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Re: [MiNT] TAS in GFA Linker

To: Lonny Pursell <atari@bright.net>
Subject: Re: [MiNT] TAS in GFA Linker
From: Andreas Schwab <schwab@linux-m68k.org>
Date: Mon, 08 Sep 2014 10:02:06 +0200
Cc: "\[MiNT\] Mailing-List" <mint@lists.fishpool.fi>
In-reply-to: <D0329A9C.26042%atari@bright.net> (Lonny Pursell's message of "Sun, 07 Sep 2014 23:28:28 -0400")
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Lonny Pursell <atari@bright.net> writes:

> ;original code
>     tas    (a1)
>     bpl.s  label
>
> ;alternate code    
> temp:       dc.w 0    ;add this somewhere or use a free register if you can
>
>     move.b  (a1),temp ;copy the byte in question (previous value)
>     bset    #7,(a1)   ;clobber bit 7 as tas would do
>     tst.b   temp      ;set Z and N based on prev value like tas
>     bpl.s   label     ;original branch instruction unchanged

This wouldn't be interrupt safe.  Testing N after tas is equivalent to
testing Z after bset #7, so no need for testing the whole byte.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
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References:
- Re: [MiNT] TAS in GFA Linker
  - From: Lonny Pursell <atari@bright.net>

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