CSC231 No-no! and Be-Careful! situations

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

---

What's wrong with the following code?

; this function will compute N results and store them 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	ebp
        mov	ebp, 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	ebp
        popad
        ret	3*4

Helpful Trick

• We can use the %define directive to assign names to our parameters living in the stack.
• More information on %define: Nasm Preprocessor
• Example:

;;; --------------------------------------------------------------
;;; openFile( filename, handle )
;;; opens a file for reading, puts handle in dword whose address
;;; is passed in stack.
;;; Does not modify the registers, but modifies flags.
openFile:
       push    ebp
       mov     ebp, esp
       pushad
        
%define of_fileName dword[ebp+12]
%define of_handle   dword[ebp+8]
       
       mov     eax,SYS_OPEN
       mov     ebx, of_fileName
       mov     ecx, O_RDONLY
       mov     edx, S_IRUSR|S_IWUSR|S_IXUSR
       int     0x80

       test    eax,eax
       jns     .fileOk
       print2  "Error opening file"
       mov     eax, SYS_EXIT
       mov     ebx, 1
       int     0x80

.fileOk:
       mov     ebx, of_handle
       mov     [ebx], eax      ; save handle
       clc                     ; clear carry if ok
       
.done:  popad                   ; restore registers
       pop     ebp
       ret     8