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Index > Main > RDMSR/WRMSR in 64-bit mode

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Hunter



Joined: 07 Jun 2006
Posts: 41
Hunter
Hello!
Does FASM allow to use 64-bit RDMSR/WRMSR (operates RDX:RAX and RCX registers) without explicit REX prefix (by any special mnemocode, e.g.)? Or I must write REX prefix explicitly? (and the same for SYSEXIT, SYSRET, XLATB).
Post 14 Jul 2006, 11:55
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Hunter



Joined: 07 Jun 2006
Posts: 41
Hunter
Ok, for XLATB the following code may be used:
xlat [ebx]
xlat [rbx]

and for other instructions prefix O64 would be very useful. Smile
Post 17 Jul 2006, 08:01
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7724
Location: Kraków, Poland
Tomasz Grysztar
Strange, Intel doesn't specify any syntax settings to differ the two forms of those instructions. But perhaps RDMSRQ/WRMSRQ should be OK, as fasm already doest it with RETQ etc.
Post 17 Jul 2006, 09:22
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Hunter



Joined: 07 Jun 2006
Posts: 41
Hunter
And SYSEXITQ, SYSRETQ ? Wink
Post 17 Jul 2006, 15:38
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MazeGen



Joined: 06 Oct 2003
Posts: 975
Location: Czechoslovakia
MazeGen
Just curious, is there real need for 64-bit RDMSR/WRMSR? I mean, is there actually any MSR with number greater than 2^32-1, which doesn't fit into ECX?
Post 24 Jul 2006, 08:08
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Hunter



Joined: 07 Jun 2006
Posts: 41
Hunter
Tomasz, thank you very much for adding WRMSRQ/RDMSRQ/SYSEXITQ to 1.67.6 Smile. And what about SYSRETQ and Intel multi-bytes NOPs? Wink
Post 24 Jul 2006, 09:08
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7724
Location: Kraków, Poland
Tomasz Grysztar
In my Intel manuals the SYSRET doesn't use such setting (there's no REX.W-prefixed variant mentioned).
Maybe that's just that I have old manuals, since multi-byte NOP are also not mention... *sigh* Perhaps a time for update...
Post 24 Jul 2006, 09:25
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MazeGen



Joined: 06 Oct 2003
Posts: 975
Location: Czechoslovakia
MazeGen
In my revision (19), it is said that SYSRET depends only on operand-size prefix.

edit: sorry, I confused it


Last edited by MazeGen on 24 Jul 2006, 09:53; edited 1 time in total
Post 24 Jul 2006, 09:32
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7724
Location: Kraków, Poland
Tomasz Grysztar
OK, you're right, the SYSRET is also affected with REX.W - when it has 32-bit operand size it goes into compatibility mode, while with 64-bit operand size it goes into long mode.
Is the SYSRETQ a good mnemonic here? Per analogy pehaps it's acceptable...
Post 24 Jul 2006, 09:48
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