21 Jan
2009
21 Jan
'09
1:04 p.m.
Subject: SC/MP 8073 addressing query
From: Doug Jackson <doug at stillhq.com>
Date: Wed, 21 Jan 2009 09:26:13 +1100
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at
classiccmp.org>
Hi list,
I have an interesting question regarding the INS8073 (National
Semiconductor SC/MP 3 with BASIC in ROM).
Finally, somebody gave me a round tuit, and I have started work on a
simple 8073 system.
Eventually, it will have 8K of ROM, 8K of RAM, an 8255, and a switch /
LED interface. Currently on the board, I have the 6264 RAM tied to the
8073, with no address decoding (ie the CS* on the RAM is tied to A15 on
the CPU). Sadly, as a simple test, this does not operate as I would
expect. I would have expected that the RAM would have been selected
anywhere in the lower 32K or the memory map, and I would have a simple
system that would spit out a console prompt. But no luck.
Now the question.... The internal ROM is located in the lowest 4K or
the memory map. When the CPU is fetching data from the lower 4K, does
it assert the NRDS line, and sample the external bus? My belief is that
it does not, as the trivial application note that I have seen simply
ties a couple of 2114 to the processor, and uses A10 as the chip select.
I built one years ago and pull it out frm time to time.
No, it's less than 4K and yes you do get NRDS.
Also you need pull up resistors on selected lines to set baus rate and
boot on rom.
I'd use 2116(2kb) or 2164s(8kB) as they are 8bits wide and really save
on parts and power over 2114. One 2164 placed at at 1000h makes for a
tiny sysem.
Finally NIBLE when it starts does a memory check for (Ep)rom at 8000h.
There are some addresses that are tested for information that have to
be respected.
If no one comes up with the circuit that National had for the demo board
they sold I can scan my copy, it's pretty poor but may help. The manual
is just too thick for me to scan or copy.
Allison
Any ideas?
Doug
--
Doug Jackson, I-RAP, MAIPM, MIEEE
Principal Information Security Consultant
EWA-AUSTRALIA
PO Box 6308 O'Connor ACT 2602
Level 1, 214 Northbourne Ave, Braddon ACT 2612
Tel: +61 (0)2 6230 6833
Fax: +61 (0)2 6230 5833
Mob: +61 (0)414 986 878
http://www.ewa-australia.com
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