KK11-A Cache Memory

The KK11-A Cache Memory was an option for the KD11-EA CPU, one which added a high-speed cache to the CPU.

Physically, it was a single hex board, the M8268, interfaced to the CPU through an 'over-the-back' connector. (This was because the PDP-11/34 could be field-upgraded from a KD11-E CPU, which did not support the cache, to a KD11-EA, which did - but the original backplane was retained.)

The KK11-A contained 1024 cache entries of high-speed DRAM, in the form of 28 1024x1 DRAM chips. The cache was a direct-mapped cache (i.e. there was only one possible cache entry in which any given word of main memory could be found), with write-through, and a block size of one word.

Each cache entry was 28 bits wide, containing two data bytes; a tag field for cache entries, 7 bits wide (covering UNIBUS address bits 17-11); 3 parity bits (one for the tag); and two valid bits (to allow the entire cache to be cleared by switching to a previously cleared set of valid bits).

Registers

The cache registers are at the same locations as some of the memory/cache registers in the PDP-11/70, but they are generally incompatible with those in the /70, except as noted below.

Register	Abbreviation	Address
Cache Memory Error Register	CME	777744
Cache Control Register	CCR	777746
Cache Maintenance Register	CMR	777750
Cache Hit Register	CHR	777752

In the register contents (below), read-write bits are shown in normal font, read-only in italics, and write-only in bold.

777744: Cache Memory Error Register (CME)

Unused								High Byte Parity	Low Byte Parity	Tag Parity	Unused				CPU Abort
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

Note: The manual says the 'CPU Abort' bit in the CME is bit 1, but the drawings show it as bit 0. Also, no indication is given as to whether these bits are read-only or not; likely they are.

777746: Cache Control Register (CCR)

Unused		Valid Store In Use	Valid Clear In Progress	Unused	Write Wrong Parity Tag	Unconditional Cache Bypass	Flush Cache	Parity Error Abort	Write Wrong Parity Data	Unused		Force Miss High	Force Miss Low	Unused	Disable Cache Parity Interrupt
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

777750: Cache Maintenance Register (CMR)

Unused															Write Tag All Ones
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

777752: Cache Hit Register (CHR)

Tag Address							Unused			Hit Register
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

Bit 0 refers to the most recent memory cycle. This field is the only one in the entire register set which is compatible with the -11/70.