On Sat, Dec 24, 2016 at 8:10 AM, Peter Corlett <ab...@cabal.org.uk> wrote:
> On Fri, Dec 23, 2016 at 04:14:28AM -0700, Eric Smith wrote: > > 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. > > 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. > The details aren't identical, but on the 6502 it's abusing the fetch of both an opcode and an immediate operand. 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. As much as I like the 6502, the Z80 is better in some ways, particularly if you want to do a lot of 16-bit add and subtract.
