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Groups > comp.sys.acorn.programmer > #447

Re: Font_DecodeMenu

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In article <4dfa29aa$0$14673$ba4acef3@reader.news.orange.fr>,
   Rick Murray <heyrickmail-usenet@yahoo.co.uk> wrote:
> I notice in hacking around the PVR code, that at some point ARM based
> Linux altered SWI handling to call SWI &0 and pass the code in R7 instead
> of calling SWI &<code>. I wonder, given how this must then be picked
> apart in the SWI handler, how much of a speed increase this makes? It
> does seem to rather break the niceness of SWI <number>, however...

I don't know if you're aware of why they do it (apart from ARM deprecating
the field in the SWI instruction which contains the RISC OS SWI number)...

It's because putting the SWI number in the instruction means the SWI handler
has to load the word of memory containing the instruction into the data
cache (it will probably only be in the instruction cache at the time it is
executed) causing a whole line of dcache to be thrown out in the process. 

This is nasty - it'd be just lovely if there was some other register (co-pro
register or something) that the ARM could automatically dump the SWI number
before it branches to the vector. That would be faster for the SWI handler
and would eliminate the dcache pollution.

Sadly, because Linux (and presumably many other OSs) never bothered to use
the SWI number field, instead deciding to pass a reference to the routine in
a register, I don't think ARM have ever cared.


Ah, I see from your next post that you already knew. :)

Steve

-- 
Steve Revill @ Home
Note: All opinions expressed herein are my own.

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Thread

Font_DecodeMenu Paul Sprangers <Paul@sprie.nl> - 2011-06-14 12:24 +0200
  Re: Font_DecodeMenu Rick Murray <heyrickmail-usenet@yahoo.co.uk> - 2011-06-14 15:54 +0200
    Re: Font_DecodeMenu Martin Wuerthner <spamtrap@mw-software.com> - 2011-06-14 20:52 +0200
    Re: Font_DecodeMenu Paul Sprangers <Paul@sprie.nl> - 2011-06-14 23:56 +0200
  Re: Font_DecodeMenu Steve Fryatt <news@stevefryatt.org.uk> - 2011-06-14 22:43 +0100
    Re: Font_DecodeMenu Rick Murray <heyrickmail-usenet@yahoo.co.uk> - 2011-06-15 06:50 +0200
      Re: Font_DecodeMenu Martin Wuerthner <spamtrap@mw-software.com> - 2011-06-15 13:18 +0200
        Re: Font_DecodeMenu Rick Murray <heyrickmail-usenet@yahoo.co.uk> - 2011-06-15 15:38 +0200
          Re: Font_DecodeMenu Steve Drain <steve@kappa.me.uk> - 2011-06-15 17:20 +0100
      Re: Font_DecodeMenu Steve Fryatt <news@stevefryatt.org.uk> - 2011-06-15 20:24 +0100
        Re: Font_DecodeMenu "Ste (news)" <steve@revi11.plus.com> - 2011-06-15 21:19 +0100
          Re: Font_DecodeMenu Steve Fryatt <news@stevefryatt.org.uk> - 2011-06-15 22:14 +0100
            Re: Font_DecodeMenu "Ste (news)" <steve@revi11.plus.com> - 2011-06-16 14:35 +0100
              Re: Font_DecodeMenu Rick Murray <heyrickmail-usenet@yahoo.co.uk> - 2011-06-16 18:10 +0200
          Re: Font_DecodeMenu Rick Murray <heyrickmail-usenet@yahoo.co.uk> - 2011-06-16 18:04 +0200
            Re: Font_DecodeMenu Rick Murray <heyrickmail-usenet@yahoo.co.uk> - 2011-06-16 18:51 +0200
            Re: Font_DecodeMenu "Ste (news)" <steve@revi11.plus.com> - 2011-06-16 18:12 +0100
              Re: Font_DecodeMenu John Tytgat <this@is.invalid> - 2011-06-21 19:19 +0200
                Re: Font_DecodeMenu Rick Murray <heyrickmail-usenet@yahoo.co.uk> - 2011-06-22 19:18 +0200
                Re: Font_DecodeMenu "Ste (news)" <steve@revi11.plus.com> - 2011-06-25 17:53 +0100
                Re: Font_DecodeMenu John Tytgat <this@is.invalid> - 2011-06-27 13:06 +0200
        Re: Font_DecodeMenu Alan Wrigley <spamhater@keepyourfilthyspamtoyourself.co.uk> - 2011-06-15 22:13 +0100

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