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Re: /etc/disktab on vax

On 07/03/2012 01:17 PM, David Laight wrote:
>> ..and here i was thinking that 20Mhz Z80 was blistering speed compared to 
>> 2-6Mhz. :>
> IIRC the Z80 required a 4x clock, so a 20MHz Z80 is about as fast as
> a 5MHz 8080 or 8085.

  I'm sorry, that's not even close.

  It's not that the Z80 "requires a 4x clock", but four T-states (clock
cycles) make up one machine cycle.  Instructions take anywhere from one
to several machine cycles to execute, depending on the instruction.

  In the 8080, six T-states make up a machine cycle, and like the Z80,
instruction execution takes from one to several machine cycles depending
on the instruction.

  I don't recall offhand the timing specs of the 8085, but I believe the
core design was almost identical to that of the 8080.  Its improvements
were primarily in the areas of bus interfacing, single power supply
voltage (the 8080 requires three), and a relatively minor and
little-used serial I/O facility.

  The Z80's instruction set is a superset of that of the 8080...as is
the 8085's, but there were only two instructions added, and they don't
do much.  So...

  Assuming the same binary program, the Z80 can be a tad bit faster than
the 8080/8085.  When the Z80s additional instructions and alternate
register set are used, a Z80-based system can have a significant
performance advantage over that of an 8080/8085.


Dave McGuire, AK4HZ
New Kensington, PA

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