Minimal assembler program for CP/M 3.1 (z80)
- by Andrew J. Brehm
I seem to be losing the battle against my stupidity.
This site explains the system calls under various versions of CP/M.
However, when I try to use call 2 (C_WRITE, console output), nothing much happens.
I have the following code.
ORG 100h
LD E,'a'
LD C,2
CALL 5
CALL 0
I recite this here from memory. If there are typos, rest assured they were not in the original since the file did compile and I had a COM file to start.
I am thinking the lines mean the following:
Make sure this gets loaded at address 100h (0h to FFh being the zero page).
Load ASCII 'a' into E register for system call 2.
Load integer 2 into C register for system call 2.
Make system call (JMP to system call is at address 5 in zero page).
End program (Exit command is at address 0 in zero page).
The program starts and exits with no problems. If I remove the last command, it hangs the computer (which I guess is also expected and shows that CALL 0 works).
However, it does not print the ASCII character. (But it does print an extra new line, but the system might have done that.)
How can I get my CP/M program to do what the system call is supposed to do? What am I doing wrong?
