flat assembler - mov vs cmove (with the identical arguments)

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Index > Windows > mov vs cmove (with the identical arguments) - OK vs Error

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fatygant

Joined: 12 Sep 2011
Posts: 30
Location: Poznan, Poland

fatygant 07 Feb 2018, 20:40

Hello!

I have another problem which makes me scratch top of my head... I would appreciate your explanation.

I have two instructions in my 64-bit Windows code (of course they do not come one after another):

Code:

mov   rax,modbus_ascii_txt
cmove rax,modbus_ascii_txt

The first one is assembled with no problems - but the second one generates the error: invalid argument. modbus_ascii_txt is just a memory address of the string defined as 'Modbus ASCII',0.

How is this possible if they work with the same type of operands according to x86_64 documentation?

Many thanks for your help!

07 Feb 2018, 20:40

alexfru

Joined: 23 Mar 2014
Posts: 80

alexfru 07 Feb 2018, 23:55

cmovcc's source operand is either a register or a memory operand. It does not take an immediate.

07 Feb 2018, 23:55

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 19099
Location: In your JS exploiting you and your system

revolution 08 Feb 2018, 03:15

It is a serious oversight on Intel's part. CMOVcc doesn't support immediate operands, as alexfru states.

One possible way around it is this:

Code:

mov rdx,modbus_ascii_txt ; use a temporary register
cmove rax,rdx

08 Feb 2018, 03:15

fatygant

Joined: 12 Sep 2011
Posts: 30
Location: Poznan, Poland

fatygant 08 Feb 2018, 08:35

Thank you alexfru and revolution!

08 Feb 2018, 08:35

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