> > While we are on the "we alone have ..." thought, look at the Clive "I=

can
do
> > magic" Sinclair ZX80 which used an ancient and venerable (and
venerated?)
> > Z80 plus a shift register to work video magic. It also managed to
pretend
to
> > be a "full" computer during the flyback period. The CISC set-and-forg=
et
> > block move instruction has it's place :-).
> =

> Correct me if I am wrong, but...
> =

> I believe the ZX80 did _not_ use a block move instruction to do the
video.
> Instead, it set a special flag that indicated that it was about to
> _execute_(!) video. When this flag was set the opcodes being fetched fr=
om
> RAM were routed to the screen. Of course this would confuse the process=
or
> something awful (or else really restrict what could be displayed). So,
the
> hardware would 'jam' a no-op onto the processor's data bus when the vid=
eo
> flag was set, regardless of what instruction was fetched from memory.

I believe it worked more like this...  One of the address lines was
connected to the interrupt request line.  There was a loop in software th=
at
accessed successive characters from RAM, apparently doing nothing with
them, but it was placed in ROM such that the address line in question wen=
t
low just at the value was read.  The character value remained on the data=

bus due to bus capacitance, during the next cycle when normally an
interrupting device would place its vector number onto the bus.  Thus, th=
e
character gets looked up in the interrupt vector table, which effectively=

becomes the character generator table.  Yes, you could change the interru=
pt
vector table pointer register to change the character set, unfortunately =
it
would only work within the ROM area, not RAM, so none of the other choice=
s
produced anything but garbage (made a nifty effect to quickly cycle throu=
gh
the possibilities, hoever!).  Note that this lookup took place even thoug=
h
the Z80 wasn't in vectored interrupt mode, one of various undocumented
features that were relied upon by the design.  The only external hardware=

used was an 8-bit shift register that picked up the translated pixel data=
,
and a 3-bit counter (or maybe it was just a latch) that held the scanline=

number within the 8-pixel high character cell.
        Jason Harper

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