I am trying to build an old 16 bit dos like os. My bootloader code: ; This is not my code. May be of Fritzos. I forgot the source. ORG 7c00h jmp Start
drive db 0 msg db " Loader Initialization",0 msg2 db "ACos Loaded",0

  print: 
  lodsb
  cmp al, 0
  je end
  mov ah, 0Eh
  int 10h
  jmp print
  end: ret   

Start:  mov [ drive ],  dl        ; Get the floppy OS booted from
; Update the segment registers
xor ax,  ax  ; XOR ax
mov ds,  ax  ; Mov AX into DS

mov si,msg
call print

; Load Kernel.
ResetFloppy:    mov ax,  0x00  ; Select Floppy Reset BIOS Function
                mov dl,  [ drive ] ; Select the floppy ADos booted from
                int 13h    ; Reset the floppy drive
                jc ResetFloppy                     ; If there was a error, try again.

ReadFloppy:     mov bx, 0x9000         ; Load kernel at 9000h.
                mov ah, 0x02         ; Load disk data to ES:BX
                mov al, 17         ; Load two floppy head full's worth of data.
                mov ch, 0  ; First Cylinder
                mov cl, 2  ; Start at the 2nd Sector to load the Kernel
                mov dh, 0  ; Use first floppy head
                mov dl, [ drive ] ; Load from the drive kernel booted from.
                int 13h   ; Read the floppy disk.
                jc ReadFloppy  ; Error, try again.

; Clear text mode screen
mov ax, 3  
int 10h     

;print starting message
mov si,msg2
call print

mov ax, 0x0
mov ss, ax
mov sp, 0xFFFF

jmp 9000h

; This part makes sure the bootsector is 512 bytes.
times 510-($-$$) db 0

;bootable sector signature
dw 0xAA55

My example kernel code:

 asm(".code16\n");
    void putchar(char);
    int main()
    {
    putchar('A');
    return 0;
    }
    void putchar(char val)
    {
       asm("movb %0, %%al\n"  
           "movb $0x0E, %%ah\n" 
           "int $0x10\n"
          :
          :"r"(val)
          ) ;
    }

This is how I compile it :

nasm -f bin -o ./bin/boot.bin ./source/boot.asm
gcc -nostdinc -fno-builtin -I./include -c -o ./bin/kernel.o ./source/kernel.c
ld -Ttext=0x9000 -o ./bin/kernel.bin ./bin/kernel.o -e 0x0
dd if=/dev/zero of=./bin/empty.bin bs=1440K count=1
cat ./bin/boot.bin ./bin/kernel.bin ./bin/empty.bin|head -c 1440K > ./bin/os
rm ./bin/empty.bin

and I run it in virtual machine.

When I make the putchar function ( in kernel code ) for constant value ....i.e like this: void putchar() { char val = 'A'; asm("movb %0, %%al\n" "movb $0x0E, %%ah\n" "int $0x10\n" : :"r"(val) ) ; }

then it works fine. But when I pass argument to it ( That is in the previous code ) , then it prints a space for any character. What should I do?