Re: Apple IIe + ProDOS = Headache

[pausch@saaf.se (Paul Schlyter)]

In article <20040625200651094+1200@News.Individual.NET>,
Roger Johnstone  <rojaws@orcon.net.nz> wrote:
 
> In <avrmd0565p2ot8f011ni2s0m87asft7rf0@4ax.com> Jeff Blakeney  wrote:
>> On 23 Jun 2004 07:00:39 GMT, Roger Johnstone <rojaws@orcon.net.nz>
>> wrote:
>> 
>>>(2) ProDOS 1.3 accidentally included a 65C02 instruction in it which 
>>>could damage disks if run on a computer with a 6502 CPU.
>> 
>> First, thank you for posting these corrections.  It saved me the time
>> of having to do so but in that last line you did mean:
>> 
>>      could damage disks if run on a computer with a 65C02 CPU.
>> 
>> right?  :-)
> 
> Not according to Apple, although maybe they were wrong too :o)
> 
> ProDOS technical note #23 says:
> 
>   ProDOS 8 1.3
> 
>   Warning: This is not a stable version of ProDOS due to an
>   illegal 65C02 instruction which was added. This version can
>   damage disks if used with a 6502 processor.
> 
> http://web.pdx.edu/~heiss/technotes/pdos/tn.pdos.23.html
> 
> I take it to mean that it was a valid 65C02 but invalid 6502 instruction, 
 
The text said "illegal 65C02 instruction".  And since the 65C02 was
backwards compatible with the 6502 at the machine code level, it was
definitely an illegal 6502 operation as well.
 
The important difference here is the way the 65C02 and the 6502
behaved when encountering illegal instructions: the 65C02 treated all
illegal instructions as a NOP (no operation), while the 6502 had some
undocumented behaviour when encountering such an operation; the
undocumented behaviour was of course different for different illegal
instructions.  That's why the illegal instruction could damage a disk
if executed on a 6502 rather than a 65C02.
 
> and was 'illegal' because it shouldn't have been used since 1.3 was not 
> supposed to require a 65C02.
> 
> -- 
> Roger Johnstone, Invercargill, New Zealand
> http://vintageware.orcon.net.nz/
-- 
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