Member Avatar for ThatGuy2244

I had tried to make a compare strings function in real mode assembly, but it didn't work, so I tried it another way and it did. It seems to me that both functions I wrote do the same thing yet one doesn't work. Can someone tell me why, the two functions are below.

This one works:

CmpString:
	mov bh, [di]
	cmp bh, [si]
	jne StringRet
	
	cmp bh, 0
	je StringEnd
	add di, 1
	add si, 1
	jmp CmpString
	
	StringEnd:
	stc
	ret
	
	StringRet:
	clc
	ret

And this one did not:

CmpString:
	mov bh, [di]
	cmp bh, [si]
	jne StringRet
	
	cmp bh, 0
	add di, 1
	add si, 1
	jne CmpString
	
	stc
	ret
	
	StringRet:
	clc
	ret
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Member Avatar for untio

Hi,
The problem is that 'add si, 1' modifies the flags. You can test if bh is equal to zero after adding 1 to si and your code will work right.

;cmp bh, 0 - Remove this line
add di, 1 ; Flags are affected.
add si, 1 ; Flags are affected.
cmp bh, 0 ; - Add this line
jne CmpString

I hope that this can be useful.

Member Avatar for ThatGuy2244

Hi,
The problem is that 'add si, 1' modifies the flags. You can test if bh is equal to zero after adding 1 to si and your code will work right.

;cmp bh, 0 - Remove this line
add di, 1 ; Flags are affected.
add si, 1 ; Flags are affected.
cmp bh, 0 ; - Add this line
jne CmpString

I hope that this can be useful.

O thanks, I didn't realize that the add instruction modified the flags thanks.

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