a classiccmp electronics project?

From: Vintage Computer Festival <vcf_at_siconic.com>
Date: Sun Aug 1 17:36:50 2004

On Sun, 1 Aug 2004, Tom Jennings wrote:

> A cute trick commonly used in the CP/M days (and later) was to put a
> boot EPROM board in the same (conflicting) memory space with RAM; the
> EPROM code would proceed to "block copy" itself in place (read: EPROM;
> write: RAM) then reset a flipflop that enabled the EPROM board.
> EPROM board read access generated many wait states; the trick was the
> EPROM board ignored write cycles but the underlying RAM didn't. I forget
> how we handled the EPROM 'read' cycle but it was buss-safe (S100) and
> simple.

I'm not sure if it's an apt analogy (or an analogy at all) but an Apple
][ with 64K has 16K ROM at the top of memory and a 16K bank of RAM shared
with the ROM. If you poked to the upper locations it would be stored to
the RAM, but if you peeked you'd get the byte in ROM. You could enable
RAM read by flipping some soft switches.

PEEK and POKE: what wonderful command names! I wish computers were that
much fun today.

Sellam Ismail                                        Vintage Computer Festival
International Man of Intrigue and Danger                http://www.vintage.org
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Received on Sun Aug 01 2004 - 17:36:50 BST

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