Gregory's ServerThe circuit is a Programmable Logic Array (PLA) holding a lookup table for the floating-point division unit (FDIV). The circuit is a small part of the Pentium chip, visible in the lower right.
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7 comments
  @kenshirriff If the PLA bits are defined in the long thin structures on the right, I wonder if the missing bits are at the bottom? That section is suspiciously devoid of the various marks the rest of the structure has. Amazing find!  @isonno Good eye to spot the unused entries at the bottom! Those aren't the missing entries, though. The mathematical table got encoded into Boolean equations, which are stored in the PLA. There is not easy mapping between the table and what you see on the die. In fact, the fixed Pentiums have *fewer* entries in the PLA due to the way the equations work out.  @kenshirriff "Rare" is a bizarre phrasing (clearly one Intel liked to use). It's trivial to reproduce this a few million times a second and indeed, it was original found on a real-world application. |
This closeup of the PLA shows the individual transistors that define the contents of the lookup table. Five entries were missing from the table, causing very rare errors in division.