comparing return values of subroutines

Question

sickly_man 0 Light Poster

15 Years Ago

hey everybody. i've got what might be a simple question about return values of subroutines in assembly. okay here goes.

say i've got a subroutine that under normal conditions will do a normal return (RET), and under one condition will return zero (RET 0). so now back in main after this function has been called, i need to compare the return value with zero....only i dont know what the syntax is, or rather, how a subroutine's return value can be referenced in main. does anyone know how to do this? can i just do cmp 0??? i havent tried it, but it doesnt seem right. thanks in advance. my code is below. i know it looks liek a lot, its mostly subroutines, and the part i need help with is towards the bottom. THANKS AGAIN

TITLE	Homework 7
;...................................................
STACKSG	SEGMENT PARA STACK 'Stack'
DB 32 dup(0)			;32 words, might want to set larger
STACKSG	ENDS
;.....................................................
DATASG SEGMENT PARA 'Data' 	;data segment

prompt1	DB	'Enter a foreground color:$'
prompt2	DB	'Enter a background color:$'

DATASG ENDS
;.................................................
CODESG SEGMENT PARA 'Code'
MAIN	 PROC FAR		;main procedure
	ASSUME SS:STACKSG, DS:DATASG, CS:CODESG
	MOV AX,DATASG		;Set address of data
	MOV DS,AX		;Segment in DS


	; CODE GOES HERE!!!!

get_fore	proc	near

		; SET CURSOR POSITOIN
		mov ah, 02H
		mov bh, 00
		mov dh, 09
		mov dl, 30
		int 10h

		; DISPLAY PROMPT TO USER
		mov ah, 09H
		lea bx, prompt1
		int 21H

		; LOOK FOR KEYBOARD INPUT...STORE IN DL
		mov ah, 0aH
		int 21h		; will store char in AL
		cmp al, ''	; compare AL with the empty string
		je empty
		mov dl, al	; char stored in DL
		ret

	empty:  ret 0		; return zero if no keyboard input
endp

get_back	proc	near

		; SET CURSOR POSITOIN
		mov ah, 02H
		mov bh, 00
		mov dh, 13
		mov dl, 30
		int 10h

		; DISPLAY PROMPT TO USER
		mov ah, 09H
		lea dx, prompt2
		int 21H

		; LOOK FOR KEYBOARD INPUT...STORE IN DH
		mov ah, 0aH
		int 21h		; will store char in AL
		cmp al, ''	; compare AL with the empty string
		je empty2
		mov dh, al	; char stored in DH
		ret

	empty2:  ret 0		; return zero if no keyboard input
endp

set_color	proc	near

		; CHANGE COLOR OF TEXT (FOREGROUND)
		mov ah, 06H
		mov al, 00H
		cmp dl, 72H	; checking for 'r' -> red
		je red_text
		cmp dl, 67H	; check for 'g' -> green
		je green_text
		cmp dl, 62H	; check for 'b' -> blue
		je blue_text
		cmp dl, 79H	; check for 'y' -> yellow
		je yellow_text
		cmp dl, 77H	; check for 'w' -> white
		je white_text

		; SET TEXT COLOR ATTRTIBUTE
red_text:	mov bh, 04H	; red text attribute
		jmp next
green_text:	mov bh, 02H	; green text attribute
		jmp next	
blue_text:	mov bh, 01H	; blue text attribute
		jmp next
yellow_text:	mov bh, 0EH	; yellow text attribute
		jmp next
white_text:	mov bh, 07H	; white text attribute
		jmp next

		; CHANGE COLOR OF BACKGROUND
next:		mov ah, 06H
		mov al, 00H
		cmp dh, 72H	; checking for 'r' -> red
		je red_bg
		cmp dh, 67H	; check for 'g' -> green
		je green_bg
		cmp dh, 62H	; check for 'b' -> blue
		je blue_bg
		cmp dh, 79H	; check for 'y' -> yellow
		je yellow_bg
		cmp dh, 77H	; check for 'w' -> white
		je white_bg

		; SET BG COLOR ATTRIBUTE
red_bg:		mov bh, 04H	; red bg attribute
		jmp finish
green_bg:	mov bh, 02H	; green bg attribute
		jmp finish	
blue_bg:	mov bh, 01H	; blue bg attribute
		jmp finish
yellow_bg:	mov bh, 0EH	; yellow bg attribute
		jmp finish
white_bg:	mov bh, 07H	; white bg attribute
		jmp finish

		; INVOKE INTERRUPT
finish:		mov cx, 0000H
		mov dx, 184FH
		int 10H

		ret
endp

clear_out	proc	near


endp

clear_in	proc	near


endp

start:		CALL get_fore
		cmp ret, 0
		jmp end_it	; end program 
		CALL get_back
		cmp ret, 0
		jmp end_it	; end program 
		CALL set_color
		jmp start
		
;........................................................

end_it: jmp end_it
	



	MOV AX,4C00H		;End processing
	INT 21H
MAIN	ENDP			;End procedure
CODESG	ENDS			;End segment
	END MAIN		;End program

assembly

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Latest Post 15 Years Ago Latest Post by sickly_man

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Ancient Dragon 5,243 Achieved Level 70

15 Years Ago

Return values are normally passed back in registers -- its standard practice to put a 16-bit int (short in C) in AX, 32-bit in EAX or DX::AX and 64-bit on EDX::EAX. But you can do it anyway you want. pointers to arrays etc are done the same way.

If those registers contain anything the calling function needs to keep then push them onto the stack before calling the function.

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sickly_man 0 Light Poster · Answer 1 · 2008-04-22T01:18:45+00:00

Return values are normally passed back in registers -- its standard practice to put a 16-bit int (short in C) in AX, 32-bit in EAX or DX::AX and 64-bit on EDX::EAX. But you can do it anyway you want. pointers to arrays etc are done the same way.
If those registers contain anything the calling function needs to keep then push them onto the stack before calling the function.

so i cant actually return a number like in C or C++? in other words, the RETURN statement in my get_fore and get_back subroutines is incorrect/illegal? so i guess i will have to put these values in registers before i do the return statement, right?

it seems to me then, that nothing is actually returned in terms of data that the subroutine computed. is that an accurate statement?

below is my updated code.

TITLE	Homework 7
;...................................................
STACKSG	SEGMENT PARA STACK 'Stack'
DB 32 dup(0)			;32 words, might want to set larger
STACKSG	ENDS
;.....................................................
DATASG SEGMENT PARA 'Data' 	;data segment

prompt1	DB	'Enter a foreground color:$'
prompt2	DB	'Enter a background color:$'

DATASG ENDS
;.................................................
CODESG SEGMENT PARA 'Code'
MAIN	 PROC FAR		;main procedure
	ASSUME SS:STACKSG, DS:DATASG, CS:CODESG
	MOV AX,DATASG		;Set address of data
	MOV DS,AX		;Segment in DS


	; CODE GOES HERE!!!!

get_fore	proc	near

		; SET CURSOR POSITOIN
		mov ah, 02H
		mov bh, 00
		mov dh, 09
		mov dl, 30
		int 10h

		; DISPLAY PROMPT TO USER
		mov ah, 09H
		lea bx, prompt1
		int 21H

		; LOOK FOR KEYBOARD INPUT...STORE IN DL
		mov ah, 0aH
		int 21h		; will store char in AL
		cmp al, ''	; compare AL with the empty string
		je empty
		mov dl, al	; char stored in DL
		ret

	empty:  mov dl, 0
		ret		; return zero if no keyboard input
endp

get_back	proc	near

		; SET CURSOR POSITOIN
		mov ah, 02H
		mov bh, 00
		mov dh, 13
		mov dl, 30
		int 10h

		; DISPLAY PROMPT TO USER
		mov ah, 09H
		lea dx, prompt2
		int 21H

		; LOOK FOR KEYBOARD INPUT...STORE IN DH
		mov ah, 0aH
		int 21h		; will store char in AL
		cmp al, ''	; compare AL with the empty string
		je empty2
		mov dh, al	; char stored in DH
		ret

	empty2: mov dh, 0 
		ret		; return zero if no keyboard input
endp

set_color	proc	near

		; CHANGE COLOR OF TEXT (FOREGROUND)
		mov ah, 06H
		mov al, 00H
		cmp dl, 72H	; checking for 'r' -> red
		je red_text
		cmp dl, 67H	; check for 'g' -> green
		je green_text
		cmp dl, 62H	; check for 'b' -> blue
		je blue_text
		cmp dl, 79H	; check for 'y' -> yellow
		je yellow_text
		cmp dl, 77H	; check for 'w' -> white
		je white_text

		; SET TEXT COLOR ATTRTIBUTE
red_text:	mov bh, 04H	; red text attribute
		jmp next
green_text:	mov bh, 02H	; green text attribute
		jmp next	
blue_text:	mov bh, 01H	; blue text attribute
		jmp next
yellow_text:	mov bh, 0EH	; yellow text attribute
		jmp next
white_text:	mov bh, 07H	; white text attribute
		jmp next

		; INVOKE INTERRUPT
next:		mov cx, 0000H
		mov dx, 184FH
		int 10H

		; CHANGE COLOR OF BACKGROUND
		mov ah, 06H
		mov al, 00H
		cmp dh, 72H	; checking for 'r' -> red
		je red_bg
		cmp dh, 67H	; check for 'g' -> green
		je green_bg
		cmp dh, 62H	; check for 'b' -> blue
		je blue_bg
		cmp dh, 79H	; check for 'y' -> yellow
		je yellow_bg
		cmp dh, 77H	; check for 'w' -> white
		je white_bg

		; SET BG COLOR ATTRIBUTE
red_bg:		mov bh, 04H	; red bg attribute
		jmp finish
green_bg:	mov bh, 02H	; green bg attribute
		jmp finish	
blue_bg:	mov bh, 01H	; blue bg attribute
		jmp finish
yellow_bg:	mov bh, 0EH	; yellow bg attribute
		jmp finish
white_bg:	mov bh, 07H	; white bg attribute
		jmp finish

		; INVOKE INTERRUPT
finish:		mov cx, 0000H
		mov dx, 184FH
		int 10H

		ret
endp

clear_out	proc	near


endp

clear_in	proc	near


endp

start:		CALL get_fore
		cmp dl, 0
		je end_it	; end program 
		CALL get_back
		cmp dh, 0
		je end_it	; end program 
		CALL set_color
		jmp start
		
;........................................................

end_it: jmp end_it
	



	MOV AX,4C00H		;End processing
	INT 21H
MAIN	ENDP			;End procedure
CODESG	ENDS			;End segment
	END MAIN		;End program

so just for your own edification, this program is supposed to prompt the user twice, once to change the foreground color with char codes (r, g, b, y, w) and once to change the background with the same codes. its supposed to keep prompting until the user enters an empty string after either of the prompts.

so now am i returning values correcntly? thanks. also if anybody has a burning desire to run my code and check it out feel free! :icon_cheesygrin:

WaltP 2,905 Posting Sage w/ dash of thyme Team Colleague · Answer 2 · 2008-04-22T07:08:56+00:00

so i cant actually return a number like in C or C++? in other words, the RETURN statement in my get_fore and get_back subroutines is incorrect/illegal? so i guess i will have to put these values in registers before i do the return statement, right?
it seems to me then, that nothing is actually returned in terms of data that the subroutine computed. is that an accurate statement?

No. As AD said, the return value is in AX. So whatever is in AX is returned.

so now am i returning values correcntly? thanks. also if anybody has a burning desire to run my code and check it out feel free! :icon_cheesygrin:

Nope. See above.

sickly_man 0 Light Poster · Answer 3 · 2008-04-22T20:16:03+00:00

No. As AD said, the return value is in AX. So whatever is in AX is returned.

Ok so let me get this straight because I want to be clear on this. Does Assembly automatically put all return values of subroutines in AX, or do I have to put it in AX manually with a MOV instruction?

Also what is the syntax for returning zero if it is automatic? can i just say RET 0....as in....

ret 0

????

I know I have a lot of questions, but I'm not an assembly pro...not anywhere near it. So please be clear and articulate with your answers cuz I want to understand rather than just know YES or NO you can't do that in assembly. thanks WaltP and Ancient Dragon for your responses so far...

comparing return values of subroutines

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