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From: davido@gordon.UUCP (David Ornstein)
Newsgroups: talk.bizarre,comp.misc
Subject: Re: What the world needs now [ is an exploding computer ]
Message-ID: <15@gordon.UUCP>
Date: Sun, 24-May-87 14:28:22 EDT
Article-I.D.: gordon.15
Posted: Sun May 24 14:28:22 1987
Date-Received: Sun, 24-May-87 21:43:46 EDT
References: <12067@topaz.rutgers.edu> <910@killer.UUCP>
Reply-To: davido@gordon.UUCP (David Ornstein)
Followup-To: comp.misc
Organization: Access Technology Inc., S. Natick MA, 01760
Lines: 32
Keywords: Originally from a friend
Summary: short circuit
Xref: mnetor talk.bizarre:1869 comp.misc:579

When I was working at the Timex Computer Corp. (you remember the little
Times/Sinclair Computers), a couple of us designed a bank-switching
mechanism that was pretty cute.  Basically, the thing was runningh on a Z80
so you could only have 64K of memory (16-bit address space).  We needed more.
So...

You had up to 256 external banks of memory, each 64K, divided up into 8K
chunks, each bank with 8 chunks.  Each bank had some controller logic
associated with it.  There was an 8-bit latch that you could write to
from the CPU to enable various chunks in any given bank.  Because the
thing needed to be so cheap, there wasn't/couldn't be any logic to make sure
that you (the programmer) didn't turn on multiple chunks in the same address
space and then access that chunk.  If you did, both (or ,potentially, all)
of the external banks would respond, turning of their tri-state buffers
and putting whatever they wanted on the bus (e.g. one 5v, the other ground).

My backgroud is mostly digital, with only a few spatterings of the analog 
stuff needed to let me do digital work, but it seemed to me that this might 
cause some nasty fireworks if taken to the extreme.  It offers the
potential for the ultimate trojan horse program!


-- 

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David Ornstein		"Never join a religion that has a water slide."

Internet:	davido@gordon
UUCP:		{mit-eddie|seismo}!mirror!gordon!davido
	     or {harvard|ames|decvax|husc6}!necntc!davido
US Snail:	Access Technology, 6 Pleasant St, Natck MA 01760
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