History: Executing a program in DOS

A common trick in the old days was to execute an application from within another. Here is some code example of how that was normally done:

.model tiny
.code

ExecBlk	Struc
	Psp	dw	?
	Cmdline	dw	?
	CmdSeg	dw	?
	FCB1	dw	?
	FCB1seg	dw	?
	FCB2	dw	?
	FCB2seg	dw	?
ExecBlk	ends

	org	100h

main	proc
	lea	bx,pspblk
	mov	sp,bx
	call	resize	

	mov	execb.psp,0		; copy our environment
	mov	execb.cmdline,80h	; pass command line as is
	mov	execb.FCB1,5ch
	mov	execb.FCB2,6ch
	mov	execb.cmdseg,cs
	mov	execb.FCB1seg,cs
	mov	execb.FCB2seg,cs
	lea	dx,execf
	push	ds
	pop	es
	lea	bx,execb
	mov	ax,4b00h
	int	21h
merr:
	mov	ax,4c00h
	int	21h
main	endp

resize	proc
	add	bx,15
	mov	cl,4
	shr	bx,cl
	mov	ax,cs
	add	bx,ax
	mov	ah,4ah
	int	21h
	jc	merr
	ret
resize	endp


execf	db	"PAT.COM",0		; ASCIIZ pathname of file to be execd
execb	ExecBlk	<>
_stack	dw	1000h dup(?)

pspblk	label	byte

	end	main