CODEWITHSUNDEEP
assemblydos
user6889435
user6889435

Reputation:

Executing .COM file with specified parameters

I am trying to execute .com file with specified arguments under MS-DOS 6.

I've found DOS 2+ - EXEC - LOAD AND/OR EXECUTE PROGRAM Int 21/AH=4Bh The problem is, I don't know how exactly should I pass these parameters. ES:BX should contain something called 'parameter block', but how to construct it? If i'd want to for example execute file named command.com with arguments /C ECHO HELLO WORLD, how would I call this interrupt?

Code that I already have:

ORG 100H
START:
    MOV AH, 4BH ;AH=>4BH
    XOR AL, AL  ;AL=>00H
    MOV DX, CMD ;DS:DX=>STRING
    MOV BX, PARAM;ES:BX=>PARAMS
    INT 21H
    MOV AH,4CH
    INT 21H
CMD: DB "COMMAND.COM$" ;NOT SURE, SHOULD END WITH $
PARAM: ;???

How do i make my program do task mentioned earlier? As MS-DOS is nearly dead, and my assembler, NASM, is barely supported on this platform, I'm going to have bad time. My code may be poorly written as there are no resources about DOS.

Whole point of it is executing batch file generated on runtime by program.

Upvotes: 1

Views: 712

Answers (1)

Martin Rosenau
Martin Rosenau

Reputation: 18521

CMD: DB "COMMAND.COM$" ;NOT SURE, SHOULD END WITH $

The only function that requires a string to end with $ is function 9 (print string). Most other functions require a string to end with NUL (which is the byte with the value 0 - not the ASCII digit '0').

Just like this:

CMD DB "COMMAND.COM"
    DB 0

The next problem you have is that a COM file gets nearly all available memory when being started.

Function 04Bh will fail because there is not enough memory (because all the memory is "used" by the .COM file).

You have to use function 04Ah to resize the memory block used by the .COM file before calling function 4Bh:

push cs  ; Only needed if ES != CS
pop  es  ; ...
mov  bx, 1000h
mov  ah, 4Ah
int  21h

PARAM: ;???

According to Ralph Brown's interrupt list this is 14 bytes long:

  • The segment containing the environment (or 0 to copy the environment) (2 bytes)
  • A pointer (offset, segment) to the command line (4 bytes)
  • A pointer (offset, segment) to the first FCB (4 bytes)
  • A pointer (offset, segment) to the second FCB (4 bytes)

AFAIK the first byte of the command line is the number of bytes following, followed by the actual letters followed by the byte 0Dh. (Check the content of cs:80h to check this information.)

The FCBs are special representations of the first two command line arguments needed for DOS 1.x programs. Because they are not needed any more since DOS 2.x you can simply pass two dummy FCBs here. FCBs are 16 bytes long. You can simply "copy" the FCBs from your program (cs:05Ch and cs:06Ch) to the control block:

PARAM:
    DW   0
    DW   CMDLINE
    DW   ???      ; Write CS here
    DW   05Ch
    DW   ???      ; Write CS here
    DW   06Ch
    DW   ???      ; Write CS here
CMDLINE:
    DB   xxx      ; Replace by the length of the command line
    DB   "xxx"    ; Replace by the actul command line
    DB   0Dh      ; Note: Not 0 but 0Dh in this case

Upvotes: 5

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