[opcode] pushing a 32bit (byte sized) value in real mode

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rain_storm

Joined: 05 Apr 2007
Posts: 67
Location: Ireland

rain_storm 20 Oct 2017, 20:18

In real mode there are two ways to push a 32 bit byte sized value. The 66 68 version encodes and entire 32bit immediate within the instruction. The 66 6A version encodes only an 8bit immediate which is sign extended to 32bits when executed.

Code:

use16
push large 0 ; 66 6A 00
push dword 0 ; 66 68 00 00 00 00

I've seen the push large syntax in IDA Pro for the opcode 66 6A 00 but it doesn't translate to FASM syntax directly. How can I force FASM to generate the 66 6A version.

20 Oct 2017, 20:18

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 20 Oct 2017, 20:25

Use PUSHD mnemonic to get the size-optimized 32-bit version in 16-bit mode.

20 Oct 2017, 20:25

rain_storm

Joined: 05 Apr 2007
Posts: 67
Location: Ireland

rain_storm 20 Oct 2017, 20:29

Brilliant! Thanks Tomasz.

20 Oct 2017, 20:29

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