SC/MP 8073 addressing query
ajp166 at bellatlantic.net
Wed Jan 21 07:04:53 CST 2009
>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>
>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
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
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.
>Doug Jackson, I-RAP, MAIPM, MIEEE
>Principal Information Security Consultant
>PO Box 6308 O'Connor ACT 2602
>Level 1, 214 Northbourne Ave, Braddon ACT 2612
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