17 Jun
2008
17 Jun
'08
12:57 a.m.
Roy J. Tellason schrieb:
I'm not familiar with many 6800 designs, but I
was somewhat surprised to see
how limited some parts are, like the 8085 in the "8085 Cookbook" where you
could really get away with very little. OTOH, the c64 doesn't use any
buffering _at all_ and yet the CPU in there seems to have little trouble
driving 3 ROM chips, a set of 8 4164s, plus all the peripherals. I'm
guessing that the Z80 is probably somewhere in between, and that the
datasheet probalby won't give me the whole story anyhow
The plain 6800 is not a
good example to compare to the 8085; from its
generation it rather relates to the 8080 three-chip system. Compare the
8085 to the 6802, and you'll get a minimum 2-chip system 6802-6846 like
the 8085-8355 pair. Using EPROMs rather than mask ROMs will expand the
chipcount similarly in both ways.
The C64 does not use the 1st generation 6502, but rather a modified 8502
CPU with different electrical specifications, so I guess it was
explicitly designed to be able to drive its special C64 peripherals. It
will, admittedly run into problems though with own extensions, e.g.
replacements of the onboard ROMs and additions to the external expansion
port.
The Z80 has also undergone several modifications throughout its first
version, which may or may not resulted in higher fanout.
For all of them, one also has to consider that nowadays one won't use
the circuitry that was used 20/30 years ago any longer, but use modern
chips. This starts with low power HCT TTL, one will avoid DRAMs of that
time like a plague now, using monolithic 512KB SRAMs instead, and maybe
replace glue logic entirely with LP-CPLDs which can drive much more loads.
Regards
--
Holger