KT11-B Paging Option

The KT11-B Paging Option is an option for the PDP-11/20, produced by DEC's Computer Special Systems group.

It allows the -11/20 to run time-sharing at the basic machine level; hardware features in the CPU prevent any user from interfering with the overall operation of the system. It does this by providing two modes for the CPU, 'User' and 'Exec', with certain operations (e.g. halting the machine) being dis-allowed in User mode.

It also allows the system to use up to 248 Kbytes of memory (the maximum allowed by the UNIBUS), although only a maximum of 64 KBytes is accessable (i.e. in the CPU's address space) at any one time. By proper programming, the mapping may be set so that user processes cannot interfere with each other's memory.

Architecture

In both Exec and User mode, when the KT11-B is enabled, the address space is divided up into 128 pages of 512 bytes each. Page maps, stored in main memory, map from virtual pages to physical pages (which always start on 512 byte boundaries).

Implementation

The KT11-B is interposed between the KA11 CPU, and the rest of the system (memory and devices). There are two UNIBUSes, joined by the KT11-B; the UNIBUS from the CPU runs into the KT11-B, which processes UNIBUS cycles before passing them through to another UNIBUS, which holds all the memory, devices, etc.

The KA11 CPU of the -11/20 is also slightly modified, and a cable carries signals between the KA11 and KT11-B.

An associative memory in the KT11-B caches the 8 most-recently-used page table entries.

Programming

The KT11-B is controlled by a bank of eleven registers:

Address	Name	Function
777572	KTWN	Window
777574	KTMR	Maintenance
777576	KTCS	Status and Control
777600	KTIC	Instruction Counter
777602	KTSP	Stack Pointer
777604	KTPC	Program Counter
777606	KTPS	Program Status
777610	KTAD	Address
777612	KTDT	Data
777614	KTUM	USER Map Control
777616	KTEM	EXEC Map Control

Note that these occupy the same locations as the standard PDP-11 Memory Management registers; 77757x are SSR0-2, and 7776xx are user PDRs.

Construction

The KT11-B is composed of a large number of single width FLIP CHIPs, most of them simple, generic ones; they are plugged into a custom wire-wrapped backplane.