64bit data 128Bit address...Re: Building a Z-80 (64bit!!!)

From: Allison J Parent <allisonp_at_world.std.com>
Date: Thu Oct 22 18:05:09 1998

< > It is good design in my opinion to have your address bus twice as wide

With a 128 bit wide data bus for a predominatly byte oriented machine
all that means is every memory cycle gets you 16bytes at memory cycle
speeds. It's a bus bandwidth saving trick as it effectively can give
what ever your word time is to put up another 16bytes. This goes back to
the core days when memory was slower than the CPU. The penelty is a
whopper when all your doing it altering or writing one byte as you have to
resort to RMW (read, modify, write) cycle at memory speeds. The alternate
is a lot of word/byte selection logic that can slow things
down.

With the upper speed of available z80s (z180s180) being 33mhz there is
plenty of speed so that it can hurry up and wait for the disk, floppy
and user.

Allison
Received on Thu Oct 22 1998 - 18:05:09 BST