More thoughts on building a Z-80 (64bit!!!)

From: Allison J Parent <allisonp_at_world.std.com>
Date: Thu Oct 22 21:33:46 1998

< WIRE_WRAP!?!?!?! ICK! I usually pull out my cheapo PCB CAD program....
< A little laser-printer transfer plastic and an exacto-knife.

the best you can do with that is two sided and that might run at 25-40MHz
and that's pushing.

< I cannot believe that there can be so many extremely complex processors
< there running up to 667Mcyc (Alpha) and there not be any simple controll
< with far less transistor counts that match or exceed those speeds.
< There is no way we can get 600Mcyc clock speeds out of a 10,000,000
< transistor chip but can't get at least that out of a 100,000 transistor
< chip.

It all that complexity that permits parallelism of functions. The simpler
machine are limited by the very direct and limiting propagation delays of
gates.

< As for the Z-80 vs Alpha features.... I like:
<
< 1) I/O ports. I hate the memory map stuff. (yes, I know it is the wa
< things are done now)

Memory mapped ops even with Z80 allow you to do things like OR data with a
device or AND data from a device or use the BIT ops to test a bit in the
device. None of which the IN or OUT can do directly.

when you only have 64kb memory mapped hurts some. When you have 4Gb
so what if you give up a meg to memory mapped IO.

FYI the PDP-11 is also memory mapped and uses it to very good advantage!

< (((((((THE Z-BUS!!!)))))))) (not the Z380 bus, that's almost as hosed a
< the Alpha bus).
< Do the Z-80 bus but with the data size control lines that I explained
< above.... (or keep a 64 bit data bus and run a local bus into a 'DMA' (f
< a lack of a better word)... Wait! A bus mastering device!!!! (<-- bett
< word).)

Zbus give advance status of the transaction to occur, that information is
handy for tweeking the memory to make it get there in time instead of
wait states. Z80 bus has a high bus bandwidth and poor bus utilization
and this shows up when you have DMA peripherls.

I'm not bombing z80, I happen to like it and have used it for 20+ years!
I'm realist enough to know it's limitations and weaknesses and I can
understand why the z380 is the way it is. Personally I happen to like
the z280 as the user/system spaces make the 64k limits far easier to
work with and also offers some speed with the 16bit bus, cache and a
paged MMU while not locking the bus. The Z180 is a good compromize and
easy to work with at low speeds, at 20+mhz it's a bear as the world
around it has to keep up. It's not like using the z80 at 4 or 6mhz
any more when you notice that the decoding dealys and the buffering
delays are are a significant part of the system timing.

Allison
Received on Thu Oct 22 1998 - 21:33:46 BST