Executing a program within a program

I have included the assembly source listing (see below) for executing another program from within a High-level Programming Language. I have used QuickBasic 4.5 as my example, but it should be easy enough to change it to C or Pascal. Just see which order they stack the parameters on the stack and you can change it quite easily.

THE SOURCE CODE:

Dosseg
.model medium, basic
.286
.stack
.data

	CmdTail	db	64			; Length of command tail
			db	64 dup(20h),13	; Actual command tail
	Parms		dw	0		; Parent environment block is OK
			dw	offset CmdTail	; Command tail address
			dw	seg CmdTail
			dw	0,0		; FCB pointers don't matter
			dw	0,0
.code
Even						; make all math even for faster execution
		public	execute			; make execute public to quick basic
Execute	proc far basic uses bp bx cx dx si di ds es, execfile:word, Prms:word
		local	savess:word, savesp:word ; (can't push ss or sp on to stack)

		push	ds			; make sure es = ds
		pop	es			;
		mov	bx,offset Prms		; Put passed parameter list
		mov	si,[bx+2]		;    into our parameter list
		mov	di,offset CmdTail	;
		inc	di			; inc past len
		mov	cx,64			; no more than 64
PutParms:	lodsb				;
		or	al,al			; end of passed list?
		jz	short DonePs		; yes then end
		stosb				;
		loop	PutParms		;
DonePs:		mov	ah,4Bh			; exec function
		xor	al,al			; load and execute
		mov	bx,offset execfile	; path name to file to execute
		mov	dx,[bx+2]
		mov	bx,offset parms		;    parameter block
		sti
		mov	savess,ss		; don't want to loose sp:ss during
		mov	savesp,sp		;    exec call
		cli
		int	21h			; call interrupt
		sti
		mov	ss,savess		; restore sp:ss from saved values
		mov	sp,savesp		;
		cli
		xor	ah,ah			; clear HI byte of RC
		jnc	short ExeDn		; if error, carry set and al = error
		xor	al,al			; if no error clear LO byte of RC
ExeDn:		ret				; r2
Execute		endp				; end of procedure
		end				; end of assembly code

To call this routine in QuickBasic4.5 use the following:

SETMEM (-10000)
DECLARE FUNCTION Execute% (ExecFile$, Prms$)

PRINT Execute%("c:\DOS\EDIT.COM" + CHR$(0), "/someparameter" + CHR$(0))

You give this function two parameters. The first is the path and file name that you want to execute. The second is the parameter list that would be added on to the command line if it was executed from the DOS prompt. Please note that both have the string of +CHR$(0) added to them. This makes them ASCIIz. MUST BE DONE.

This function returns in Execute% an error code of:
0 = OK
1 = invalid function
2 = file not found
3 = path not found
4 = too many files open
5 = access denied
8 = insufficient memory
10 = invalid environment block
11 = invalid format

As QuickBasic allocates all of the available memory as startup, you must free some of that memory to allow this function to work. You must free enough memory so that the wanted program can fit and all of the memory that it wants can fit. To do this, you can use the following line:

SETMEM(- somevalue)

If you have any other questions or see a mistake that I made, please e-mail me and I will do my best to help out.