24 Dec
2016
24 Dec
'16
3:10 p.m.
On Fri, Dec 23, 2016 at 04:14:28AM -0700, Eric Smith wrote:
On Fri, Dec 23, 2016 at 3:59 AM, Peter Corlett
<abuse at cabal.org.uk> wrote:
Interesting. I was of the understanding that the Z80 video hack didn't work on
the 6502 due to the latter being slightly pipelined and so the instruction
fetch cycle couldn't be abused in the same way. Perhaps Sinclair couldn't get
it to work on some dodgy 6502s that fell off the back of a lorry.
People who know Uncle Clive's unwillingness
to spend a penny more than he
has to on bulding computers may wonder why they selected the relatively
expensive Z80 over the 6502, but it was because they managed to trick the
Z80's address-fetcting and instruction decoding cycle into generating video
on the ZX80 and ZX81, and thus saved more money elsewhere.
The same trick works
perfectly well with a 6502, and in fact was invented by
Don Lancaster using a 6502 years before the ZX80 was designed. That doesn't
really explain the choice of the Z80. Also, by the time the ZX80 was introduced, both the
Z80 and 6502 were
basically dirt cheap. Any premium price the Z80 had once commanded had long
since evaporated.
If there was nothing in it, then it's a mystery why the Z80 was selected.
However, Sinclair wasn't exactly one for making rational design decisions based
on technical merit or industry best practice...