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).

Ok, for XLATB the following code may be used:
xlat [ebx]
xlat [rbx]

and for other instructions prefix O64 would be very useful.

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.

And SYSEXITQ, SYSRETQ ?

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?

Tomasz, thank you very much for adding WRMSRQ/RDMSRQ/SYSEXITQ to 1.67.6 . And what about SYSRETQ and Intel multi-bytes NOPs?

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...

In my revision (19), it is said that SYSRET depends only on operand-size prefix.

edit: sorry, I confused it

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...