[Coladam] Question about RAM
oltmansg at bellsouth.net
Mon Feb 14 22:27:53 CET 2011
There are 64kB of system RAM, and 16kB of Video RAM. That's where the 80kB
figure comes from. Most other 64kB machines of that vintage have memory mapped
displays that would eat up some of your system memory (C-64, Atari 8-bit
machines for example). Practically all of the ADAM's 64kB of system memory is
useable for anything... I think the only memory mapped device is the DCBs for
the Master 6801 for ADAMNet since it was a DMA'able interface.
The TI 99/4A also uses the same video chip as the ADAM. They advertised that
machine as having 16kB of RAM. It actually has a little bit more, but guess
what? All of that 16kB is used by the VDP chip directly, and they relied on very
slow IO port access to fetch instructions out of the VRAM for the CPU to use.
The TMS 9900 processor didn't have built-in registers like virtually all CPUs
have, instead they needed to have a small amount of RAM for registers, stack,
and some workspace (about 512 *bytes*). Very odd design, and also quite slow,
even by yesterday's standards. Especially sad since the TMS9900 processor was a
true 16-bit processor. The designers hobbled it with an 8-bit data bus, and the
limited amount of working RAM.
From: Justin Salvato <justin76here at hotmail.com>
To: coladam at adamcon.org
Sent: Mon, February 14, 2011 3:18:01 PM
Subject: [Coladam] Question about RAM
It’s been bugging me for a while. The advertisements for the ADAM as well as the
box it comes in, mentions that it has 80 kilobytes of RAM, but many sources
online and in magazines say that it has 64 kilobytes. Which is it? Thanks!
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