1 Aug
2004
1 Aug
'04
10:36 a.m.
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
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