Re: Does an enhanced //e _ALWAYS_ have 80 col. support?

[Don Bruder <dakidd@sonic.net>]

In article <20041123133113.21939.00001199@mb-m06.aol.com>,
 mjmahon@aol.com (Michael J. Mahon) wrote:

> Don Bruder wrote:
> 
> >In article <Udzod.178761$df2.88821@edtnps89>,
> > "Craig Bower \(Bender MX\)" <NOSPAMnitroacd@telusplanet.NAYSPAM.net> 
> > wrote:
> 
> <snip>
> 
> >> For Example from assembly:
> >> 
> >> LDA #$00                   Load Accum with #$00 (soft swtich bait)
> >> STA $C00A                Enable main ROM from $C300-$C3FF
> >> JSR $C300                  PR#3
> >> LDA #$00                   Reset Accum in case JSR $C300 scrambled it.
> >> <<STA $C00C>>    << Might be optional -try it with & w/out
> >> STA $C00F                Turn on Alternate character set.
> >
> >ERROR! ERROR! ERROR! ERROR!!!
> >
> >When playing with soft-switches, *ALWAYS* *READ* from them, unless 
> >specifically told that writing to activate them is OK. Some of them 
> >don't care. Others will toggle twice when written. (A read involves one 
> >access, a write involves two accesses - one to "locate" the address, the 
> >second to do the actual write) 
> 
> *** ERROR: ERRONEOUS ERROR MESSAGE ABOVE! ***  ;-)
> 
> Many of the //e softswitches can be written to.  Some _must_ be
> written to since they have different functions depending on whether
> they are read or written.

Right, that's what I was referring to. When in doubt, the "rule" I 
always knew was "read unless specifically told otherwise". 

> 
> $C00F is a switch that should be written to.
> 
> The only time a "store" accesses the effective address twice is
> in the indexed form.  I think this myth got started because of
> the warnings about POKE vs. PEEK, since POKE uses an
> indexed store.

Hmmm... That'd be a new one on me. Wasn't aware of it being limited to 
the indexed forms. I've always been under the impression that all writes 
did what amounts to "look at the address (counts as one access) store 
the value (counts as second access)" 

> 
> Jim Sather has a nice table showing exactly what bus activity
> results from the execution of each opcode.

Sounds like an interesting item to have... Might just have to go looking 
for it if Jim doesn't offer to ship it direct. :) 
 
> BTW, the read-modify-write instructions reference the effective
> address three times (as you might expect  ;-)!

That's a big surprise... NOT! :)

-- 
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