18 Jun
2008
18 Jun
'08
8:24 a.m.
On Tuesday 17 June 2008 03:57, Holger Veit wrote:
Roy J. Tellason schrieb:
Yeah, I'm not familiiar enough with the members of the 68xx family to know
what the 6802 is all about without looking it up. But neither of these parts
is what I'm into here anyway. I have some familiarity with the 8080, 8085,
and z80. The 8080 with its requirement for support chips and multiple power
supplies I'd rather avoid, the 8085 is okay but limited, and I have more
z80 parts than anything else, so...
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.
I don't recall ever seein "8502" anywhere, the actual number in there being
6510, which has the onboard I/O port as being the major part of the
differences as far as I'm aware. I don't know about differences in drive
specifications and such. Nor do I have a clue as to where those 85xx numbers
came from, only that I ran into some of them in the later c64s being used
for CPU, SID, and VIC chips. Mostly though I have a lot of 6502s and 6522s
from 1541s, 6526s, and some small number of other
parts. I'm not that
familiar with those parts from the programming side of
things though. Nor do
I have any plans to support onboard video like the 64 did.
It will, admittedly run into problems though with own
extensions, e.g.
replacements of the onboard ROMs and additions to the external expansion
port.
Interesting to know. One thing I've always been curious about is when you
look at the 64 schematics some of them shows lines on some of the connectors
labeled "Z80" and "DMA" and I've never seen anything that actually
used
either of these.
The Z80 has also undergone several modifications
throughout its first
version, which may or may not resulted in higher fanout.
Hm. Perhaps I'll need to dig out what actual parts I have on hand and see who
made them and which versions they are, then.
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.
Well, my inclination is to use what I have on hand, rather than buying
stuff, if at all possible. What I have includes 2K and 8K static RAM (I
wouldn't mind having some bigger parts :-), and an assortment of EPROMs of
various sizes from 2716 on up, plus an assortment of Z80-family peripheral
chips, and a lot of LSTTL. Maybe *some* 74HCT parts as well.
I figure a CPU, one or two RAM chips (or more?), an EPROM, at least one
serial chips, at least one parallel chip, perhaps a CTC and DMA, and we'll
see where it goes from there. Just something to fool around with, mostly.
The EPROM will get some kind of homebrew monitor and some code in there to
load stuff by way of one or another port, I have no plans to tie a disk
drive of any sort into this either.
I'm particularly *not* inclined to try and go the route that I've seen a
number of people go, where they want floppy drives, video, etc.
essentially clonining what was out there to a large extent in the pre-PC
days. I have plenty of PC hardware around if I want that sort of thing, and
could even fire up my old Executive if I needed to get some of that feeling
back again, but that's not where I'm inclined to go with this.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin