CSC231 No-no! and Be-Careful! situations
--D. Thiebaut 13:15, 10 November 2010 (UTC)
What's wrong with the following code?
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.
- 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
