MM8-E Memory System

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The MM8-E Memory System was the standard main memory system on DEC PDP-8/E; it was core memory. The MM8-E was the first PDP-8 family core memory system to use the shared sense/inhibit line technical innovation. The initial MM8-E version was 4K words; a later 4KW variant was the MM8-EH. That was augmented by an 8KW-sized variant, the MM8-EJ.


All versions consisted of sets of three quad boards:

  • X-Y Driver and Current Source G227/G234 (4K); G233 (8K)
  • Sense/Inhibit G104/G115 (4K); G111 (8K)
  • Memory Stack Assembly H211/H220 (4K); H212 (8K)

The Engineering Drawings indicate that the two X-Y Driver and Sense/Inhibit variants can be mixed in any combination in an 8KW set; for the 4KW set, it calls for the G115-G211 combination.

They were connected with four H851 over the back Edge Connectors, in two sets of two; the Stack Assembly, in the center, has four groups (often called 'rows' in DEC documentation) of contact fingers on the back edge of the boards, and the other two each have two sets of contact fingers.


Setting the field of the MM8-E is done with three jumpers on the G104/G111/G115 Sense/Inhibit board: EMA0, EMA1 and EMA2 (which set the bits of the starting field in the extended address of the MM8-E). Since a field is 4KW long, an 8KW set will use two fields: the one selected as the starting field, and the one above that one.

0 In In In
1 In In Out
2 In Out In
3 In Out Out
4 Out In In
5 Out In Out
6 Out Out In
7 Out Out Out

A jumper is inserted for a '0', and removed for a '1'.

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