Comparing two words in memory

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Trinitek

Joined: 06 Nov 2011
Posts: 257

Trinitek 06 Nov 2011, 22:38

Excuse me if I sound like I don't know what I'm doing, but I'm having a bit of trouble with comparing two words stored in memory. I'm trying to compare a BMP's magic number, which is stored as a word at the very beginning of the file (424Dh), and the same hexadecimal value stored in a variable called "magic".

Code:

MOV     BX,image
CMP     word [BX],[magic]
magic   DW 424Dh
image   FILE '..\snowym~1.bmp'

I've tried every combination in the book. With brackets, without, using an immediate value, everything!

06 Nov 2011, 22:38

magicSqr

Joined: 27 Aug 2011
Posts: 105

magicSqr 06 Nov 2011, 22:51

you can't compare 2 memory values, you need to place one in a register first.

Code:


mov ax, [bx]
cmp ax, [magic]

or, since you know the value you're looking for...

Code:

cmp   word [magic], 0x424D

Quote:

Code:

MOV     BX,image            <------ ???
CMP     word [BX],[magic]
magic   DW 424Dh
image   FILE '..\snowym~1.bmp'

Not sure what you're trying to do at the arrowed part tho ¿

You need to open the file for reading, read the first word from the file, then compare it to 0x424D.

magic²

06 Nov 2011, 22:51

Trinitek

Joined: 06 Nov 2011
Posts: 257

Trinitek 07 Nov 2011, 01:00

Oh.

Nevermind. I forgot that it swaps the placement of the bits when I got the word from the image header.

Thanks for telling me that I had to put the value into a register first. I completely forgot about that as well.

Everything seems to be in working order as of now...

07 Nov 2011, 01:00

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 07 Nov 2011, 18:28

and FASM also allows you to do this:

Code:

cmp word[bx],"BM" ; Now there's no "magic", just constant

07 Nov 2011, 18:28

Trinitek

Joined: 06 Nov 2011
Posts: 257

Trinitek 08 Nov 2011, 00:07

Madis731 wrote:

and FASM also allows you to do this:
Code:
cmp word[bx],"BM" ; Now there's no "magic", just constant 
    

Thanks Madis, but I already figured it out. Laughing

Apparently, there's a problem with how I load the binary file using FILE. I'll have to look into that, but it's working as of now.

08 Nov 2011, 00:07

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