Macros and ROR, ROL

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blh42

Joined: 19 Jun 2015
Posts: 2

blh42 19 Jun 2015, 13:54

Hi!

I'm somewhat new to FASM and was mostly intrigued by the powerful macro capabilities. Now my problem is how to use ROR and ROL with "local" variables.

My assumption was that this code world work but it only renders an error

Code:

macro rotr addr {
    local ..addr
    ..addr = addr ror 0xdd
    mov eax, ..addr
}

Quote:

error: extra characters on line.

How would i solve this?

blh

19 Jun 2015, 13:54

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20516
Location: In your JS exploiting you and your system

revolution 19 Jun 2015, 14:04

When using "local" in macros they are not declaring variables, instead they are just label names that are local to the macro context.

fasm does not support ror in the expression parser. You would have to simulate it with "shl", "shr" and "or".

Code:

macro rotr addr {
    mov eax,((addr shr 0x1d) and 0xffffffff) or ((addr shl (0x20 - 0x1d)) and 0xffffffff)
}

The extra are brackets are just for clarity

19 Jun 2015, 14:04

blh42

Joined: 19 Jun 2015
Posts: 2

blh42 19 Jun 2015, 15:11

Ah right. So then what am I doing wrong with the rotl?

Code:

macro rotr dst,src,shift {
    mov dst,((src shr shift) and 0xffffffff) or ((src shl (0x20 - shift)) and 0xffffffff)
}

macro rotl dst,src,shift {
    mov dst,((src shl shift) and 0xffffffff) or ((src shr (0x20 - shift)) and 0xffffffff)
}

It doesnt seem to produce the result im expecting :S

blh

19 Jun 2015, 15:11

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20516
Location: In your JS exploiting you and your system

revolution 19 Jun 2015, 17:26

What values and results did you get? I can't see anything obviously wrong.

19 Jun 2015, 17:26

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