CSC231 No-no! and Be-Careful! situations

--D. Thiebaut 13:15, 10 November 2010 (UTC)

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What's wrong with the following code?

;------------------------------------------------------------------
; function that pushes several results in the stack
;------------------------------------------------------------------
compute:	
        pushad
        mov	ecx, N		;loop some number of times                                                              
        mov	eax, data1	;get some data                                                                          
        mov	ebx, data2
.for:   call	doSomething	;operate on eax and ebx                                                                 
                                ; on return, eax contains                                                               
                                ; result we're interested in                                                            
        push	eax		;save result in stack                                                                   
        loop	.for

;;; we're done                                                                                                          
        popad
        ret

What's tricky about the following code?

compute:	
        pushad
	push	ebx
        mov	ebx, esp

;;; get parameters from the stack                                                                                       

        mov     ecx,[ebp+XX]    ;XX is some offset                                                                      
        mov	eax,[ebp+YY]	;YY is some offset                                                                      
        mov	ebx,[ebp+ZZ]    ;ZZ is some offset                                                                      
.for:   call	doSomething	;operate on eax and ebx                                                                 
                                ; on return, eax contains                                                               
                                ; result we're interested in                                                            
        loop    .for

;;; we're done                                                                                                          
        pop	ebx
        popad
        ret	3*4