Thread: Examining the Assembly Listing

This is my third project in this class involving having to identify what machine instruction is doing what, and I just want to see if I'm doing it right or wrong because that would suck to get three F's! I'm not trying to cheat, so try giving me advice without just giving me the answer. Thanks!

The following is the assembly code generated from a short program I wrote to give the sum of an int and a char:

Code:

; Listing generated by Microsoft (R) Optimizing Compiler Version 15.00.21022.08 

	TITLE	c:\Users\C\Documents\Visual Studio 2008\Projects\proj6vcsolut\proj6vc\proj6.cpp
	.686P
	.XMM
	include listing.inc
	.model	flat

INCLUDELIB MSVCRTD
INCLUDELIB OLDNAMES

PUBLIC	?z@@3HA						; z
PUBLIC	?x@@3HA						; x
PUBLIC	?y@@3DA						; y
_BSS	SEGMENT
?z@@3HA	DD	01H DUP (?)				; z
?x@@3HA	DD	01H DUP (?)				; x
?y@@3DA	DB	01H DUP (?)				; y
_BSS	ENDS
PUBLIC	?sumfunc@@YAHHD@Z				; sumfunc
EXTRN	__RTC_Shutdown:PROC
EXTRN	__RTC_InitBase:PROC
;	COMDAT rtc$TMZ
; File c:\users\c\documents\visual studio 2008\projects\proj6vcsolut\proj6vc\proj6.cpp
rtc$TMZ	SEGMENT
__RTC_Shutdown.rtc$TMZ DD FLAT:__RTC_Shutdown
rtc$TMZ	ENDS
;	COMDAT rtc$IMZ
rtc$IMZ	SEGMENT
__RTC_InitBase.rtc$IMZ DD FLAT:__RTC_InitBase
; Function compile flags: /Odtp /RTCsu /ZI
rtc$IMZ	ENDS
;	COMDAT ?sumfunc@@YAHHD@Z
_TEXT	SEGMENT
_z$ = -8						; size = 4
_x$ = 8							; size = 4
_y$ = 12						; size = 1
?sumfunc@@YAHHD@Z PROC					; sumfunc, COMDAT

; 9    : int sumfunc(int x, char y){

	push	ebp
	mov	ebp, esp
	sub	esp, 204				; 000000ccH
	push	ebx
	push	esi
	push	edi
	lea	edi, DWORD PTR [ebp-204]
	mov	ecx, 51					; 00000033H
	mov	eax, -858993460				; ccccccccH
	rep stosd

; 10   :      int z;
; 11   :      z= x + y;

	movsx	eax, BYTE PTR _y$[ebp]
	add	eax, DWORD PTR _x$[ebp]
	mov	DWORD PTR _z$[ebp], eax

; 12   :      return z;

	mov	eax, DWORD PTR _z$[ebp]

; 13   :      }

	pop	edi
	pop	esi
	pop	ebx
	mov	esp, ebp
	pop	ebp
	ret	0
?sumfunc@@YAHHD@Z ENDP					; sumfunc
_TEXT	ENDS
PUBLIC	??_C@_05PDJBBECF@pause?$AA@			; `string'
PUBLIC	??_C@_0CC@CNFEEKMM@The?5sum?5of?5the?5two?5numbers?5is?5?$CFi@ ; `string'
PUBLIC	??_C@_02DPKJAMEF@?$CFd?$AA@			; `string'
PUBLIC	??_C@_0BO@PFLLPLLJ@Please?5type?5another?5integer?4?6?$AA@ ; `string'
PUBLIC	??_C@_02IKAHHCAI@?$CFi?$AA@			; `string'
PUBLIC	??_C@_0BJ@BLCGGBIN@Please?5type?5an?5integer?4?6?$AA@ ; `string'
PUBLIC	_main
EXTRN	__imp__system:PROC
EXTRN	__imp__scanf:PROC
EXTRN	__imp__printf:PROC
EXTRN	__RTC_CheckEsp:PROC
;	COMDAT ??_C@_05PDJBBECF@pause?$AA@
CONST	SEGMENT
??_C@_05PDJBBECF@pause?$AA@ DB 'pause', 00H		; `string'
CONST	ENDS
;	COMDAT ??_C@_0CC@CNFEEKMM@The?5sum?5of?5the?5two?5numbers?5is?5?$CFi@
CONST	SEGMENT
??_C@_0CC@CNFEEKMM@The?5sum?5of?5the?5two?5numbers?5is?5?$CFi@ DB 'The su'
	DB	'm of the two numbers is %i', 0aH, 00H	; `string'
CONST	ENDS
;	COMDAT ??_C@_02DPKJAMEF@?$CFd?$AA@
CONST	SEGMENT
??_C@_02DPKJAMEF@?$CFd?$AA@ DB '%d', 00H		; `string'
CONST	ENDS
;	COMDAT ??_C@_0BO@PFLLPLLJ@Please?5type?5another?5integer?4?6?$AA@
CONST	SEGMENT
??_C@_0BO@PFLLPLLJ@Please?5type?5another?5integer?4?6?$AA@ DB 'Please typ'
	DB	'e another integer.', 0aH, 00H		; `string'
CONST	ENDS
;	COMDAT ??_C@_02IKAHHCAI@?$CFi?$AA@
CONST	SEGMENT
??_C@_02IKAHHCAI@?$CFi?$AA@ DB '%i', 00H		; `string'
CONST	ENDS
;	COMDAT ??_C@_0BJ@BLCGGBIN@Please?5type?5an?5integer?4?6?$AA@
CONST	SEGMENT
??_C@_0BJ@BLCGGBIN@Please?5type?5an?5integer?4?6?$AA@ DB 'Please type an '
	DB	'integer.', 0aH, 00H				; `string'
; Function compile flags: /Odtp /RTCsu /ZI
CONST	ENDS
;	COMDAT _main
_TEXT	SEGMENT
_main	PROC						; COMDAT

; 15   : int main(void){

	push	ebp
	mov	ebp, esp
	sub	esp, 192				; 000000c0H
	push	ebx
	push	esi
	push	edi
	lea	edi, DWORD PTR [ebp-192]
	mov	ecx, 48					; 00000030H
	mov	eax, -858993460				; ccccccccH
	rep stosd

; 16   :    
; 17   :     printf("Please type an integer.\n");

	mov	esi, esp
	push	OFFSET ??_C@_0BJ@BLCGGBIN@Please?5type?5an?5integer?4?6?$AA@
	call	DWORD PTR __imp__printf
	add	esp, 4
	cmp	esi, esp
	call	__RTC_CheckEsp

; 18   :     scanf("%i", &x);

	mov	esi, esp
	push	OFFSET ?x@@3HA				; x
	push	OFFSET ??_C@_02IKAHHCAI@?$CFi?$AA@
	call	DWORD PTR __imp__scanf
	add	esp, 8
	cmp	esi, esp
	call	__RTC_CheckEsp

; 19   :     printf("Please type another integer.\n");

	mov	esi, esp
	push	OFFSET ??_C@_0BO@PFLLPLLJ@Please?5type?5another?5integer?4?6?$AA@
	call	DWORD PTR __imp__printf
	add	esp, 4
	cmp	esi, esp
	call	__RTC_CheckEsp

; 20   :     scanf("%d", &y);

	mov	esi, esp
	push	OFFSET ?y@@3DA				; y
	push	OFFSET ??_C@_02DPKJAMEF@?$CFd?$AA@
	call	DWORD PTR __imp__scanf
	add	esp, 8
	cmp	esi, esp
	call	__RTC_CheckEsp

; 21   :     printf("The sum of the two numbers is %i\n", sumfunc(x,y));

	movzx	eax, BYTE PTR ?y@@3DA			; y
	push	eax
	mov	ecx, DWORD PTR ?x@@3HA			; x
	push	ecx
	call	?sumfunc@@YAHHD@Z			; sumfunc
	add	esp, 8
	mov	esi, esp
	push	eax
	push	OFFSET ??_C@_0CC@CNFEEKMM@The?5sum?5of?5the?5two?5numbers?5is?5?$CFi@
	call	DWORD PTR __imp__printf
	add	esp, 8
	cmp	esi, esp
	call	__RTC_CheckEsp

; 22   :     
; 23   :     
; 24   :     system("pause");    

	mov	esi, esp
	push	OFFSET ??_C@_05PDJBBECF@pause?$AA@
	call	DWORD PTR __imp__system
	add	esp, 4
	cmp	esi, esp
	call	__RTC_CheckEsp

; 25   :     return 0;

	xor	eax, eax

; 26   : }

	pop	edi
	pop	esi
	pop	ebx
	add	esp, 192				; 000000c0H
	cmp	ebp, esp
	call	__RTC_CheckEsp
	mov	esp, ebp
	pop	ebp
	ret	0
_main	ENDP
_TEXT	ENDS
END

And I answered the questions required as follows:

a. What machine instruction is used to actually invoke (run, transfer control to) your function?
"call"

b. What code got generated to return the result of the function to main?
"mov"

c. What machine instruction is used to assign a value to a variable? (corresponding to the =
assignment operator)?
"mov"

d. What assembly language instruction is used to pass parameter values to your function?
(Hint: it involves the concept of the stack).
"movzx"

More like this? 3/4 is better than I anticipated but would be nice to get them all right, lol.

a. What machine instruction is used to actually invoke (run, transfer control to) your function?
"call ?sumfunc@@YAHHD@Z"

b. What code got generated to return the result of the function to main?
"mov eax, DWORD PTR _z$[ebp]"

c. What machine instruction is used to assign a value to a variable? (corresponding to the = assignment operator)?
"mov DWORD PTR _z$[ebp], eax"

d. What assembly language instruction is used to pass parameter values to your function? (Hint: it involves the concept of the stack).
"movzx eax, BYTE PTR ?y@@3DA
push eax
mov ecx, DWORD PTR ?x@@3HA"

meh- I don't know its the only instruction after "return z;" Am I looking in the wrong place? Since the result would be in the z=x+y then should I be looking at the instructions under it: movsx eax, BYTE PTR _y$[ebp]
add eax, DWORD PTR _x$[ebp]
mov DWORD PTR _z$[ebp], eax
??

For "b. What code got generated to return the result of the function to main?"

Am I getting warmer or colder by stating "The value is stored by a function in the eax register which is taken to be the return value of the function" ??

I found that answer here:
From C To Assembly Language LG #94

Originally Posted by quintenmater

For "b. What code got generated to return the result of the function to main?"

Am I getting warmer or colder by stating "The value is stored by a function in the eax register which is taken to be the return value of the function" ??

I found that answer here:
From C To Assembly Language LG #94

As for part b:
Yes, what you said is right but because he's asking "what code got generated?" I'd also provide the actual machine code. And for you that's going to be the following:
mov eax, DWORD PTR _z$[ebp]

And because your function call is in a printf statement, you don't really see (or at least I cannot tell) when or where the actual value of z (which got moved to eax, in the code above) is being dealt with. Maybe its because its happening within the printf call and we can't see it?

Anyway, good luck!