3 Sep
2006
3 Sep
'06
10:09 a.m.
On Saturday 02 September 2006 11:56 pm, Jim Brain wrote:
Roy J. Tellason wrote:
Oh really? I was wondering what they were trying to do there. I was also of
the impression that the +4 was supposed to be a replacement for the c64, and
that the c16 (which I think I may have seen _one_ of perhaps) was supposed to
replace the vic20. But I'm not really sure.
People should also be aware of the misleading
stuff in the way the speed
of that part is presented. They called it "a 4 MHz Z80" but even though
it was indeed that chip the way they had it configured in there was so
tightly coupled with the rest of the system that it only ran some of the
time, rather than all of the time, giving an effective clock rate of
somewhere around 2.5 MHz.
Misleading it was, though we should assign blame to the Marketing folks.
When the C128 was being developed, Marketing wanted to hit the business
market (they had tried to position the Plus/4 there and failed). Someone remember the old C64 CP/M cartridge and
decided to play that up
(the C128 was C64 compatible, they reasoned, so the CP/M cart could be
used to offer CP/M support).
I read somewhere that the CP/M cartridge never actually worked all that well.
If it worked at all.
The problem was, the CP/M cart sucked, and would only
work on the
earliest models of the C64. It was a direct copy of the Apple CP/M
cart, according to some reports, but didn;t take into account the
specifics of the C64 expansion port.
I have the 64's service manual around here someplace, and maybe I should look
at the differences between the various boards, I don't remember anything in
particular there that would account for this. I do remember a couple of
lines of that port being labeled "Z80" and "DMA", which I never saw
anything
use.
Brian Bagnall documents the specifics in his book, but
in general, Bil
Herd got the CP/M cart to work by "accidentally" designing the Z80 into
the motherboard.
However, as the design-in was accidental, there was little time (or
ability) to do a proper design-in. If it had been a sanctioned design,
the full 4MHz would have been offered (and probably more), but CBM was
cheap, and Bil was trying to hit a mandated requirement, not a speedy
CP/M design.
Although the book does not document it, I think Bil had to fight tooth
and nail to keep the accidental work in place.
However, as Bil later notes, the Z80 ended up saving the CBM bacon, as
it was used to deal with badly misbehaving cartridges that did funky
things with the C64 MMU lines, which the C128 could not deal with on
machine startup. Thus, the Z80 actually starts the C128 up, and then
passes control to the 8502 (6502/10 compatible) CPU.
I also have the 128 manual on hand as well, but never really dug into the
workings of that MMU chip. As a service center and wanting to have a
complete set of spares on hand I did get a hold of some of those, and some
of the PLA chips that were used in the 128, as well as a number of the parts
peculiar to the 1571, and ended up never using any of them. I don't know if
they'd be useful in anything else offhand.
--
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