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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Byte Addressability And Beyond
Date: Mon, 6 May 2024 19:26:16 +0000
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BGB wrote:

> On 5/5/2024 9:30 PM, Lawrence D'Oliveiro wrote:
>> On Sun, 5 May 2024 12:13:27 +0300, Michael S wrote:
>> 
>>> PEXTR/PDEP has no immediate form, which makes it inconvenient for
>>> 'C'-style fixed bit fields.
>> 
>> Fixed bit fields are a limitation of the C language. Why should it
>> constrain the design of machine architectures?

> If it lacks an immediate form, one is harder pressed to beat out 
> shift+and or shift+shift on the performance front...

> Though, to be useful, it needs an immediate large enough to express both 
> the shift amount and the width of the bitfield, and also a 3RI encoding.

My 66000 has 12-bits of immediate for shifts, and a slot in the 3-operand
instruction group.

> Bitfield insert would a little easier to get a performance advantage (vs 
> bitfield extract), since insertion is a more complex operation, but is 
> also likely require a more complex implementation and is also less 
> common than bitfield extract.

Without SR <w:o>; one needs two shifts and a container sized mask

     SR     Rt,Rc,#64-11             // get rid of excess significance
     SL     Rt,Rt,#64-11-12          // position field to container
     AND    Rk,Rk,#ox0007FF0000      // EMPTY field in Kontainer
     OR     Rk,Rk,Rt                 // insert field

With:

     SR     Rt,Rc,<11:12>
     AND    Rk,Rk,#ox0007FF0000      // EMPTY field in Kontainer
     OR     Rk,Rk,Rt                 // insert field

With insert::

     INS    Rk,Rk,Rc,<11:12>

> ....