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List:       debian-user
Subject:    Re: Best Keeps Getting Bigger
From:       Mark Allums <mark () allums ! com>
Date:       2009-12-31 7:16:20
Message-ID: 4B3C4FC4.8070705 () allums ! com
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On 12/30/2009 1:11 PM, Stan Hoeppner wrote:

> My x86-64 Architectural Programmers Manuals are packed away somewhere so I'll
> have to wing it here only stating what I know to be factual.
>
> 1.  The original eight 32 bit x86 GPRs were increased in width to 64 bits
> 2.  Eight additional 64 bit GPRs were added for a total of 16 GPRs
> 3.  In 64 bit long mode, all registers are available
> 4.  In 32 bit mode, only the first eight are available
> 5.  In 32 bit mode, only the first 4 bytes of each register are available
>
> New 64 bit mode instructions were created in essence duplicating most/all of the
> previous 32 bit x86 instructions.  I'm unsure of the actual instruction encoding
> so I don't know the bit length of each/any/all the 64 bit instructions.  I do
> know that all of them are a few bits larger than the 32 bit cousins and some
> many bits larger.  I was being simplistic in my previous post for brevity.  The
> instruction word bit/byte length isn't doubled across the board, but it is
> always larger on average, so 64 bit code is always going to have a larger
> footprint, as you state, but it won't be twice the size in most cases.  Wild ass
> guess, I'd say it's probably 30-50% larger on average for user space code and
> slightly greater for kernel code since it deals so heavily with programming the
> hardware and managing memory.


Yes, I'm sorry, I got a little pedantic, didn't I?  A figure for the 
binary size increase I saw was 70% larger code texts.  Data space can 
grow as well, by virtue of the increase in integer size in some cases, 
from 32 to 64 bits.

Mark Allums




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