Assembler: using debug.exe to write DOS programs

C:\WINDOWS\system32\debug.exe

Windows (XP) still comes with Debug.

C:\test>DEBUG
-a
0C8A:0100 mov ax,0200
0C8A:0103 mov dx,0041
0C8A:0106 int 21
0C8A:0108 int 20
0C8A:010A
-h 010A 0100
020A  000A
-n PCHAR.COM
-rcx
CX 0000
:000A
-w
Writing 0000A bytes
-q

C:\test>PCHAR.COM
A

The above is the console output for writing a 10 byte executable in assembly language. All the application does is print out the A character (ASCII value 41h, sort of - it is more likely to be Code Page 850 on a Western European PC).

Registers and Interrupts

API: Interrupt Services

The common registers (AX, BX, CD, DX) are 16bit. The processor might be 32bit or 64bit, but it will be emulating 16bit here. A register like AX can be considered as two bytes and documentation will often refer to its separate byte values, AH and AL (high and low).

AX
      AH       AL
00000010 00000000

Thus, a value of AH=02h and AL=00h can be expressed as AX=0200h.

INT 21h,2h

Description: outputs character to STDOUT
Inputs:
   AH = 02h
   DL = char value
Outputs:
   none

INT 20h

Description: program terminate (deprecated: use INT 21h,4Ch to terminate with return code)
Inputs:
   none
Outputs:
   none

Understanding the Console Output

C:\test>DEBUG	start Debug
-a	assemble; note that the default start address is 0100h
0C8A:0100 mov ax,0200	set the register AX with hexadecimal value 0200h; the interrupt sub-function in AH
0C8A:0103 mov dx,0041	set the register DX with the hex value 0041h; the character to be output in DL
0C8A:0106 int 21	call interrupt 21h; since the value 02h is in AH, this tells DOS to output a character
0C8A:0108 int 20	call interrupt 20h; this tells DOS to terminate the program
0C8A:010A	press ENTER again to finish
-h 010A 0100	calculate the difference between the end address and the start address
020A  000A	hexadecimal; the sum is 020Ah;the difference is 000Ah (10)
-n PCHAR.COM	name the output file
-rcx	register CX
CX 0000	the current value in register CX
:000A	set the value of CX to the difference from above (000Ah)
-w	write the file
Writing 0000A bytes	the number of bytes written (10)
-q	quit
C:\test>PCHAR.COM	execute the resultant program
A