It was always somewhat of a nuissance dealing with the Z-80 because of the
way it presented its interrupt acknowledge, which was a combined I/O and M1
cycle, as long as the I/O cycle, but with M1. That meant, in my case, that
I had to insert a wait state in the M1 cycle in order to avoid having to use
much faster RAM, since the M1 signal appeared before either the IORQ or the
MEMRQ signals. By either synchronizing the resulting signal with the clock,
thereby eliminating the glitch, or by waiting for either of these two
steering signals, the resulting cycle was too short. By that I mean that it
was almost an entire clock period shorter than the normal M1 cycle would
have been, had I been able to ignore the IACK protocol in the Z-80
peripheral devices. I was stuck with them as a result of decisions already
made. <sigh>
This same protocol made it unwise to try to use memory mapped I/O, since the
cycle length of an I/O cycle differed from that of a memory cycle.
Dick
-----Original Message-----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Tuesday, March 30, 1999 2:20 PM
Subject: Re: Rebirth of IMSAI
>> >One of the problems with the S100 bus is that it is _very_ dependant on
>> >the 8080 signals. Even using a Z80 is a bit of work - things like PINTE
>> >(which indicates if interrupts are enabled) and SSTACK (address lines
>> >contain the value of the stack pointer) don't exist as external signals
>> >on the Z80.
>> >
>> Of course if you wire your own Z-80, you can ignore most of the weird
>> signals if you are using static ram. Just use a memory write signal for
>> writes, and another to gate read data onto the buss.
>
>True, until you find you have some strange (commercial) card that depends
>on one of these signals for some strange reason. I know that many
>commercial Z80 CPU boards did have extra logic at least for PINTE.
>
>> There is nothing in the Z-80, etc. to prevent you from using memory
mapped
>
>-tony
>
Received on Tue Mar 30 1999 - 16:38:03 BST