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Date: Tue, 12 May 2015 15:32:00 -0000
From: Michael Matz <matz@suse.de>
To: "H.J. Lu" <hjl.tools@gmail.com>
cc: Jan Beulich <JBeulich@suse.com>, Binutils <binutils@sourceware.org>
Subject: Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
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References: <20150511212331.GA1838@intel.com> <5551F4E70200007800079575@mail.emea.novell.com> <CAMe9rOpDbkeFbmNbQh0a1AKhAQy-cH4HJu20o_ERQaoR6sTxbQ@mail.gmail.com> <55520C440200007800079718@mail.emea.novell.com> <CAMe9rOpFgSyJm-oceuDkrBYnBQGv01ywCc43WySqX21NTJYi4Q@mail.gmail.com> <555216370200007800079773@mail.emea.novell.com> <CAMe9rOpOx=SSUZnCFimn4fBzFqNRDch8QYLn3Os_y7EfQH65Qw@mail.gmail.com> <alpine.LSU.2.20.1505121548100.4883@wotan.suse.de> <CAMe9rOqg_H9YXHdoWv-k4zoQm77OoQ19j3djHZ74pgUORiDECw@mail.gmail.com> <alpine.LSU.2.20.1505121559480.4883@wotan.suse.de> <CAMe9rOpmOBJoRNy8QGQn03eQJrzsUk+MTzJJuQu_DD1kxXV5pg@mail.gmail.com> <alpine.LSU.2.20.1505121713290.4883@wotan.suse.de> <CAMe9rOrwY-MmZcPPxEo7NsO1O6yR4K6rO08j-SvmivOYXGuNag@mail.gmail.com>
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Hi,

On Tue, 12 May 2015, H.J. Lu wrote:

> >> I thought it did jump to "(nextip + offset) & 0xffff" on AMD.  Can 
> >> you verify if it is true?
> >
> > Sorry, yes, this is true, the 16bit operand size prefix truncates RIP 
> > to 16 bit.  Nevertheless it's a valid instruction and works as 
> > documented.
> 
> It doesn't work on Intel processors.  Can you show me a real usage for 
> this?

Why should I?  Obviously in linux there are no uses, but perhaps in BIOS 
or boot loader code, or micro-OSes and the like.  Can you instead show me 
why you want to remove support for it?


Ciao,
Michael.