How CPU's work (was Re: Hi, I'm new...) - test-drb@ccmp.vtda.org

8 Aug 2006

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Subject: Re: How CPU's work (was Re: Hi, I'm new...)
   From: "Chuck Guzis" <cclist at sydex.com>
   Date: Tue, 08 Aug 2006 09:25:19 -0700
     To: cctalk at classiccmp.org

On 8/8/2006 at 11:11 AM Dave McGuire wrote:

>   I like the index registers; I use them with some frequency.  The 
>alternate register set, though...ugh.  It'd be much more useful if the 
>designers had provided a way to determine which set is currently in use! 
Index regs are nice, alternate registers...more pain than gain.

...

I've wondered about something for years, however.  Did anyone ever make use
of the fact that an INI or OTI instruction placed the contents of both the
B and C registers on the Z80 address bus?  It would seem to be a simple way
of expanding the I/O space to 64K ports. 
Yes, but why I'll never know.  Even that case they actually never used more
100 or so port addresses.  I did use it myself to put z80 on XT ISA 
(no 8088 at all) but that was to fool the 10bit device IO addresses. It 
was for hardware savings rather than more addresses.  Generally the 
problem is to write a zero to every possible port of a z80 (outir) will
take over .3sec at 4mhz.  Every time I hear "I want to have all 64k 
ports" I ask what in the world requires 512kbits of slow IO!

Allison