[Simh] PDP11 microstore

[Paul Koning]

> On Jul 1, 2019, at 8:11 PM, Bob Supnik <bob at supnik.org> wrote:
> 
> The PLA scheme (another invention from the fertile mind of Bill Roberts, architect of the LSI11 and UDA50 proto and founder of Emulex) was basically a microcode and gate conservation scheme. It provided enormous compression for the decode phase of the PDP11 and then got exploited to a fare-thee-well by clever microcoders like Burt Hashizume (F11) and Keith Henry (J11).

Using logic to represent what you might think of as a table of values (a ROM) appears in other places too.  The CDC 6000 series console display, from 1964, has tables of stroke data.  Those are documented as lists of 5 bit data, up to 23 rows for each character, 45 character codes total.

In the later 170 series controller this is indeed done in a ROM, but the 6000 series controller used discrete transistor logic and a ROM wasn't a possible option then (not when a new value was needed every 100 ns).  So instead it uses a rather large tree of gate logic to produce the data.  Large as in roughly 130 "cordwood" plug-in modules, each with 30 or so transistors on them.

	paul