问题
I'm trying to write a bubble sort in x86 (masm32). The sort doesn't work. I've tested some of the code and it appears to be messed up in the compare and swap sections. For some reason, the compare function always assigns 2 to EAX. If I can figure out why I can get the program working.
Thanks for your help in advance.
.data
aa DWORD 10 DUP(5, 7, 6, 1, 4, 3, 9, 2, 10, 8)
count DWORD 0
; DB 8-bits, DW 16-bit, DWORD 32, WORD 16 BYTE 8
.code ; Tell MASM where the code starts
; «««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««
start: ; The CODE entry point to the program
mov esi, count ;loop count
outer:
inc esi
mov edi, count
cmp esi, 9
je end_loop
inner: ;this loop represents a full pass on the entire array
inc edi
cmp edi, 9 ;after 9 passes goes to outer loop
je outer
compare:
mov eax, [aa + edi * 4h] ;higher indexed one
mov ebx, [aa + edi * 4h - 4h]
;testing print chr$(13,10)
;testing print str$(eax)
;testing print chr$(13, 10)
;testing print str$(ebx)
;testing print chr$(13, 10)
cmp ebx, eax
jg swap
swap:
mov [aa + edi * 4h], eax
mov [aa + edi * 4h + 4], ebx
jmp inner
end_loop:
;print out array elements
sub esi, esi
mov esi, [aa]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 2]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 3]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 4]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 5]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 6]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 7]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 8]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 9]
print str$(esi)
print chr$(" ")
sub esi, esi
exit
; «««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««
end start ; Tell MASM where the program ends
回答1:
Figured out what was wrong - the print statements in the middle of the program were hosing my memory. Here is the working sort. Thanks for the help everyone!
.data
aa DWORD 10 DUP(5, 7, 6, 1, 4, 3, 9, 2, 10, 8)
count DWORD -1
; DB 8-bits, DW 16-bit, DWORD 32, WORD 16 BYTE 8
.code ; Tell MASM where the code starts
; «««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««
start: ; The CODE entry point to the program
mov esi, count ;loop count
outer:
inc esi
mov edi, count
cmp esi, 10
je end_loop
inner: ;this loop represents a full pass on the entire array
inc edi
cmp edi, 9 ;after 9 passes goes to outer loop
je outer
compare:
mov eax, [aa + edi * 4h]
mov ebx, [aa + edi * 4h + 4] ;want to make this one the higher indexed-one
;print chr$(13,10) These print calls were hosing the memory before.
;print str$(eax)
;print chr$(13, 10)
;print str$(ebx)
;print chr$(13, 10)
cmp eax, ebx
jle inner
swap:
mov [aa + edi * 4h], ebx
mov [aa + edi * 4h + 4], eax
jmp inner
end_loop:
;print out array elements
sub esi, esi
mov esi, [aa]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 2]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 3]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 4]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 5]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 6]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 7]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 8]
print str$(esi)
print chr$(" ")
sub esi, esi
mov esi, [aa + 4h * 9]
print str$(esi)
print chr$(" ")
sub esi, esi
exit
; «««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««««
end start ; Tell MASM where the program ends
回答2:
One obvious problem is right here:
cmp ebx, eax
jg swap
swap:
mov [aa + edi * 4h], eax
mov [aa + edi * 4h + 4], ebx
jmp inner
With this, it jumps to swap if the g flag is set -- but it falls through to swap if the g flag is not set, so it executes exactly the same code either way. At a guess, what you want is probably something like jle inner to skip the swap if the two items are already in order.
Edit: Looking at things again, it looks like you have another fairly obvious problem. Let's think in terms of something like C, and use base as the address from whcih you load eax (at compare). You (conceptually) do:
eax = *base;
ebx = *(base + 4);
Then at swap you do:
*base = eax;
*(base - 4) = ebx;
This is clearly wrong as well. To swap the two variables, you want something like:
*base = ebx;
*(base + 4) = eax;
来源：https://stackoverflow.com/questions/17802947/bubble-sort-in-x86-masm32-the-sort-i-wrote-doesnt-work