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From: bazyar@ernie (Jawaid Bazyar)
Newsgroups: comp.sys.apple2
Subject: BRK Reality
Message-ID: <1991Jan17.072144.12928@ux1.cso.uiuc.edu>
Date: 17 Jan 91 07:21:44 GMT
References: <278d0eda.563e@petunia.CalPoly.EDU> <91015.000214AABENSON@MTUS5.BITNET> <11204@ucrmath.ucr.edu>
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Reply-To: bazyar@cs.uiuc.edu
Organization: Mutation Testing Facility/UIUC
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In article <11204@ucrmath.ucr.edu> rhyde@koufax.ucr.edu (randy hyde) writes:
>>> On the II gs BRK has an operand.
   The above poster is quite correct.

>This is an artifact of the disassembler.  The "operand" for the
>break instruction goes all the way back to the 6502.  The original
>chip had a bug which pushed the return address of the *next* instruction
>(rather than next instruction minus one).  BRK handlers on the older
>chips always compensated for this.  The GS' disassembler simply treated
>BRK as a single byte instruction.  If you look at all manufacturer's 
>literature, BRK is always listed as a single byte instruction.  It was
>Apple who decided it should be a two-byte instruction.  COP, OTOH, is
>listed in WDC's literature as a two-byte instruction.

  That's really strange, since I'm sitting here staring the WDC literature
in the face, and it plainly states that in native mode a signature byte
is fetched for both COP AND BRK.  In emulation mode, only a single byte
is fetched for BRK (the opcode).

--
Jawaid Bazyar               | Girl don't stop the screamin',
Senior/Computer Engineering |  you're sounding so sincere
bazyar@cs.uiuc.edu          | There's so much beauty
   Apple II Forever!        |  in the tracks of your tears	(BOC)