[GOODIE] A2Emulator

Duane Maxwell dmaxwell at san.rr.com
Fri Nov 23 08:25:35 UTC 2001


>The interesting thing about text mode was that each line (in 40 or 80
>column mode) was 48 bytes long (48 in each bank in 80-column mode). Since
>there were only 40 characters displayed, the extra 8 bytes at the end of
>each line were "wasted". I seem to recall some of them (on the //c at
>least) being registers that actually did stuff when you accessed the
>memory...

According to my memory, and confirmed by checking out the ROM listings and
other resources on the web, the extra eight bytes per scanline are used as
miscellaneous memory cubbyholes for cards to use for their drivers.  The
driver would typically keep the slot number of the card in the X or Y
register, and would use absolute index mode into the "waste" to store stuff.
These locations had no other hardware interpretation, ie. they didn't change
the state of the machine the way the $C0xx range did.

One bizarro thing I recall about certain cards (the serial card being one
example I remember), was that one of the status bits was actually mapped to
a bit in an actual instruction in the card's ROM, resulting in a instruction
that changed - this worked because the 6502 had no instruction cache.





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