Programming instructions

CHAPTER 3

INPUT-OUTPUT EQUIPMENT

FUNCTIONS AND PROGRAMMING

PDP-4 is capable of operating with the ten input-output devices described

in Chapter 1 and with a variety of others. The computer can operate with

most of the devices simultaneously. The Interface, consisting of the Real-

Time Connection or the Real-Time Option, issues commands to the devices,

monitors their state of availability, transfers information to them, and

receives information from them. Since the Internal Processor can store

or read out data much faster than the devices can operate, the Interface

and the individual devices provide buffering to minimize the amount of

program time consumed in transfers.

The Real-Time Connection, furnished as standard equipment, provides

communication between the Internal Processor and the Perforated-Tape

Reader, the Perforated-Tape Punch, and the Keyboard-Printer. 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. The Real-Time Option consists of the Device Selector, the

Information Collector, the Information Distributor, the Input-Output Skip

Facility, the Program Interrupt Control, the Data Interrupt Control, and

the Clock/Timer (see Figure 8).

The coupling of input-output equipment to PDP-4 is similar for all devices.

The electrical characteristics of the coupling are discussed in Chapter 4.

The logical functions and programming instructions are given below.

Input-Output Commands

DEVICE SELECTOR (DS)

The input-output transfer (iot) augmented instruction causes the Interface

to produce pulses which select IO devices and transfer information. Upon

receipt of an instruction, the Device Selector in the Interface performs one

of the following functions:

(a) Starts a device (e.g.asks for a line of perforated tape to be read and

assembled into a word, a card to be moved to a reading or punching

station, etc.)