PDP-11 Memory Management

PDP-11s which provide memory management use a standard PDP-11 Memory Management architecture. It provides several different operating modes for the CPU: Kernel, Supervisor, and User; each mode has its own set of memory management settings.

When memorymanagement is enable, the basic 64 Kbyte address space of the PDP-11 architecture is divided into 8 segments, each of which can be set to any length between 0 bytes and 8 Kbtes, in 0100 (64.) byte increments. Segments can grow either up from their base address, or down; this is to accomodate PDP-11 stacks, which typically grow down (from higher addresses to lower).

An additional enhancement is that instruction and data fetches can be set to go to separate 64 Kbyte address spaces, the so-called Split I+D space capability; this increases the memory available to the operating system, and each user, to 128 Kbytes. (Since immediate operands and absolute addresses are stored contiguous with their instructions, fetches of them are considered to be instruction fetches, for the purposes of deciding which space to use for them.)

Registers

The memory management is entirely controlled by groups of registers in the CPU.

Each segment (48 in total; 8 each Instruction and Data, for the three different modes) is described by a pair of registers, a Page Descriptor Register (PDR) and a Page Address Register (PAR).

The PAR contains the base physical address for the segment (in units of 0100 bytes); the PDR contains the segment's length, along with the direction of growth, the access control field (read/only, read/write, etc), a 'dirty' bit, etc.

Their addresses are:

Address	Name	Function
772200	SID0	Supervisor I-Space PDR #0
...
772216	SID7	Supervisor I-Space PDR #7
772220	SDD0	Supervisor D-Space PDR #0
...
772236	SDD7	Supervisor D-Space PDR #7
772240	SIA0	Supervisor I-Space PAR #0
...
772256	SIA7	Supervisor I-Space PAR #7
772260	SDA0	Supervisor D-Space PAR #0
...
772276	SDA7	Supervisor D-Space PAR #7
772300	KID0	Kernel I-Space PDR #0
...
772316	KID7	Kernel I-Space PDR #7
772320	KDD0	Kernel D-Space PDR #0
...
772336	KDD7	Kernel D-Space PDR #7
772340	KIA0	Kernel I-Space PAR #0
...
772356	KIA7	Kernel I-Space PAR #7
772360	KDA0	Kernel D-Space PAR #0
...
772376	KDA7	Kernel D-Space PAR #7
777600	UID0	User I-Space PDR #0
...
777616	UID7	User I-Space PDR #7
777620	UDD0	User D-Space PDR #0
...
777636	UDD7	User D-Space PDR #7
777640	UIA0	User I-Space PAR #0
...
777656	UIA7	User I-Space PAR #7
777660	UDA0	User D-Space PAR #0
...
777676	UDA7	User D-Space PAR #7

When split I+D is not enabled in any mode, a single 64 Kbyte address space is provided, using only the I-space registers for that mode.

In addition, there are four registers which control the overall operation of the memory management:

Address	Name	Function
777572	SR0	Control and status
777574	SR1	Register increment/descrement record
777576	SR2	Virtual address (usually the instruction address)
777516	SR3	D-space enable/disable (per mode)

Simplified subset

A number of the smaller PDP-11's (notably the PDP-11/40, PDP-11/34 and PDP-11/23) only provide a limited subset of the standard memory management facilities.

In these machines, there is no support for Split I+D, and no Supervisor mode (just Kernel and User).