of enhancements to the basic DRAM architecture have been explored, and some of these are now on the market including… SDRAM (Synchronous DRAM) DDR-DRAM RDRAM SDRAM (Synchronous DRAM) SDRAM exchanges data with the processor synchronized to an external clock signal and running at the full speed of the processor/memory bus without imposing wait states. SDRAM employs a burst mode to eliminate the address setup time and row and column line precharge time after the first access In burst mode a series of data bits can be clocked out rapidly after the first bit has been accessed SDRAM has a multiple bank internal architecture that improves opportunities for on chip parallelism SDRAM performs best when it is transferring large blocks of data serially There is now an enhanced version of SDRAM known as double data rate SDRAM or DDR-SDRAM that overcomes the once-per-cycle limitation of SDRAM

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• The challenge of giving a positive No

  - by MarkPearl

  I find it ironic that the more I am involved in the software industry, the more apparent it becomes that soft skills are just as if not more important than the technical abilities of a developer. One of the biggest challenges I have faced in my career is in managing client expectations to what one can deliver and being able to work with multiple clients. If I look at where things commonly go pear shaped, one area features a lot is where I should have said "No" to a request, but because of the way the request was made I ended up saying yes. Time and time again this has caused immense pain. Thus, when I saw on Amazon that they had a book titled "The power of a positive no" by William Ury I had to buy it and read it. In William's book he explains an approach to saying No that while extremely simple does change the way a No is presented. In essence he talks of a pattern the Yes! > No > Yes? Pattern. 1. Yes! -> positively and concretely describing your core interests and values 2. No. -> explicitly link your no to this YES! 3. Yes? -> suggest another positive outcome or agreement to the other person Let me explain how I understood it. If you are working on a really important project and someone asks you to do add a quick feature to another project, your Yes! would be to the more important project, which would mean a No to the quick feature, and an option for your Yes? may be an alternative time when you can look at it.. An example of an appropriate response would be... It is really important that I keep to the commitment that I made to this customer to finish his project on time so I cannot work on your feature right now but I am available to help you in a weeks time. William then goes on to explain the type of behaviour a person may display when the no is received. He illustrates this with a diagram called the curve of acceptance. William points out that if you are aware of the type of behaviour you can expect it empowers you to stay true to your no. Personally I think reading and having an understanding of the “soft” side of things like saying no is invaluable to a developer.

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• Refactoring FizzBuzz

  - by MarkPearl

  A few years ago I blogger about FizzBuzz, at the time the post was prompted by Scott Hanselman who had podcasted about how surprized he was that some programmers could not even solve the FizzBuzz problem within a reasonable period of time during a job interview. At the time I thought I would give the problem a go in F# and sure enough the solution was fairly simple – I then also did a basic solution in C# but never posted it. Since then I have learned that being able to solve a problem and how you solve the problem are two totally different things. Today I decided to give the problem a retry and see if I had learnt anything new in the last year or so. Here is how my solution looked after refactoring… Solution 1 – Cheap and Nasty public class FizzBuzzCalculator { public string NumberFormat(int number) { var numDivisibleBy3 = (number % 3) == 0; var numDivisibleBy5 = (number % 5) == 0; if (numDivisibleBy3 && numDivisibleBy5) return String.Format("{0} FizzBuz", number); else if (numDivisibleBy3) return String.Format("{0} Fizz", number); else if (numDivisibleBy5) return String.Format("{0} Buz", number); return number.ToString(); } } class Program { static void Main(string[] args) { var fizzBuzz = new FizzBuzzCalculator(); for (int i = 0; i < 100; i++) { Console.WriteLine(fizzBuzz.NumberFormat(i)); } } } My first attempt I just looked at solving the problem – it works, and could be an acceptable solution but tonight I thought I would see how far  I could refactor it… The section I decided to focus on was the mass of if..else code in the NumberFormat method. Solution 2 – Replacing If…Else with a Dictionary public class FizzBuzzCalculator { private readonly Dictionary<Tuple<bool, bool>, string> _mappings; public FizzBuzzCalculator(Dictionary<Tuple<bool, bool>, string> mappings) { _mappings = mappings; } public string NumberFormat(int number) { var numDivisibleBy3 = (number % 3) == 0; var numDivisibleBy5 = (number % 5) == 0; var mappedKey = new Tuple<bool, bool>(numDivisibleBy3, numDivisibleBy5); return String.Format("{0} {1}", number, _mappings[mappedKey]); } } class Program { static void Main(string[] args) { var mappings = new Dictionary<Tuple<bool, bool>, string> { { new Tuple<bool, bool>(true, true), "- FizzBuzz"}, { new Tuple<bool, bool>(true, false), "- Fizz"}, { new Tuple<bool, bool>(false, true), "- Buzz"}, { new Tuple<bool, bool>(false, false), ""} }; var fizzBuzz = new FizzBuzzCalculator(mappings); for (int i = 0; i < 100; i++) { Console.WriteLine(fizzBuzz.NumberFormat(i)); } Console.ReadLine(); } } In my second attempt I looked at removing the if else in the NumberFormat method. A dictionary proved to be useful for this – I added a constructor to the class and injected the dictionary mapping. One could argue that this is totally overkill, but if I was going to use this code in a large system an approach like this makes it easy to put this data in a configuration file, which would up its OC (Open for extensibility, closed for modification principle). I could of course take the OC principle even further – the check for divisibility by 3 and 5 is tightly coupled to this class. If I wanted to make it 4 instead of 3, I would need to adjust this class. This introduces my third refactoring. Solution 3 – Introducing Delegates and Injecting them into the class public delegate bool FizzBuzzComparison(int number); public class FizzBuzzCalculator { private readonly Dictionary<Tuple<bool, bool>, string> _mappings; private readonly FizzBuzzComparison _comparison1; private readonly FizzBuzzComparison _comparison2; public FizzBuzzCalculator(Dictionary<Tuple<bool, bool>, string> mappings, FizzBuzzComparison comparison1, FizzBuzzComparison comparison2) { _mappings = mappings; _comparison1 = comparison1; _comparison2 = comparison2; } public string NumberFormat(int number) { var mappedKey = new Tuple<bool, bool>(_comparison1(number), _comparison2(number)); return String.Format("{0} {1}", number, _mappings[mappedKey]); } } class Program { private static bool DivisibleByNum(int number, int divisor) { return number % divisor == 0; } public static bool Divisibleby3(int number) { return number % 3 == 0; } public static bool Divisibleby5(int number) { return number % 5 == 0; } static void Main(string[] args) { var mappings = new Dictionary<Tuple<bool, bool>, string> { { new Tuple<bool, bool>(true, true), "- FizzBuzz"}, { new Tuple<bool, bool>(true, false), "- Fizz"}, { new Tuple<bool, bool>(false, true), "- Buzz"}, { new Tuple<bool, bool>(false, false), ""} }; var fizzBuzz = new FizzBuzzCalculator(mappings, Divisibleby3, Divisibleby5); for (int i = 0; i < 100; i++) { Console.WriteLine(fizzBuzz.NumberFormat(i)); } Console.ReadLine(); } } I have taken this one step further and introduced delegates that are injected into the FizzBuzz Calculator class, from an OC principle perspective it has probably made it more compliant than the previous Solution 2, but there seems to be a lot of noise. Anonymous Delegates increase the readability level, which is what I have done in Solution 4. Solution 4 – Anon Delegates public delegate bool FizzBuzzComparison(int number); public class FizzBuzzCalculator { private readonly Dictionary<Tuple<bool, bool>, string> _mappings; private readonly FizzBuzzComparison _comparison1; private readonly FizzBuzzComparison _comparison2; public FizzBuzzCalculator(Dictionary<Tuple<bool, bool>, string> mappings, FizzBuzzComparison comparison1, FizzBuzzComparison comparison2) { _mappings = mappings; _comparison1 = comparison1; _comparison2 = comparison2; } public string NumberFormat(int number) { var mappedKey = new Tuple<bool, bool>(_comparison1(number), _comparison2(number)); return String.Format("{0} {1}", number, _mappings[mappedKey]); } } class Program { static void Main(string[] args) { var mappings = new Dictionary<Tuple<bool, bool>, string> { { new Tuple<bool, bool>(true, true), "- FizzBuzz"}, { new Tuple<bool, bool>(true, false), "- Fizz"}, { new Tuple<bool, bool>(false, true), "- Buzz"}, { new Tuple<bool, bool>(false, false), ""} }; var fizzBuzz = new FizzBuzzCalculator(mappings, (n) => n % 3 == 0, (n) => n % 5 == 0); for (int i = 0; i < 100; i++) { Console.WriteLine(fizzBuzz.NumberFormat(i)); } Console.ReadLine(); } }   Using the anonymous delegates I think the noise level has now been reduced. This is where I am going to end this post, I have gone through 4 iterations of the code from the initial solution using If..Else to delegates and dictionaries. I think each approach would have it’s pro’s and con’s and depending on the intention of where the code would be used would be a large determining factor. If you can think of an alternative way to do FizzBuzz, add a comment!

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• Yippy &ndash; the F# MVVM Pattern

  - by MarkPearl

  I did a recent post on implementing WPF with F#. Today I would like to expand on this posting to give a simple implementation of the MVVM pattern in F#. A good read about this topic can also be found on Dean Chalk’s blog although my example of the pattern is possibly simpler. With the MVVM pattern one typically has 3 segments, the view, viewmodel and model. With the beauty of WPF binding one is able to link the state based viewmodel to the view. In my implementation I have kept the same principles. I have a view (MainView.xaml), and and a ViewModel (MainViewModel.fs).     What I would really like to illustrate in this posting is the binding between the View and the ViewModel so I am going to jump to that… In Program.fs I have the following code… module Program open System open System.Windows open System.Windows.Controls open System.Windows.Markup open myViewModels // Create the View and bind it to the View Model let myView = Application.LoadComponent(new System.Uri("/FSharpWPF;component/MainView.xaml", System.UriKind.Relative)) :?> Window myView.DataContext <- new MainViewModel() :> obj // Application Entry point [<STAThread>] [<EntryPoint>] let main(_) = (new Application()).Run(myView) You can see that I have simply created the view (myView) and then created an instance of my viewmodel (MainViewModel) and then bound it to the data context with the code… myView.DataContext <- new MainViewModel() :> obj If I have a look at my viewmodel (MainViewModel) it looks like this… module myViewModels open System open System.Windows open System.Windows.Input open System.ComponentModel open ViewModelBase type MainViewModel() = // private variables let mutable _title = "Bound Data to Textbox" // public properties member x.Title with get() = _title and set(v) = _title <- v // public commands member x.MyCommand = new FuncCommand ( (fun d -> true), (fun e -> x.ShowMessage) ) // public methods member public x.ShowMessage = let msg = MessageBox.Show(x.Title) () I have exposed a few things, namely a property called Title that is mutable, a command and a method called ShowMessage that simply pops up a message box when called. If I then look at my view which I have created in xaml (MainView.xaml) it looks as follows… <Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="F# WPF MVVM" Height="350" Width="525"> <Grid> <Grid.RowDefinitions> <RowDefinition Height="Auto"/> <RowDefinition Height="Auto"/> <RowDefinition Height="*"/> </Grid.RowDefinitions> <TextBox Text="{Binding Path=Title, Mode=TwoWay}" Grid.Row="0"/> <Button Command="{Binding MyCommand}" Grid.Row="1"> <TextBlock Text="Click Me"/> </Button> </Grid> </Window>   It is also very simple. It has a button that’s command is bound to the MyCommand and a textbox that has its text bound to the Title property. One other module that I have created is my ViewModelBase. Right now it is used to store my commanding function but I would look to expand on it at a later stage to implement other commonly used functions… module ViewModelBase open System open System.Windows open System.Windows.Input open System.ComponentModel type FuncCommand (canExec:(obj -> bool),doExec:(obj -> unit)) = let cecEvent = new DelegateEvent<EventHandler>() interface ICommand with [<CLIEvent>] member x.CanExecuteChanged = cecEvent.Publish member x.CanExecute arg = canExec(arg) member x.Execute arg = doExec(arg) Put this all together and you have a basic project that implements the MVVM pattern in F#. For me this is quite exciting as it turned out to be a lot simpler to do than I originally thought possible. Also because I have my view in XAML I can use the XAML designer to design forms in F# which I believe is a much cleaner way to go rather than implementing it all in code. Finally if I look at my viewmodel code, it is actually quite clean and compact…

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• Computer Networks UNISA - Chap 15 &ndash; Network Management

  - by MarkPearl

  After reading this section you should be able to Understand network management and the importance of documentation, baseline measurements, policies, and regulations to assess and maintain a network’s health. Manage a network’s performance using SNMP-based network management software, system and event logs, and traffic-shaping techniques Identify the reasons for and elements of an asset managements system Plan and follow regular hardware and software maintenance routines Fundamentals of Network Management Network management refers to the assessment, monitoring, and maintenance of all aspects of a network including checking for hardware faults, ensuring high QoS, maintaining records of network assets, etc. Scope of network management differs depending on the size and requirements of the network. All sub topics of network management share the goals of enhancing the efficiency and performance while preventing costly downtime or loss. Documentation The way documentation is stored may vary, but to adequately manage a network one should at least record the following… Physical topology (types of LAN and WAN topologies – ring, star, hybrid) Access method (does it use Ethernet 802.3, token ring, etc.) Protocols Devices (Switches, routers, etc) Operating Systems Applications Configurations (What version of operating system and config files for serve / client software) Baseline Measurements A baseline is a report of the network’s current state of operation. Baseline measurements might include the utilization rate for your network backbone, number of users logged on per day, etc. Baseline measurements allow you to compare future performance increases or decreases caused by network changes or events with past network performance. Obtaining baseline measurements is the only way to know for certain whether a pattern of usage has changed, or whether a network upgrade has made a difference. There are various tools available for measuring baseline performance on a network. Policies, Procedures, and Regulations Following rules helps limit chaos, confusion, and possibly downtime. The following policies and procedures and regulations make for sound network management. Media installations and management (includes designing physical layout of cable, etc.) Network addressing policies (includes choosing and applying a an addressing scheme) Resource sharing and naming conventions (includes rules for logon ID’s) Security related policies Troubleshooting procedures Backup and disaster recovery procedures In addition to internal policies, a network manager must consider external regulatory rules. Fault and Performance Management After documenting every aspect of your network and following policies and best practices, you are ready to asses you networks status on an on going basis. This process includes both performance management and fault management. Network Management Software To accomplish both fault and performance management, organizations often use enterprise-wide network management software. There various software packages that do this, each collect data from multiple networked devices at regular intervals, in a process called polling. Each managed device runs a network management agent. So as not to affect the performance of a device while collecting information, agents do not demand significant processing resources. The definition of a managed devices and their data are collected in a MIB (Management Information Base). Agents communicate information about managed devices via any of several application layer protocols. On modern networks most agents use SNMP which is part of the TCP/IP suite and typically runs over UDP on port 161. Because of the flexibility and sophisticated network management applications are a challenge to configure and fine-tune. One needs to be careful to only collect relevant information and not cause performance issues (i.e. pinging a device every 5 seconds can be a problem with thousands of devices). MRTG (Multi Router Traffic Grapher) is a simple command line utility that uses SNMP to poll devices and collects data in a log file. MRTG can be used with Windows, UNIX and Linux. System and Event Logs Virtually every condition recognized by an operating system can be recorded. This is typically done using event logs. In Windows there is a GUI event log viewer. Similar information is recorded in UNIX and Linux in a system log. Much of the information collected in event logs and syslog files does not point to a problem, even if it is marked with a warning so it is important to filter your logs appropriately to reduce the noise. Traffic Shaping When a network must handle high volumes of network traffic, users benefit from performance management technique called traffic shaping. Traffic shaping involves manipulating certain characteristics of packets, data streams, or connections to manage the type and amount of traffic traversing a network or interface at any moment. Its goals are to assure timely delivery of the most important traffic while offering the best possible performance for all users. Several types of traffic prioritization exist including prioritizing traffic according to any of the following characteristics… Protocol IP address User group DiffServr VLAN tag in a Data Link layer frame Service or application Caching In addition to traffic shaping, a network or host might use caching to improve performance. Caching is the local storage of frequently needed files that would otherwise be obtained from an external source. By keeping files close to the requester, caching allows the user to access those files quickly. The most common type of caching is Web caching, in which Web pages are stored locally. To an ISP, caching is much more than just convenience. It prevents a significant volume of WAN traffic, thus improving performance and saving money. Asset Management Another key component in managing networks is identifying and tracking its hardware. This is called asset management. The first step to asset management is to take an inventory of each node on the network. You will also want to keep records of every piece of software purchased by your organization. Asset management simplifies maintaining and upgrading the network chiefly because you know what the system includes. In addition, asset management provides network administrators with information about the costs and benefits of certain types of hardware or software. Change Management Networks are always in a stage of flux with various aspects including… Software changes and patches Client Upgrades Shared Application Upgrades NOS Upgrades Hardware and Physical Plant Changes Cabling Upgrades Backbone Upgrades For a detailed explanation on each of these read the textbook (Page 750 – 761)

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• F# Objects &ndash; Integrating with the other .Net Languages &ndash; Part 1

  - by MarkPearl

  In the next few blog posts I am going to explore objects in F#. Up to now, my dabbling in F# has really been a few liners and while I haven’t reached the point where F# is my language of preference – I am already seeing the benefits of the language when solving certain types of programming problems. For me I believe that the F# language will be used in a silo like architecture and that the real benefit of having F# under your belt is so that you can solve problems that F# lends itself towards and then interact with other .Net languages in doing the rest. When I was still very new to the F# language I did the following post covering how to get F# & C# to talk to each other. Today I am going to use a similar approach to demonstrate the structure of F# objects when inter-operating with other languages. Lets start with an empty F# class … type Person() = class end   Very simple, and all we really have done is declared an object that has nothing in it. We could if we wanted to make an object that takes a constructor with parameters… the code for this would look something like this… type Person =     {         Firstname : string         Lastname : string     }   What’s interesting about this syntax is when you try and interop with this object from another .Net language like C# - you would see the following…   Not only has a constructor been created that accepts two parameters, but Firstname and Lastname are also exposed on the object. Now it’s important to keep in mind that value holders in F# are immutable by default, so you would not be able to change the value of Firstname after the construction of the object – in C# terms it has been set to readonly. One could however explicitly state that the value holders were mutable, which would then allow you to change the values after the actual creation of the object. type Person = { mutable Firstname : string mutable Lastname : string }   Something that bugged me for a while was what if I wanted to have an F# object that requires values in its constructor, but does not expose them as part of the object. After bashing my head for a few moments I came up with the following syntax – which achieves this result. type Person(Firstname : string, Lastname : string) = member v.Fullname = Firstname + " " + Lastname What I haven’t figured out yet is what is the difference between the () & {} brackets when declaring an object.

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• Computer Networks UNISA - Chap 14 &ndash; Insuring Integrity &amp; Availability

  - by MarkPearl

  After reading this section you should be able to Identify the characteristics of a network that keep data safe from loss or damage Protect an enterprise-wide network from viruses Explain network and system level fault tolerance techniques Discuss issues related to network backup and recovery strategies Describe the components of a useful disaster recovery plan and the options for disaster contingencies What are integrity and availability? Integrity – the soundness of a networks programs, data, services, devices, and connections Availability – How consistently and reliably a file or system can be accessed by authorized personnel A number of phenomena can compromise both integrity and availability including… security breaches natural disasters malicious intruders power flaws human error users etc Although you cannot predict every type of vulnerability, you can take measures to guard against the most damaging events. The following are some guidelines… Allow only network administrators to create or modify NOS and application system users. Monitor the network for unauthorized access or changes Record authorized system changes in a change management system’ Install redundant components Perform regular health checks on the network Check system performance, error logs, and the system log book regularly Keep backups Implement and enforce security and disaster recovery policies These are just some of the basics… Malware Malware refers to any program or piece of code designed to intrude upon or harm a system or its resources. Types of Malware… Boot sector viruses Macro viruses File infector viruses Worms Trojan Horse Network Viruses Bots Malware characteristics Some common characteristics of Malware include… Encryption Stealth Polymorphism Time dependence Malware Protection There are various tools available to protect you from malware called anti-malware software. These monitor your system for indications that a program is performing potential malware operations. A number of techniques are used to detect malware including… Signature Scanning Integrity Checking Monitoring unexpected file changes or virus like behaviours It is important to decide where anti-malware tools will be installed and find a balance between performance and protection. There are several general purpose malware policies that can be implemented to protect your network including… Every compute in an organization should be equipped with malware detection and cleaning software that regularly runs Users should not be allowed to alter or disable the anti-malware software Users should know what to do in case the anti-malware program detects a malware virus Users should be prohibited from installing any unauthorized software on their systems System wide alerts should be issued to network users notifying them if a serious malware virus has been detected. Fault Tolerance Besides guarding against malware, another key factor in maintaining the availability and integrity of data is fault tolerance. Fault tolerance is the ability for a system to continue performing despite an unexpected hardware or software malfunction. Fault tolerance can be realized in varying degrees, the optimal level of fault tolerance for a system depends on how critical its services and files are to productivity. Generally the more fault tolerant the system, the more expensive it is. The following describe some of the areas that need to be considered for fault tolerance. Environment (Temperature and humidity) Power Topology and Connectivity Servers Storage Power Typical power flaws include Surges – a brief increase in voltage due to lightening strikes, solar flares or some idiot at City Power Noise – Fluctuation in voltage levels caused by other devices on the network or electromagnetic interference Brownout – A sag in voltage for just a moment Blackout – A complete power loss The are various alternate power sources to consider including UPS’s and Generators. UPS’s are found in two categories… Standby UPS – provides continuous power when mains goes down (brief period of switching over) Online UPS – is online all the time and the device receives power from the UPS all the time (the UPS is charged continuously) Servers There are various techniques for fault tolerance with servers. Server mirroring is an option where one device or component duplicates the activities of another. It is generally an expensive process. Clustering is a fault tolerance technique that links multiple servers together to appear as a single server. They share processing and storage responsibilities and if one unit in the cluster goes down, another unit can be brought in to replace it. Storage There are various techniques available including the following… RAID Arrays NAS (Storage (Network Attached Storage) SANs (Storage Area Networks) Data Backup A backup is a copy of data or program files created for archiving or safekeeping. Many different options for backups exist with various media including… These vary in cost and speed. Optical Media Tape Backup External Disk Drives Network Backups Backup Strategy After selecting the appropriate tool for performing your servers backup, devise a backup strategy to guide you through performing reliable backups that provide maximum data protection. Questions that should be answered include… What data must be backed up At what time of day or night will the backups occur How will you verify the accuracy of the backups Where and for how long will backup media be stored Who will take responsibility for ensuring that backups occurred How long will you save backups Where will backup and recovery documentation be stored Different backup methods provide varying levels of certainty and corresponding labour cost. There are also different ways to determine which files should be backed up including… Full backup – all data on all servers is copied to storage media Incremental backup – Only data that has changed since the last full or incremental backup is copied to a storage medium Differential backup – Only data that has changed since the last backup is coped to a storage medium Disaster Recovery Disaster recovery is the process of restoring your critical functionality and data after an enterprise wide outage has occurred. A disaster recovery plan is for extreme scenarios (i.e. fire, line fault, etc). A cold site is a place were the computers, devices, and connectivity necessary to rebuild a network exist but they are not appropriately configured. A warm site is a place where the computers, devices, and connectivity necessary to rebuild a network exists with some appropriately configured devices. A hot site is a place where the computers, devices, and connectivity necessary to rebuild a network exists and all are appropriately configured.

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• Stacks in C++

  - by MarkPearl

  So some more basics… One of the things you will be taught at any college after conquering arrays is different derivatives of collections. Stack is one of the simplest of those and very useful… A stack is a LIFO (last in first out) data structure and has at least two basic method calls – push & pop. Push, “pushes” an item on the top of the stack. Pop, removes the top most item off the stack. Because all elements on a stack are of the same type, one can use an array to implement a stack or a linked list. With the array based approach, the first element in a stack would be the first element in the array, the second on the stack would be the second on the array, etc. One limitation with an array implementation of a stack is that unless the array is dynamic, one would have to have a preset max stack size (based on the bounds of the array). Linked lists is another approach that gets past this boundary by allowing you to dynamically grow or shrink a collection of data. Stacks have many applications… a typical computer science example would be Postfix Expression Calculator, where the LIFO principle is maintained.

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• Microsoft Dev Days &ndash; Johannesburg 2010

  - by MarkPearl

  So I am half way through dev days in Johannesburg. It has been quite an interesting day… Maybe it is me, but this year it hasn’t been as OMG as at previous conferences. A few things that stood out though… 1) This is the first time I have had to queue in a line to use the gents toilets before – yes, a true sign that we are at a typically male dominated industry event in this country – the men’s toilets were jam packed – the ladies if there were any there didn’t have a problem. 2) Bart De Smet presentation still rocks – I am a fan of Bart’s and once again his presentation was great. Something that I am going to look into in more depth which I think is a new feature in .Net is called Code Contracts. 3) I have got to get into Silverlight more… I have known this for a long time and have dabbled in it for a while, but Silverlight in my opinion will become the main platform for “hosting” applications. So… 3 things so far, hopefully I get some OMG’s from the rest of the day…

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• Organization &amp; Architecture UNISA Studies &ndash; Chap 6

  - by MarkPearl

  Learning Outcomes Discuss the physical characteristics of magnetic disks Describe how data is organized and accessed on a magnetic disk Discuss the parameters that play a role in the performance of magnetic disks Describe different optical memory devices Magnetic Disk The way data is stored on and retried from magnetic disks Data is recorded on and later retrieved form the disk via a conducting coil named the head (in many systems there are two heads) The writ mechanism exploits the fact that electricity flowing through a coil produces a magnetic field. Electric pulses are sent to the write head, and the resulting magnetic patterns are recorded on the surface below with different patterns for positive and negative currents The physical characteristics of a magnetic disk   Summarize from book   The factors that play a role in the performance of a disk Seek time – the time it takes to position the head at the track Rotational delay / latency – the time it takes for the beginning of the sector to reach the head Access time – the sum of the seek time and rotational delay Transfer time – the time it takes to transfer data RAID The rate of improvement in secondary storage performance has been considerably less than the rate for processors and main memory. Thus secondary storage has become a bit of a bottleneck. RAID works on the concept that if one disk can be pushed so far, additional gains in performance are to be had by using multiple parallel components. Points to note about RAID… RAID is a set of physical disk drives viewed by the operating system as a single logical drive Data is distributed across the physical drives of an array in a scheme known as striping Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure (not supported by RAID 0 or RAID 1) Interesting to note that the increase in the number of drives, increases the probability of failure. To compensate for this decreased reliability RAID makes use of stored parity information that enables the recovery of data lost due to a disk failure.   The RAID scheme consists of 7 levels…   Category Level Description Disks Required Data Availability Large I/O Data Transfer Capacity Small I/O Request Rate Striping 0 Non Redundant N Lower than single disk Very high Very high for both read and write Mirroring 1 Mirrored 2N Higher than RAID 2 – 5 but lower than RAID 6 Higher than single disk Up to twice that of a signle disk for read Parallel Access 2 Redundant via Hamming Code N + m Much higher than single disk Highest of all listed alternatives Approximately twice that of a single disk Parallel Access 3 Bit interleaved parity N + 1 Much higher than single disk Highest of all listed alternatives Approximately twice that of a single disk Independent Access 4 Block interleaved parity N + 1 Much higher than single disk Similar to RAID 0 for read, significantly lower than single disk for write Similar to RAID 0 for read, significantly lower than single disk for write Independent Access 5 Block interleaved parity N + 1 Much higher than single disk Similar to RAID 0 for read, lower than single disk for write Similar to RAID 0 for read, generally  lower than single disk for write Independent Access 6 Block interleaved parity N + 2 Highest of all listed alternatives Similar to RAID 0 for read; lower than RAID 5 for write Similar to RAID 0 for read, significantly lower than RAID 5  for write   Read page 215 – 221 for detailed explanation on RAID levels Optical Memory There are a variety of optical-disk systems available. Read through the table on page 222 – 223 Some of the devices include… CD CD-ROM CD-R CD-RW DVD DVD-R DVD-RW Blue-Ray DVD Magnetic Tape Most modern systems use serial recording – data is lade out as a sequence of bits along each track. The typical recording used in serial is referred to as serpentine recording. In this technique when data is being recorded, the first set of bits is recorded along the whole length of the tape. When the end of the tape is reached the heads are repostioned to record a new track, and the tape is again recorded on its whole length, this time in the opposite direction. That process continued back and forth until the tape is full. To increase speed, the read-write head is capable of reading and writing a number of adjacent tracks simultaneously. Data is still recorded serially along individual tracks, but blocks in sequence are stored on adjacent tracks as suggested. A tape drive is a sequential access device. Magnetic tape was the first kind of secondary memory. It is still widely used as the lowest-cost, slowest speed member of the memory hierarchy.

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• Computer Arithmetic - Binary for Decimal Numbers

  - by MarkPearl

  This may be of use to someone else doing this course… The Problem In the section on Computer Arithmetic it gives an example of converting -7.6875 to IEEE floating point format. I understand all the steps except for the first one, where it does the following... 7.6875 (base 10) = 111.1011 (base 2) I don't understand the conversion - I realize that 111 (base 2) = 7 (base 10), but how does the .6875 part relate to the .1011? Or am I totally off track with this? The Solution The fractional part of the decimal to binary conversion is done as follows: 0.6875 x 2 = 1.375 = 0.375 + 1 (Keep the 1 separate) 0.375 x 2   = 0.75   = 0.75    + 0 0.75 x 2    = 1.5      = 0.5      + 1 0.5 x 2     = 1.0       = 0.0       + 1 The bit pattern of 0s and 1s on the right-hand side gives you the fractional part. So 0.6875 (base 10) = .1011 (Base 2) See also Stallings, chapter 19.

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• Does F# kill C++?

  - by MarkPearl

  Okay, so the title may be a little misleading… but I am currently travelling and so have had very little time and access to resources to do much fsharping – this has meant that I am right now missing my favourite new language. I was interested to see this post on Stack Overflow this evening concerning the performance of the F# language. The person posing the question asked 8 key points about the F# language, namely… How well does it do floating-point? Does it allow vector instructions How friendly is it towards optimizing compilers? How big a memory foot print does it have? Does it allow fine-grained control over memory locality? Does it have capacity for distributed memory processors, for example Cray? What features does it have that may be of interest to computational science where heavy number processing is involved? Are there actual scientific computing implementations that use it? Now, I don’t have much time to look into a decent response and to be honest I don’t know half of the answers to what he is asking, but it was interesting to see what was put up as an answer so far and would be interesting to get other peoples feedback on these questions if they know of anything other than what has been covered in the answer section already.

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• 5 Lessons learnt in localization / multi language support in WPF

  - by MarkPearl

  For the last few months I have been secretly working away at the second version of an application that we initially released a few years ago. It’s called MaxCut and it is a free panel/cut optimizer for the woodwork, glass and metal industry. One of the motivations for writing MaxCut was to get an end to end experience in developing an application for general consumption. From the early days of v1 of MaxCut I would get the odd email thanking me for the software and then listing a few suggestions on how to improve it. Two of the most dominant suggestions that we received were… Support for imperial measurements (the original program only supported the metric system) Multi language support (we had someone who volunteered to translate the program into Japanese for us). I am not going to dive into the Imperial to Metric support in todays blog post, but I would like to cover a few brief lessons we learned in adding support for multi-language functionality in the software. I have sectioned them below under different lessons. Lesson 1 – Build multi-language support in from the start So the first lesson I learnt was if you know you are going to do multi language support – build it in from the very beginning! One of the power points of WPF/Silverlight is data binding in XAML and so while it wasn’t to painful to retro fit multi language support into the programing, it was still time consuming and a bit tedious to go through mounds and mounds of views and would have been a minor job to have implemented this while the form was being designed. Lesson 2 – Accommodate for varying word lengths using Grids The next lesson was a little harder to learn and was learnt a bit further down the road in the development cycle. We developed everything in English, assuming that other languages would have similar character length words for equivalent meanings… don’t!. A word that is short in your language may be of varying character lengths in other languages. Some language like Dutch and German allow for concatenation of nouns which has the potential to create really long words. We picked up a few places where our views had been structured incorrectly so that if a word was to long it would get clipped off or cut out. To get around this we began using the WPF grid extensively with column widths that would automatically expand if they needed to. Generally speaking the grid replacement got round this hurdle, and if in future you have a choice between a stack panel or a grid – think twice before going for the easier option… often the grid will be a bit more work to setup, but will be more flexible. Lesson 3 – Separate the separators Our initial run through moving the words to a resource dictionary led us to make what I thought was one potential mistake. If we had a label like the following… “length : “ In the resource dictionary we put it as a single entry. This is fine until you start using a word more than once. For instance in our scenario we used the word “length’ frequently. with different variations of the word with grammar and separators included in the resource we ended up having what I would consider a bloated dictionary. When we removed the separators from the words and put them as their own resources we saw a dramatic reduction in dictionary size… so something that looked like this… “length : “ “length. “ “length?” Was reduced to… “length” “:” “?” “.” While this may not seem like a reduction at first glance, consider that the separators “:?.” are used everywhere and suddenly you see a real reduction in bloat. Lesson 4 – Centralize the Language Dictionary This lesson was learnt at the very end of the project after we had already had a release candidate out in the wild. Because our translations would be done on a volunteer basis and remotely, we wanted it to be really simple for someone to translate our program into another language. As a common design practice we had tiered the application so that we had a business logic layer, a ui layer, etc. The problem was in several of these layers we had resource files specific for that layer. What this resulted in was us having multiple resource files that we would need to send to our translators. To add to our problems, some of the wordings were duplicated in different resource files, which would result in additional frustration from our translators as they felt they were duplicating work. Eventually the workaround was to make a separate project in VS2010 with just the language translations. We then exposed the dictionary as public within this project and made it as a reference to the other projects within the solution. This solved out problem as now we had a central dictionary and could remove any duplication's. Lesson 5 – Make a dummy translation file to test that you haven’t missed anything The final lesson learnt about multi language support in WPF was when checking if you had forgotten to translate anything in the inline code, make a test resource file with dummy data. Ideally you want the data for each word to be identical. In our instance we made one which had all the resource key values pointing to a value of test. This allowed us point the language file to our test resource file and very quickly browse through the program and see if we had missed any linking. The alternative to this approach is to have two language files and swap between the two while running the program to make sure that you haven’t missed anything, but the downside of dual language file approach is that it is much a lot harder spotting a mistake if everything is different – almost like playing Where’s Wally / Waldo. It is much easier spotting variance in uniformity – meaning when you put the “test’ keyword for everything, anything that didn’t say “test” stuck out like a sore thumb. So these are my top five lessons learnt on implementing multi language support in WPF. Feel free to make any suggestions in the comments section if you feel maybe something is more important than one of these or if I got it wrong!

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• Computer Networks UNISA - Chap 8 &ndash; Wireless Networking

  - by MarkPearl

  After reading this section you should be able to Explain how nodes exchange wireless signals Identify potential obstacles to successful transmission and their repercussions, such as interference and reflection Understand WLAN architecture Specify the characteristics of popular WLAN transmission methods including 802.11 a/b/g/n Install and configure wireless access points and their clients Describe wireless MAN and WAN technologies, including 802.16 and satellite communications The Wireless Spectrum All wireless signals are carried through the air by electromagnetic waves. The wireless spectrum is a continuum of the electromagnetic waves used for data and voice communication. The wireless spectrum falls between 9KHZ and 300 GHZ. Characteristics of Wireless Transmission Antennas Each type of wireless service requires an antenna specifically designed for that service. The service’s specification determine the antenna’s power output, frequency, and radiation pattern. A directional antenna issues wireless signals along a single direction. An omnidirectional antenna issues and receives wireless signals with equal strength and clarity in all directions The geographical area that an antenna or wireless system can reach is known as its range Signal Propagation LOS (line of sight) uses the least amount of energy and results in the reception of the clearest possible signal. When there is an obstacle in the way, the signal may… pass through the object or be obsrobed by the object or may be subject to reflection, diffraction or scattering. Reflection – waves encounter an object and bounces off it. Diffraction – signal splits into secondary waves when it encounters an obstruction Scattering – is the diffusion or the reflection in multiple different directions of a signal Signal Degradation Fading occurs as a signal hits various objects. Because of fading, the strength of the signal that reaches the receiver is lower than the transmitted signal strength. The further a signal moves from its source, the weaker it gets (this is called attenuation) Signals are also affected by noise – the electromagnetic interference) Interference can distort and weaken a wireless signal in the same way that noise distorts and weakens a wired signal. Frequency Ranges Older wireless devices used the 2.4 GHZ band to send and receive signals. This had 11 communication channels that are unlicensed. Newer wireless devices can also use the 5 GHZ band which has 24 unlicensed bands Narrowband, Broadband, and Spread Spectrum Signals Narrowband – a transmitter concentrates the signal energy at a single frequency or in a very small range of frequencies Broadband – uses a relatively wide band of the wireless spectrum and offers higher throughputs than narrowband technologies The use of multiple frequencies to transmit a signal is known as spread-spectrum technology. In other words a signal never stays continuously within one frequency range during its transmission. One specific implementation of spread spectrum is FHSS (frequency hoping spread spectrum). Another type is known as DSS (direct sequence spread spectrum) Fixed vs. Mobile Each type of wireless communication falls into one of two categories Fixed – the location of the transmitted and receiver do not move (results in energy saved because weaker signal strength is possible with directional antennas) Mobile – the location can change WLAN (Wireless LAN) Architecture There are two main types of arrangements Adhoc – data is sent directly between devices – good for small local devices Infrastructure mode – a wireless access point is placed centrally, that all devices connect with 802.11 WLANs The most popular wireless standards used on contemporary LANs are those developed by IEEE’s 802.11 committee. Over the years several distinct standards related to wireless networking have been released. Four of the best known standards are also referred to as Wi-Fi. They are…. 802.11b 802.11a 802.11g 802.11n These four standards share many characteristics. i.e. All 4 use half duplex signalling Follow the same access method Access Method 802.11 standards specify the use of CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) to access a shared medium. Using CSMA/CA before a station begins to send data on an 802.11 network, it checks for existing wireless transmissions. If the source node detects no transmission activity on the network, it waits a brief period of time and then sends its transmission. If the source does detect activity, it waits a brief period of time before checking again. The destination node receives the transmission and, after verifying its accuracy, issues an acknowledgement (ACT) packet to the source. If the source receives the ACK it assumes the transmission was successful, – if it does not receive an ACK it assumes the transmission failed and sends it again. Association Two types of scanning… Active – station transmits a special frame, known as a prove, on all available channels within its frequency range. When an access point finds the probe frame, it issues a probe response. Passive – wireless station listens on all channels within its frequency range for a special signal, known as a beacon frame, issued from an access point – the beacon frame contains information necessary to connect to the point. Re-association occurs when a mobile user moves out of one access point’s range and into the range of another. Frames Read page 378 – 381 about frames and specific 802.11 protocols Bluetooth Networks Sony Ericson originally invented the Bluetooth technology in the early 1990s. In 1998 other manufacturers joined Ericsson in the Special Interest Group (SIG) whose aim was to refine and standardize the technology. Bluetooth was designed to be used on small networks composed of personal communications devices. It has become popular wireless technology for communicating among cellular telephones, phone headsets, etc. Wireless WANs and Internet Access Refer to pages 396 – 402 of the textbook for details.

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• C# Concisely by Judith Bishop &amp; Nigel Horspool

  - by MarkPearl

  In my quest to read all the books I have lying on my bookshelf I have finally got round to finishing C# Concisely (ISBN 0-321-15418-5). While this book was fairly old, I found it to be quite useful for a student wanting to learn C# for the first time, and a nice way to review and make I hadn’t missed something when I was learning the language. The book is simple and explains the basic concepts in a clean manner, but is really intended for the beginner programmer – it also had a few chapters dedicated to winforms, which was an indication of its age. None the less, I will keep it on the bookshelf so when I come across someone who is wanting to learn the language I can give them it as a gift.

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• 5 ways to stop code thrashing&hellip;

  - by MarkPearl

  A few days ago I was programming on a personal project and hit a roadblock. I was applying the MVVM pattern and for some reason my view model was not updating the view when the state changed??? I had applied this pattern many times before and had never had this problem. It just didn’t make sense. So what did I do… I did what anyone would have done in my situation and looked to pass the blame to someone or something else. I tried to blame one of the inherited base classes, but it looked fine, then to Visual Studio, but it seemed to be fine and eventually to any random segment of code I came across. My elementary problem had now mushroomed into one that had lost any logical basis and I was in thrashing mode! So what to do when you begin to thrash? 1) Do a general code cleanup – Now there is a difference between cleaning code and changing code . When you thrash you change code and you want to avoid this. What you really want to do is things like rename variables to have better meaning and go over your comments. 2) Do a proof of concept – if cleaning code doesn’t help. The  you want to isolate the problem and identify the key concepts. When you isolate code you ideally want it to be in a totally separate project with as little complexity as possible. Make the building blocks and try and replicate the functionality that you are getting in your current application. 3) Phone a friend – I have found speaking to someone else about the problem generally helps me solve any thrashing issues I am having. Usually they don’t even have to say anything to solve the problem, just you talking them through the problem helps you get clarity of mind. 4) Let the dust settle – Sometimes time is the best solution. I have had a few problems that no matter who I discussed them with and no matter how much code cleaning I had done I just couldn’t seem to fix it. My brain just seemed to be going in circles. A good nights rest has always helped and often just the break away from the problem has helped me find a solution. 5) Stack overflow it – So similar to phone a friend. I am really surprised to see what a melting pot stack overflow has been and what a help it has been in solving technology specific problems. Just be considerate to those using the site and explain clearly exactly what problem you are having and the technologies you are using or else you will probably not get any useful help…

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• F# &ndash; It&rsquo;s time to grow up baby.

  - by MarkPearl

  In the last few months since I started learning F# I have begin to notice an increase in the number of people blogging about the language. Sure, it could just be that I am noticing it more because I am actively looking out for questions and blog posts, but even in my day to day reading of Code Project Daily News and Stack Overflow questions there seems to be increased activity around the language. So what sparked this post? Well, today today I logged in and saw that the latest podcast by DNR was on F# and then immediately afterwards I received an email from CodeProject with a great article comparing F# and Scala. Currently, as of this blog posting (21 May 2010) F# is ranked on the tiobe site at position 43, but I am willing to put money on it that in the next few tiobe ratings this ranking will continue to rise till F# will be a top 20.

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• South African MVPs deserve their title.

  - by MarkPearl

  Recently I read a post by someone who felt the Microsoft MVP program had failed. My local experience with the MVP program would tend for me to disagree. On Saturday I attended a free Windows Phone 7 event organized by Robert MacLean and Rudi Grobler both of whom are local MVP’s. First of all, kudos to them for organizing the event which included a free lunch and flash stick and had some great content for a free event. Secondly, this is not the first time that either of these two MVP’s have organized events. They are active in the community, present at the majority of local events and are always approachable and give an “honest” opinion. For me, that is what an MVP stands for and at least in my region I feel that the MVP program is a real success.

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