From: Spiro Trikaliotis (trik-news_at_gmx.de)
Date: 2003-06-12 20:04:21
Hello Laze,
On Thu, Jun 12, 2003 at 06:44:42PM +0200, Laze Ristoski wrote:
> In fact, I wasn't going to replace the chips.
> I was just interested what generates this pattern.
> Why is it different among the C64s?
RAM is build from Flip-Flops (FF), which have two states: 1 and 0 (or: on and
off). Well, this is not really true with the C64's RAM, because it is DRAM
(and not SRAM), but the effect is the same.
FFs are build to remember the last state they were set to. Unfortunetely, at
the moment you put voltage on the circuit, there is no "previous" state. So,
they decide "randomly" which state to reach.
You might say: "If it is randomly, why is there a remarkable pattern?" Well,
randomly does not necessarily mean that they choose a value by random. In
fact, in most times, they get the same value whenever you switch them on. It
depends on many things, like internal organization, manufacturer, process
used, phase of the moon, temperature of your beer, and-so-on. ;-)
What can be remarked is (at least in every situation I have worked with
RAMs) that they tend to generate a 1-0-1-0 pattern, i.e., "neighbored"
addresses are most likely to have opposite states.
Since the RAMs inside of an (older) C64 are responsible for one bit on an
address, and the RAM chips are of the same type, you get the typical
FF/00-pattern in many cases, but not in all.
Just my EUR 0.02,
Spiro.
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