🚀 go-pugleaf

I have been trying for DAYS to find out how to use MS-DOS
function calls from within an interrupt service routine (ISR).
I am a novice programmer who has been attempting to overcome
the difficulties of learning 8086/8088 assembly language programming
using the Microsoft Macro-Assembler, version 1.27. I've been
trying my programs out on an IBM-PC/XT, AT&T 6300, and an
H/Z-150 PC (Heathkit). All use MS-DOS version 2.0 or higher.

My recent efforts have been concerned with writing a memory resident
ISR containing DOS fuction calls to print strings, control the cursor,
etc. These work fine for me when I use them in a regular .COM or .EXE
file, but when I try to incorporate them in an ISR, the program consistently
bombs in all sorts of horrible ways.

PLEASE HELP!!! - I am in danger of losing my mind at this point, or of
burning my assembly language manuals to a crisp, in frustration (#%!??).

Below is a simplified example of the technique I've been using. 
When I assemble and try to run the program, the string is printed out, 
but the computer "hangs". As I stated before, I am a NOVICE programmer, 
and take no responsibility for the sanity of this approach. 
Any help that anyone can offer is greatly appreciated.

----------------------------- cut here -------------------------------------

TITLE	PROBLEM.ASM
PAGE	60,132
;
;PROBLEM.ASM -- Program to demonstrate problem with attempting to use
;		a DOS function call from within a memory resident
;		interrupt handler. Computer "hangs".
;
;		This programs attempts to print a message on the screen
;		when the Shift-PrtSc keys are pressed.
;
;		Installation code (at initialize:) is discarded to
;		reduce size of program in memory.
;
print_s	equ	9h	;print string function
get_vtr equ	35h	;get vector function
set_vtr equ	25h	;set vector function

r_intr	equ	05h	;interrupt vector to be replaced (print screen)
doscall equ	21h	;DOS interrupt number for funct calls
stayres	equ	27h	;terminate-but-stay-resident interrupt

;**********************************************************

sample	segment			;define code segment

	assume	cs:sample,ds:sample
	org	100h		;for COM files

start:		;starting execution address

	jmp	initialize	;install code

msg	db	"Hello there!",13,10,"$"      
oldvtr	dd	;old interrupt vector 

;start of interrupt service routine

ISR:	push	DS	;save everything!
	push	ES
	push	SI
	push	DI
	push	BP
	push	DX
	push	CX
	push	BX
	push	AX
	push	CS
	pop	DS

	CALL	MAIN

	pop	AX	;put everything back
	pop	BX
	pop	CX
	pop	DX
	pop	BP
	pop	DI
	pop	SI
	pop	ES
	pop	DS

	iret		;return from interrupt

;----------------------------------------------------------
MAIN	proc	near

     	sti				;enable interrupts

;display message on screen
	mov   dx,offset msg		;put string address in DX
	mov   ah,print_s		;print string function
	int   doscall			;call DOS

;	jmp DWORD PTR oldvtr		;chain to previous int vector
					;(*** not used in example ***)
	ret

MAIN	endp
;----------------------------------------------------------

initialize:
	mov	bx,cs			;set DS = CS
	mov	ds,bx

;get old interrupt vector and save it
	mov	al,r_intr		;put intr number in AL
	mov	ah,get_vtr		;funct number in AH
	int	doscall			;funct call interrupt
	cli				;disable interrupts temporarily
	mov	WORD PTR oldvtr,bx	;old vector is returned in ES:BX
	mov	WORD PTR oldvtr[2],es	;save it in variable
	sti				;enable interrupts 

;set intr vector to point to this routine
	mov	bx,cs			;put segment:offset of routine
	mov	ds,bx			;in DS:DX
	mov	dx,offset ISR
	mov	al,r_intr		;vector num to be replaced in AL
	mov	ah,set_vtr		;set-vector funct num in AH
	int	doscall

;terminate and stay resident
	mov	bx,cs			;first usable address is that of
	mov	ds,bx			;initialize, so put seg:off 
	mov	dx,offset initialize	;in DS:DX
	int	stayres			;end program, but leave sample 
					;routine in memory
	
sample	ends				;end of code segment

;**********************************************************

	end	start		;end assembly