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SubjectRE: Intel vs AMD64
Thanks for the clarification.

Yes, "implementation specific" is one of the differences between IA-32e
and AMD64, i.e. that behavior is architecturally defined on AMD64, but
on IA-32e (as I posted):
Near branch with 66H prefix:
As documented in PRM the behavior is implementation specific and
avoid using 66H prefix on near branches.

>-----Original Message-----
>From: [mailto:linux-kernel-
>] On Behalf Of
>Sent: Wednesday, February 25, 2004 8:28 PM
>Subject: RE: Intel vs AMD64
>Not sure about other architectures, but in the
>AMD64 architecture, the 66h and 67h prefixes
>can be applied to the near branch
>instructions and have an *architecturally*
>defined action (rather than implementation-defined
>action) which all AMD64 processors follow. It's all
>described in the AMD64 Architecture Programmer's
>Manuals ...
>But, I definitely agree that it is sorta hard to figure
>out what a 64-bit general purpose compiler would
>actually *do* with some of them. However, there are
>embedded/special-purpose scenarios where this might
>be just fine.
>For example, for JMP (near):
>In 64-bit mode, if the JMP target is specified by a
>displacement in the instruction, the signed displacement is
>added to the rIP (of the following instruction), and the
>result is truncated to 16 or 64 bits depending on operand
>size. [rb: 64-bit is default, 66h forces 16-bit]. The
>signed displacement can be 8 bits, 16 bits, or 32 bits,
>depending on the opcode and the operand size. [rb: 8-bit
>has its own opcode (EB); for the E9 opcode: 32-bit is
>default and 66h forces 16-bit].
>] -Rich ...
>] AMD Fellow
>] richard.brunner at amd com
>> -----Original Message-----
>> From: Nakajima, Jun []
>> Sent: Wednesday, February 25, 2004 5:20 PM
>> To: H. Peter Anvin; Timothy Miller
>> Cc:
>> Subject: RE: Intel vs AMD x86-64
>> Yes, that's the very reason I said "useless for compilers." The way
>> IP/RIP is updated is different (and implementation specific) on those
>> processors if 66H is used with a near branch. For example, RIP may be
>> zero-extended to 64 bits (from IP), as you observed before.
>> Jun
>> >-----Original Message-----
>> >From: H. Peter Anvin []
>> >Sent: Wednesday, February 25, 2004 4:14 PM
>> >To: Timothy Miller
>> >Cc: Nakajima, Jun;
>> >Subject: Re: Intel vs AMD x86-64
>> >
>> >Timothy Miller wrote:
>> >>
>> >>
>> >> Nakajima, Jun wrote:
>> >>
>> >>> For near branches (CALL, RET, JCC, JCXZ, JMP, etc.), the operand
>> >>> forced to 64 bits on both processors in 64-bit mode, basically
>> >>> RIP is updated.
>> >>>
>> >>> Compilers would typically use a JMP short for "intraprocedural
>> >>> which requires just an 8-bit displacement relative to RIP.
>> >>
>> >> I see. It's too bad you can't have a 16-bit displacement.
>> >>
>> >> Ummm... so if 66H were used with a near branch, would that affect
>> >> size of the immediate operand which gets added to RIP, or would
>> >> affect the the portion of IP/EIP/RIP affected? If it's the
>> >> that's pretty silly.
>> >>
>> >
>> >Yes, that would be pretty silly.
>> >
>> >I honestly don't remember off the top of my head what "o16 jmp blah"
>> >does on i386; I have a vague memory that it zero-extends %eip to 32
>> >bits, which makes it useless, of course.
>> >
>> > -hpa
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