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

From: Charles A Davis <cad_at_gamewood.net>
Date: Fri Oct 23 22:44:09 1998

Tony Duell wrote:
>
> > I HATE partial decodes --- it's something else waiting to 'bite you in
> > the A..' sometime in the future.
>
> Who said anything about partial decodes?
>
> What I was suggesting was to AND the top 100 address lines (if you can
> make a CPU clocking at 1GHz, then you can make a 100 input AND gate).
> That gives you an 'I/O page selected' signal, which can be system-wide.
> Now decode the bottom 28 address lines fully, AND with the I/O page
> selected signal, and use the result as the chip select signal for an I/O
> chip.
>
> That's fully decoded. The I/O is in its own page at the top of memory
> (like on a PDP11, where it works very well), and you can use the same
> instructions to access memory and I/O.
>
> >
> > Chuck
> >
> >
> >
>
> -tony

Hi Tony:
O.K. You have described _complete_ decoding, but the description
_implies_ that you _may_ just check a few lines, and make a decision on
the basis of that.

The possible problem that I was seeing, is that IF the description of
how things are separated is obvious that _partial_ decoding will work
for most things, then people (some) will be lazy and, save a few clock
cycles, or speed up a routine, 'for now' --- ignoring the problems
later.

Chuck
Received on Fri Oct 23 1998 - 22:44:09 BST