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-   -   comparing return values of subroutines (http://www.daniweb.com/forums/thread120266.html)

sickly_man Apr 21st, 2008 1:55 pm
comparing return values of subroutines
 
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

Ancient Dragon Apr 21st, 2008 3:41 pm
Re: comparing return values of subroutines
 
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.

sickly_man Apr 21st, 2008 4:18 pm
Re: comparing return values of subroutines
 
Quote:
Originally Posted by Ancient Dragon (Post 590054)
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 Apr 21st, 2008 10:08 pm
Re: comparing return values of subroutines
 
Quote:
Originally Posted by sickly_man (Post 590080)
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.


Quote:
Originally Posted by sickly_man (Post 590080)
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 Apr 22nd, 2008 11:16 am
Re: comparing return values of subroutines
 
Quote:
Originally Posted by WaltP (Post 590228)
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...


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