4164/41256 RAM Modification
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Williams MPUs shipped with 24 × Mostek 4116 DRAM (16,384 × 1-bit), introduced in 1977 — only a few years before Defender.
These chips need three voltages (triple-rail):
- +5 V — TTL input/output buffers
- +12 V — primary power
- −5 V — substrate bias / shield
If −5 V drops while +5 and +12 are still present, the chip overheats and fails quickly.
That triple-rail requirement made the 4116 a weak link on these boards, especially as the linear supplies aged and the rails got noisy.
4164 First Successor
In the early 1980s the direct successor appeared: the 4164 (65,536 × 1-bit).
An internal bias generator supplies the −5 V, so the part runs from a single +5 V rail. Pins that used to carry the extra supplies became another address bit, which is how capacity went up. 4164s generally run cooler and tolerate supply variation better.
41256
By the late 1980s density had increased again. Using the last unused supply pin as another address line gave the 41256 (262,144 × 1-bit).
Pinout comparison
Here is a comparison in pinout:
