Port-vax archive

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Old Index]

Re: /etc/disktab on vax

On 2012-07-03 20:54, Mouse wrote:
IIRC the Z80 required a 4x clock, so a 20MHz Z80 is about as fast as
a 5MHz 8080 or 8085.
It's not that the Z80 "requires a 4x clock", but four T-states (clock
cycles) make up one machine cycle.  [...]

Also - and this memory is seriously fuzzy, so it could be entirely
wrong - I think the 8080 required a two-phase clock, so that there are
four, not two, state changes on the clock pins per clock cycle.  This
could easily turn into "requires a 4x clock", so if my memory is right
and the Z80 copied that particular bit of design....

It didn't. If I remember right, that was one reason the Z80 became so much more popular. It just has a simple clock in. (The other reason being the built-in RAM refresh...) And instructions take anything from four to a random, very big number of T-states. It is not even a multiple of four. And the number of T-states also vary depending on what kind of argument an instruction gets.


Home | Main Index | Thread Index | Old Index