Date   : Tue, 10 May 2005 11:34:38 +0100
From   : Richard Gellman <splodge@...>
Subject: Re: Jack Attack & undocumented opcode

Mike Tomlinson wrote:

>In article <gemini.ig8pqi001eh9x01w4.emulators@...>, Michael
>Borcherds <emulators@...> writes
>
>  
>
>>I've tracked it down to the fact that I run a hybrid processor - a
>>compromise between 6502 in the BBC and 65C102 in the Master 128.
>>    
>>
>
>That's interesting.  I tried a 65C02 in my BBC B, which allowed me to
>use the BASIC 4 ROM (which uses the extra opcodes) from the Master.  It
>caused too many incompatibilities with other software, however, so I
>reverted to the 6502.
>  
>
65C(1)02's in BBC Bs cause their own problems. Software that is 
opcode-optimised looks at the OS version (OSBYTE 0), sees a BBC B, and 
enables all the sneaky opcodes.

When your 65C02 comes along to execute them, it goes "ERrrr?!?" and 
falls over.

Now when the same software receives "3" back from OSBYTE 0, it disables 
the sneaky opcodeage, allowing it to run fine.

To quote from an article in an Acorn User mag: "I tried a 65C02 in my 
BBC B, and thought it was great. Then I found Revs wouldn't work, so I 
swapped [the SY6502A] back in again".

-- Richard