Preliminary Specifications: Programmed Data Processor Model Three Part 5

_Test Switch_ - When the Test Switch is on, the computer will perform the instruction indicated in the Test Address location. It will repeat this instruction either at the normal speed rate or at a single cycle rate if the Single Cycle Switch is up. This switch is primarily useful for maintenance purposes.

_Sense Switches_ - There are six switches on the Console which are present for manual intervention.

STORAGE

The internal Memory System for PDP-3 consists of modules of 4096 words of coincident current magnetic core storage. Each word has 36 bits. The memory modules operate with a read-rewrite cycle time of 5 microseconds.

The driving currents of the memory are automatically adjusted to compensate for normal room temperature variations.

Each core memory module consists of the memory stack, the required X and Y switches, the X and Y current sources and sense amplifiers for that stack.

The Memory Address Register, the Memory Buffer Register, and the Memory Timing Controls are considered to be part of the Central Processor. The standard PDP-3 Memory Address Register configuration is built to allow up to 8 modules of core memory (32,768 words). There is a s.p.a.ce in the addressing section of the machine to allow expansion of the addressing by a factor of eight for a total addressing capacity of 262,144 memory registers.

The Core Memory may be supplemented by Magnetic Tape Storage. This is described under Input-Output.

STANDARD INPUT-OUTPUT

The PDP-3 is designed to accommodate a variety of input-output equipment. Standard input-output units include a Paper Tape Reader, Paper Tape Punch and an Electric Typewriter.

A single instruction, In-Out Transfer (see Central Processor), performs all in-out operations through the 36 bit In-Out Register. The address portion of this instruction specifies the in-out function. One bit of the instruction selects an in-out halt as required.

PAPER TAPE READER

The Paper Tape Reader of the PDP-3 is a photoelectric device capable of reading 300 lines per second. Six lines form the standard 36 bit word when reading binary punched eight hole tape. Five, six and seven hole tape may also be read.

The reader will operate in one of two basic modes or in a third special mode.

Alphanumeric Mode rpa iot 1

In this mode, one line of tape is read for each In-Out Transfer. All eight holes of the line are read. The information is left in the right eight bits of the In-Out Register, the remainder of the register being left clear. The standard PDP alphanumeric paper tape code includes an odd parity bit which may be checked by the program. Tape of non-standard width would be read in this mode.

Binary Mode rpb iot 2

For each In-Out Transfer instruction, six lines of paper tape are read and a.s.sembled in the In-Out Register to form a full computer word. For a line to be recognized in this mode, the eighth hole must be punched; i.e., lines with no eighth hole will be skipped over. The seventh hole is ignored. The pattern of holes in the binary tape is arranged so as to be easily interpreted visually in terms of machine instruction.

Read-In Mode

This is a special mode activated by the "Read-In" Switch on the Console.

It provides a means of entering programs which neither rely on read-in programs in memory nor require a plug board. Pushing the "Read-In"

Switch starts the reader in the binary mode. The first group of six lines and alternate succeeding groups of six lines are interpreted as "Read-In" mode instructions. Even-numbered groups of 6 lines are data.

The "Read-In" mode instructions must be either "deposit in-out" (dio Y) or "jump" (jmp Y). If the instruction is dio Y, the next group of six binary lines will be stored in memory location Y and the reader continues moving. If the instruction is jmp Y, the "Read-In" mode is terminated and the computer will commence operation at the address of the jump instruction.

PAPER TAPE PUNCH

The standard PDP-3 Paper Tape Punch has a nominal speed of 20 lines per second. It can operate in either the alphanumeric mode or the binary mode.

Alphanumeric Mode ppa iot 5

For each In-Out Transfer instruction one line of tape is punched. In-Out Register bit 35 conditions hole #1. Bit 34 conditions hole #2, etc. Bit 28 conditions hole #8.

Binary Mode ppb iot 6

For each In-Out Transfer instruction one line of tape is punched. In-Out Register bit five conditions hole #1. Bit four conditions hole #2, etc.

Bit zero conditions hole #6. Hole #7 is left blank. The #8 hole is always punched in this mode.

TYPEWRITER

The Typewriter will operate in the input mode or the output mode.

Output Mode tyo iot 3

For each In-Out Transfer instruction one character is typed. The character is specified by the right six bits of the In-Out Register.

Input Mode tyi iot 4

This operation is completely asynchronous and is therefore handled differently than any of the preceding in-out operations.

When a Typewriter key is struck, Program Flag Number One is set. At the same time the code for the struck key is presented to gates connected to the right six bits of the In-Out Register. This information will remain at the gate for a relatively long time by virtue of the slow mechanical action. A program designed to accept typed-in data would periodically check the status of Program Flag One. If at any time Program Flag One is found to be set, an In-Out Transfer instruction with address four must be executed for information to be transferred. This In-Out Transfer normally should not use the optional in-out halt. The information contained in the Typewriter's coder is then read into the right six bits of the In-Out Register.

OPTIONAL INPUT-OUTPUT

The PDP-3 is designed to accommodate a variety of input-output equipment. Of particular interest is the ease with which new, and perhaps unusual, external equipment can be added to the machine.

Optional in-out devices include Cathode Ray Tube Display, Magnetic Tape, Real Time Clock, Line Printer and a.n.a.log to Digital Converters. The method of operation of PDP-3 with these optional devices is similar to the standard input-output equipment.

SEQUENCE BREAK SYSTEM

An optional in-out control is available for PDP-3. This control, termed the Sequence Break System, allows concurrent operation of several in-out devices and the main sequence. The system has, nominally, 16 automatic interrupt channels arranged in a priority chain.

A break to a particular sequence may be initiated by the completion of an in-out device, the program, or an external signal. If this sequence has priority, the C(AC), C(IO), C(PC), and C(IA) are stored in three fixed memory locations unique to that sequence. Since the C(PC) and C(IA) are eighteen bits each, these two registers are stored in one memory location. The next instruction is taken from a fourth location.

This instruction is usually a jump to a suitable routine. The program is now operating in the new sequence. This new sequence may be broken by a higher priority sequence. A typical program loop for handling an in-out sequence would contain 3 to 5 instructions, including the appropriate iot. These are followed by load AD and load IO from the fixed locations and a special indirect jump through the location of the previous C(PC).

This special jump also loads the IA. This last instruction terminates the sequence.

HIGH SPEED IN-OUT CHANNEL

The device connected to an in-out channel communicates directly with memory through the Memory Buffer Register. At the completion of each machine instruction, a check is made to see if the in-out channel has a word for, or needs a word from, the memory. When necessary, a memory cycle is taken to serve the channel. The operation is initiated by an in-out command. The in-out transfer command indicates the nature of the transfer. The left half of the In-Out Register must contain the starting address of the transfer, and the right half must contain the number of words to be transferred. If the Sequence Break System is connected, the completion of the transfer will signal the proper sequence. If no Sequence Break System is connected, the completion of the in-out channel transfer sets a program flag.