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NASM Forum => Using NASM => Topic started by: wanghua131 on September 26, 2018, 01:01:02 AM

Title: How to set the REX.W=1?
Post by: wanghua131 on September 26, 2018, 01:01:02 AM

The assembler code is: PCMPESTRI XMM6, XMM13, 0x45, I want to generate  the code which the REX.W =1, but the NASM generate the the code is : 66410F3A61F545, 0x41 is the REX prefix, the REX.W = 0, how to set the REX.W=1 when the NASM generates the code?

Title: Re: How to set the REX.W=1?
Post by: vitsoft on September 28, 2018, 07:38:44 PM

Instructions PCMPESTRM (http://www.felixcloutier.com/x86/PCMPESTRM.html) and PCMPESTRI (http://www.felixcloutier.com/x86/PCMPESTRI.html) expect the length of input strings provided in registers EAX and EDX. When the instruction is encoded with prefix REX.W, RAX and RDX are used instead.
Value in length-registers is saturated to 8 or 16, so the need of REX.W can be bypassed with manual extension of 32bit value to 64 bits:
      XCHG RAX,RDX
    CDQE
    XCHG RDX, RAX
    CDQE
    PCMPESTRI XMM6,XMM13,0x45

But you are right, AMD and Intel do not suggest any specification how to ask for 64bit version of length-registers, so it depends on assembler manufacturers, and it is often overlooked. Thank you for reminding. Possible solutions which occured me:

• Explicit prefix request:
    REX.W PCMPESTRI XMM6,XMM13,0x45
  
• Explicit notation of hardwired length-registers as additional operands:

PCMPESTRI XMM6,XMM13,0x45,RAX,RDX

• Modification of mnemonic name:

   PCMPESTRI64 XMM6,XMM13,0x45
or PCMPESTRIQ  XMM6,XMM13,0x45

• Using data modifier:

PCMPESTRI XMM6,XMM13,0x45,DWORD[/li][/list]

Current version of NASM doesn't allow any of this possibilities, AFAIK. I completed €ASM with the fourth variant today, see this test example (https://euroassembler.eu/eatests/t3820.htm).[/list]

Title: Re: How to set the REX.W=1?
Post by: wanghua131 on October 04, 2018, 09:11:44 AM

Thank you very much!