CODEWITHSUNDEEP
assemblydos
Roman
Roman

Reputation: 1466

How to end assembly correctly?

I have a problem correctly terminating a 16bit DOS program written in Assembly. Here's part of code:

.386P
.model flat

stack_s segment stack 'stack' 
        db 256 dup(0)
stack_s ends

data segment use16
data ends

code segment 'code' use16
assume cs:code, ds:data

main proc
    mov ax, data
    mov ds, ax

    iretd
main endp

code ends
end main

The problem is, that the program doesn't terminate in a correct way. DOSBox just freezes. I tried to understand what happens using debugger, and it seems the program just ends up in an infinite loop after iretd is performed. Why does this happen? How can do I terminate a 16bit DOS app correctly?

Upvotes: 2

Views: 18442

Answers (3)

Daishi
Daishi

Reputation: 14319

If you want to exit without any particular error code this is the way to go:

mov ax,4C00h
int 21h

But if you want to return a particular error code, this is a better way:

; ...
ErrorCode db 0   ; set a default error code
; ...
mov ErrorCode,1h ; change the error code if needed
; ...
mov ah,4Ch       ; prepare to exit
mov al,ErrorCode ; and return the error code
int 21h

Upvotes: 0

Brendan
Brendan

Reputation: 37262

The most correct way to end a DOS program is to use the "terminate" DOS function; followed by adequate comments so that people understand that this function won't return.

For example:

pleaseKillMeNow:
    mov ah,0x4C          ;DOS "terminate" function
    int 0x21

Upvotes: 5

John Elliot V
John Elliot V

Reputation: 302

Brendan's answer shows how to exit but it leaves the error level undefined (it will be whatever is in the AL register...)

If you want to exit with error level 0:

mov ax,0x4c00
int 0x21

If you want to exit with error level 1:

mov ax,0x4c01
int 0x21

Upvotes: 7

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