Programming instructions

Keyboard and Control, Type 65. The Real-Time Option, Type 25 gives the

system the additional capability to operate efficiently over a wide range

of information handling rates (from seconds per event to 125,000 words

per second) and with a large variety of input-output devices (see Figure 2).

The Real-Time Option consists of a Device Selector, an Information Col-

lector, an Information Distributor, an Input-Output Skip connection, a

Program Interrupt facility, a Data Interrupt facility, and a Clock/Timer.

See Chapter 3 for details of functions.

ARITHMETIC

AND CONTROL

PROCESSOR*

ELEMENT

--w--------w-

c-L-----------e---II

INTERFACE I I

-m-s-..-

INPUT-OUTPUT

EDUIPMENT

Ill I

I CARD READER I

III I

*included in a

TYPE 40.4

TYPE 62

RELAY BUFFER,

Equipment

Figure 2 - PDP-4 System with Real-Time Option

THE DEVICE SELECTOR consists of decoding elements to select and

establish the state of an external device when the program issues an

input-output transfer instruction. The direction of information transfer

(in or out of the Internal Processor) is controlled by signals produced

by the Device Selector. Up to 64 input-output devices can be selected

and these, in turn, may cause the selection of many more. The stand-

ard Device Selector has provisions for twenty selector elements.

THE INFORMATION COLLECTOR receives information from input

devices (selected by the Device Selector) and transfers the informa-

tion to the Internal Processor. Up to 18 bits of information can be

collected simultaneously; 8

18 bits of information may be collected,

broken into variable-sized words.