[fasmg] Far return in 64-bit mode missing REX.W

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Index > Compiler Internals > [fasmg] Far return in 64-bit mode missing REX.W

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bitRAKE

Joined: 21 Jul 2003
Posts: 4602
Location: vpcmpistri

bitRAKE 06 Sep 2026, 02:31

REX.W is required in long mode for 64-bit retf.

SDM Vol. 2B 4-560 wrote:

In 64-bit mode, the default operation size of this instruction is the stack-address size, i.e., 64 bits. This applies to near returns, not far returns; the default operation size of far returns is 32 bits.

... they hide that little detail down at the bottom.

... and AMD hides it somewhere else - it's not on the instruction page.

Last edited by bitRAKE on 06 Sep 2026, 07:54; edited 2 times in total

06 Sep 2026, 02:31

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 21091
Location: In your JS exploiting you and your system

revolution 06 Sep 2026, 05:26

Please add more detail. It shows this for 1.73.31

Code:

use64
retf
ret

Code:

flat assembler  version 1.73.31  (16384 kilobytes memory)
1 passes, 3 bytes.
00000000  48CB              retfq  ; <--- REX.W is set
00000002  C3                ret

Looks fine to me.

06 Sep 2026, 05:26

bitRAKE

Joined: 21 Jul 2003
Posts: 4602
Location: vpcmpistri

bitRAKE 06 Sep 2026, 07:51

Not in fasm2.

flat assembler version g.l8vn

iirc, the version is not updated for changes to x86-2.inc

Code:

iterate instr, retf,retfq
        calminstruction instr? operand
                check   x86.mode = 64
                jno     forward
                emit    1,48h
        forward:
                call    instr, operand
        end calminstruction
end iterate

... work around is easy.

06 Sep 2026, 07:51

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