11 Apr
1999
11 Apr
'99
11:48 a.m.
On Apr 11, 10:20, Richard Erlacher wrote:
Subject: Re: stepping machanism of Apple Disk ][ drive
(was Re: Heatkit
51
I have to disagree with your comparison of the 2 MHz
6502 with a 4 MHz
Z-80A. My thought here is that the 4MHz Z-80 used in the conventional
way,
had a memory cycle of 750 nanoseconds (3 clock ticks),
while the 6502, at
whatever rate, again, used in the conventional way, had a memory cycle of
one clock tick. Now, some instructions involve several memory cycles,
but
that was true of both processor families. What I
often cursed, was that
the
textbook application of the 650x core left memory
available (idle) half
the
time. That was a blessing up to a point (2.5 MHz to
be exact) because it
allowed for DRAM "RAS-precharge." The Apple and others like it proved
that
at around 1 MHz, the 6502's memory could be used
for an entirely separate
purpose, e.g. video refresh.
I wasn't talking about precisely 2MHz vs 4MHz, just a ballpark figure (as
opposed to "about the same" or "about ten times" clock speeds). So,
given
the rest of your message, I think we're in broad agreement. BTW, BBC
Micros have a 2MHz clock on the 6502, and interleaved video and processor
access quite happily in 1980. The video took care of the refresh
requirement.
I believe there are entirely too many subjective,
architecture-related,
factors to allow an absolute comparison/contrast of the two processors.
Agreed :-) That's why lies, damned lies, and benchmarks are so much fun
:-)
In my "gut" I still believe the 4 MHz Z-80
is about
comparable to a 1.5 MHz 6502.
Well, that's not very far from what I wrote, is it? I was just pointing
out that although Allison seemed to imply that a 6 or 8MHz Z80 was much
faster than a 4MHz(? I haven't got the original message any more) 6502, I
believe that to be far from the case.
--
Pete Peter Turnbull
Dept. of Computer Science
University of York