cmp with big immediate operand

flat assembler
Message board for the users of flat assembler.

Index > Windows > cmp with big immediate operand

Author

Thread

alessandro95

Joined: 24 Mar 2013
Posts: 62

alessandro95 11 Apr 2013, 01:23

Why, while writing 64bit code, the range allowed for cmp's immediate operand is a 32bit signed integer?

Code:

format PE64 GUI 5.0

cmp rax, 10000000000

for example, gives an out of range error

11 Apr 2013, 01:23

HaHaAnonymous

Joined: 02 Dec 2012
Posts: 1178
Location: Unknown

HaHaAnonymous 11 Apr 2013, 03:06

[ Post removed by author. ]

Last edited by HaHaAnonymous on 28 Feb 2015, 21:06; edited 1 time in total

11 Apr 2013, 03:06

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20445
Location: In your JS exploiting you and your system

revolution 11 Apr 2013, 07:09

alessandro95 wrote:

Why, while writing 64bit code, the range allowed for cmp's immediate operand is a 32bit signed integer?

HaHaAnonymous is correct. The x86 CPU design does not have any way to encode numbers >= 2^31 in the immediate field of cmp. fasm gives you the correct response to say that you can't encode such a thing. IIRC only the mov instruction allows 64-bit immediate values.

11 Apr 2013, 07:09

< Last Thread | Next Thread >

Forum Rules:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum