KE11-A Extended Arithmetic Element

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The KE11-A Extended Arithmetic Element is an autonomous UNIBUS peripheral which can perform a variety of arithmetic or logical operations:

  • multiplication
  • division
  • multi-bit shifts

(The last is used for implementation of floating point in software.)

It is implemented as several quad and dual boards, plugged into a custom four-slot system unit (details below).

The KE11-A was originally produced for use with the PDP-11/20, whose KA11 CPU did not have multiply, divide, or multi-bit shift operations. As a UNIBUS peripheral, not integrated into the CPU, it was also usable with other models, e.g. the PDP-11/05 and PDP-11/04, which likewise did not implement multiplication in hardware, etc.

The KE11-A can be operated with an optional KM11-A Maintenance Set in order to diagnose faults.

It was replaced by the KE11-B Extended Arithmetic Element, a program compatible re-implementation on a single hex card.


The device has a number of registers, which can be configured to sequential locations in the I/O page; most are words, but there are two byte registers. It is normally configured to addresses 777300-777316; a second KE11-A, if present, is configured to addresses 777320-777336. The operation to be performed is selected by the register which data is written to. Registers which hold results are readable, but many of the operation selection registers are write-only. In the register table (below), those which are read/write are shown in normal font; those which are read-only are in italics; those which are write-only in bold (these return '0' when read).

Register Abbreviation Address Size
Divide DIV 777300 W
Accumulator AC 777302 W
Multiplier-Quotient MQ 777304 W
Multiply MUL 777306 W
Step Counter$ SC 777310 B
Status Register% SR 777311 B
Normalize NOR 777312 W
Logical Shift LSH 777314 W
Arithmetic Shift ASH 777316 W

$ The SC is read-only as an individual byte, but can written as a word together with the SR.
% The SR is read-only as an individual byte, but can written as a word together with the SC.

Division takes as one operand a 32-bit value held in the AC (high order) and MQ (low order); a remainder is left in the AC. Multiplication produces a 32-bit product in the AC (high order) and MQ (low order). Both arithmetic and logical shifts operate on a 32-bit value held in the AC (high order) and MQ (low order).

Status Register (SR)

Overflow XOR Negative Negative AC = -1 AC = 0 MQ = 0 AC = MQ = 0 AC = MQ15 Carry
07 06 05 04 03 02 01 00


The KE11-A consists of 3 quad boards:

  • M7210 Data Control
  • M7211 Register Control
  • M827 Clock and States

and a pair of M234 Register dual-height boards, plugged into the quad custom backplane.

Board locations

These are the board locations for the KE11-A (as seen from the board side of the backplane):

Slot A B C D E F
1 UNIBUS In M7210 Data Control
2 Unused KM11-A M234 Register
(low byte)
M234 Register
(high byte)
3 G772
Reserved M7211 Register Control
4 UNIBUS Out M827 Clock & States