MCA25 KL Cache/Paging Upgrade
The MCA25 KL Cache/Paging Upgrade was an optional improvement for the KL10 PDP-10 CPU; it replaced the earlier MCA20 Cache. It increased the size of both the main memory cache, and the paging cache; it also improved the functionality of the latter. It required CPU backplane modifications (whether additional wires, or a new backplane, is not yet clear). A KL10 with an MCA25 installed was denominated as a 'KL10-PW'.
The main memory cache of the MCA25, used to hold instructions and operands, had 1024 4-word location sets (twice as many as the MCA20), divided into 4 'quarters'. Any location in main memory was associated with one entry in each quarter (i.e. this cache is 'four-way set associative'); when allocating a location set for a new entry, the least recently used of the 4 was chosen.
There were two improvements to the 512-entry paging cache (which cached entries from the page table, kept in main memory):
- it was improved to a two-way associative (so that references to two virtual addresses that mapped to the same paging cache entry did not cause it to be constantly reloaded), i.e. doubling the number of entries overall;
- a 'keep' bit was added to each entry, so that page table entries that would be the same in multiple processes could be retained in the paging cache when it was flushed when switching processes.
The MCA25 consisted of 9 hex cards, of 6 types:
- M852 Cache Directory
- M853 Cache Extension
- M854 Physical Memory Address
- M855 2-Way Associative Pager
- 4 x M856 Cache Data
- M857 MBox Control
All of these are different from the boards in the MCA20.