Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!microsoft!brianw
From: brianw@microsoft.UUCP (Brian Willoughby)
Newsgroups: comp.sys.apple
Subject: Re: Zip-Chip/Rocket Chip
Summary: Clarification and a question about the Monitor
Message-ID: <8041@microsoft.UUCP>
Date: 13 Oct 89 21:43:58 GMT
References: <8910110859.AA13733@trout.nosc.mil>
Reply-To: brianw@microsoft.UUCP (Brian Willoughby)
Organization: Microsoft Corp., Redmond WA
Lines: 55

In article <8910110859.AA13733@trout.nosc.mil> dale@pro-colony.cts.com (System Operator) writes:
>
>> From: pnet01!pro-tcc!jerryk@crash.cts.com (Jerry E. Kindall)
>[stuff deleted]
>
>> $C030.  You can use the standard TransWarp POKE to turn the chip on and off
>> (and you don't have to reset the computer after changing speeds -- although
>> you don't have to with the Zip either, you do with the TransWarp).
>
>You only have to reset the TransWarp if you turn the card off.  You do not
>have to reset the card to change from one speed to another.

To be even more exact, you don't have to reset the TransWarp at all.  Its
the motherboard 65x02 that you have to reset.  Realize that the normal
speed processor has been hanging around dormant the whole time the
TransWarp was active, and turning the TransWarp off means you need to
cold boot the motherboard.  None of the 65x02 registers have been
initialized, and who knows where the Program Counter is pointing.  RESET
just forces your regular processor to a known state.  I have had the
unpredictable luck of switching off the TransWarp and not having to
RESET, probably because the main 65x02 just happened to execute a BRK
instruction, or somehow decided to reset on its own.

I have had the monitor do some strange things when attempting to change
TransWarp speeds (but keeping the TransWarp active).  Typing $C074:1 for
slow speed or $C074:0 for high speed will sometimes lock up the computer.
I have had trouble with using the monitor to do direct writes to hardware
before, so I wrote two BIN programs to do the 'POKES' for me.  These
never lock the machine up when changing speeds.

The problems I had with directly writing to hardware from the monitor
were related to a 6581 (Commodore) SID chip (Sound Interface Device)
board that I built.  I made a habit of directly storing sound commands to
the chip, but occasionally the monitor would screw up the volume register
and I wouldn't hear the sound output anymore.  So I would have to
re-write a new output volume register value.  I assumed that this was
related to the 6502 bug/feature where index register addressing modes
would sometimes do an access on a partially incomplete address.  I.e. the
monitor stores to STA (A3L),Y and (A3L) is put on the address bus (a read
access) the cycle before the 6502 computes A3L + Y for the actual write
address.  This extra read doesn't hurt RAM addresses, but extra I/O reads
can sometimes do damage.  The confusing part was that I seem to remember
having a R65C02 at the time - and I thought that the 65C02 corrected this
anomaly?  Also, why would a random read of a 6581 register or the
TransWarp speed register change the value stored there if its not a write
access?

Anybody have any more details on why the monitor might screw up I/O
accesses?

Brian Willoughby
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