Replacing 6550s

From: Philip.Belben_at_pgen.com <(Philip.Belben_at_pgen.com)>
Date: Tue Dec 1 09:14:36 1998

>> > Why do POKE and PEEK fail there? Was that done on purpose or is it
just
>> > the result of something lame like using a signed value to represent
>> > addresses?
>> No, it's software. It was a feature that was supposed to prevent
>> inquisitive geeks disassembling the BASIC ROM between $C000 and (I
think)
>> $E7FF. The OS ROMs, above $F000, were peekable, though, as was the I/O
>
> It was totally useless for that. The sort of person who could disassemble
> and make sense of the BASIC ROM was the sort of person who could also
> write enough machine code to copy the ROM into (peekable) RAM a few K at
> a time...

I just added a little machine code routine to my disassembler that peeked
the byte it was looking at for it. I was really annoyed to find that I
needed one for the assembler as well to do the poking...

BTW BASIC programs up in the ROM expansion space didn't work. The machine
relied on the MSB of the address not being set for one or two things, I
can't remember what.

Philip.
Received on Tue Dec 01 1998 - 09:14:36 GMT