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From: Torbjorn Lindgren <tl@none.invalid>
Newsgroups: comp.arch
Subject: Re: Why VAX Was the Ultimate CISC and Not RISC
Date: Sun, 9 Mar 2025 01:27:19 -0000 (UTC)
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Anton Ertl <anton@mips.complang.tuwien.ac.at> wrote:
>EricP <ThatWouldBeTelling@thevillage.com> writes:
>>Looking at the Signetics 82S100 in 1976 has max access of 50 ns, 600 mw
>>in a 28 pins dip.
>
>The Commodore 64 used a 82S100 or compatible for various purposes,
>especially for producing various chip select and RAM control signals
>from the addresses produced by the CPU or the VIC (graphics chip).
>Thomas Giesel wrote a very detailed report
><http://skoe.de/docs/c64-dissected/pla/c64_pla_dissected_a4ds.pdf> on
>the original PLAs and their behaviour, in order to replace it
>(apparently it's a chip that was failure-prone).

AFAIK the Signetics 82S100 isn't failure-prone in C64, the MOS
Technology (IE Commodore) *clone* that they switched to due to cost
reasons IS known to be failure prone. Those are the reason there's
lots of PLA replacement projects.

If you have any actual Signetics device in a C64 it'll very likely
fine unless the power supply failed and fed everything too high
voltages which is unfortunately a common failure mode on the C64 power
brick. This PSU failure usually destroys most or all of the memory
chips, the SID and one or more likely multiple of CPU, PLA and ROMs.

Other things with known high failure rates are the MOS Technology 74xx
clones and the MT memory. These failure also include MT branded memory
chips of that specific type when used in non-Commodore items like PC
clones so it's not just C64.

Notice the common factor here - MT/Commodore was making a lot of
"working but only barely" chips and Commodore used them internally to
save money and also sold them to others which used them because, well,
they were frequently the cheapest.