S-100 phantom (Was: RE: Northstar Horizon)

From: allisonp_at_world.std.com <(allisonp_at_world.std.com)>
Date: Tue Nov 2 15:09:55 1999

Yes, Phantom/ was generated by a lot of cards. The mech for it was if
that card was enabled (in an IO latch) and it's address came up it would
assert (active low) phantom/ and kick everyone off the bus for that cycle
and if the board didn't select it was not an issue.

So My Compupro RAM16 (64k static) in the NS* (system A) has all banks
enabled and if we hit e800H the FDC forces phantom/ and the ram16 is
deselected for that address.


On Tue, 2 Nov 1999, Bill Sudbrink wrote:

> I'm getting to this point with my IMSAI restoration. The
> ROM on the Cromemco 4FDC can be disabled via an I/O
> write... the SDS Expandoram will enable/disable based on
> phantom (line 67). Unfortunatly, this seems to control the
> whole board. How was phantom used historically? Are there

It would but only if the board like the 4FDC asserted phantom/
when the rom is active. The fact that the board (expando)
is board level selected is unimportant as Phantom/ is a DESELECT
that is generated by some other card using the same space.

It is possible (and was done) to use phantom/ for memory banking
such that if a bord generated phantom/ it didn't look it at (or it get
circular). I used that trick with 4 16k static cards all jumperd for
0000-->3fffh range. Only one would be active based on a selct register
and they would kick off the main 64k ram card when their addresses were
active. from a software perspective that gave my 5 16k banks all starting
at 0000 and 48k of main ram that was allways the same(common). It was my
first banked system and allowed background tasks running in each of the
16k banks (also caches, extended bios and extras).

> common boards which generate phantom as factory built? I'm
> now using a Cromemco ZPU which seems to work fine, but I have
> no docs for it. Does it have a "phantom latch"?

Most of the CCS, Compupro, Cromemco and others that were post 79ish did
use and respect that signal. However early ones (all MITS and IMSAI)
didn't use that signal at all. by 1981 it was defacto useage for roms and
memory mapped Video ram, FDC or other.

Received on Tue Nov 02 1999 - 15:09:55 GMT

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