INTRODUCTION

GENERAL DESCRIPTION

The DEC Programmed Data Processor Model Three (PDP-3) is a high performance, large scale digital computer featuring reliability in operation together with economy in initial cost, maintenance and use. This combination is achieved by the use of very fast, reliable, solid state circuits coupled with system design restraint. The simplicity of the system design excludes many marginal or superfluous features and thus their attendant cost and maintenance problems.

The average internal instruction execution rate is about 100,000 operations per second with a peak rate of 200,000 operations per second. This speed, together with its economy and reliability, recommends PDP-3 as an excellent instrument for complex real time control applications and as the center of a modern computing facility.

PDP-3 is a stored program, general purpose digital computer. It is a single address, single instruction machine operating in parallel on 36 bit numbers. It features multiple step indirect addressing and indexing of addresses. The main memory makes 511 registers available as index registers.

The main storage is coincident current magnetic core modules of 4096 words each. The computer has a built-in facility to address 8 modules and can be expanded to drive 64 modules. The memory has a cycle time of five microseconds.

SYSTEM BLOCK DIAGRAM

The flow of information between the various registers of PDP-3 is shown in the System Block Diagram (Fig. 1). There are four registers of 36 bit length. Their functions are described below.

Memory Buffer

The Memory Buffer is the central switching register. The word coming from or going to memory is retained in this register. In arithmetic operations it holds the addend, subtrahend, multiplicand, or divisor. The left 6 bits of this register communicate with the Instruction Register. The address portion of the Memory Buffer Register communicates with the Index Adder, the Memory Address Register, and the Program Counter. In certain instructions, the address portion of the control word does not refer to memory but specifies variations of an instruction, thus, the address portion of the Memory Buffer is connected to the Control Element.

Accumulator

The Accumulator is the main register of the Arithmetic Element. Sums and differences are formed in the Accumulator. At the completion of multiplication it holds the high order digits of the product. In division it initially contains the high order digits of the dividend and is left with the remainder.

The logical functions AND, inclusive OR, and exclusive OR, are formed in the Accumulator.

Carry Storage Register

The Carry Storage Register facilitates high-speed multiply and is properly part of the Accumulator.

In-Out Register

The In-Out Register is the main path of communication with external equipment. It is also part of the Arithmetic Element. In multiplication it ends with the low order digits of the product. In division it starts with the low order parts of the dividend and ends with the quotient.

The In-Out Register has a full set of shifting properties, (arithmetic and logical).

There are three registers of 15 bit length which deal exclusively with addresses. The design allows for expansion to 18 bits. These registers are:

Memory Addressing

The Memory Address Register holds the number of the memory location that is currently being interrogated. It receives this number from the Program Counter, the Index Adder or the Memory Buffer.

Program Counter

The Program Counter holds the memory location of the next instruction to be executed.

Index Adder

The Index Adder is a 15 bit ring accumulator. The sum of an instruction base address, Y, and the contents of an index register, C(x), are formed in this register. This register holds the previous content of the Program Counter in the "jump and save Program Counter," jps, instruction. The Index Adder also serves as the step counter in shift, multiply, and divide.

The Control Element contains two six bit registers and several miscellaneous flip-flops. The latter deal with indexing, indirect addressing, memory control, etc. The six bit registers are:

Instruction Register

The Instruction Register receives the first six bits of the Memory Buffer Register during the cycle which obtains the instruction from memory (cycle zero). This information is the primary input to the Control Element.

Program Flags

The six Program Flags act as convenient program switches. They are used to indicate separate states of a program. The program can set, clear, or sense the individual flip-flops. The program can also sense or make the state "All Flags ZERO." They can also be used to synchronize various input devices which occur at random times (see Input-Output, Typewriter Input).

Three toggle switch registers are connected to the Central Processor (see Manual Controls).

Test Address

The fifteen Test Address Switches are used to indicate start points and to select memory registers for manual examination or change.

Test Word

The thirty-six Test Word Switches indicate a new number for manual deposit into memory. They may also be used for insertion of constants while a program is operating by means of the operate instruction.

Sense Switches

The six Sense Switches allow the operator to manually select program options or cause a jump to another program in memory. The program can sense individual switches or the state "All Switches ZERO."

ELECTRICAL DESCRIPTION

The PDP-3 circuitry is the static type using saturating transistor flip-flops and, for the most part, transistor switch elements. The primary active elements are Micro-Alloy and Micro-Alloy-Diffused transistors. The flip-flops have built-in delay so that a logic net may be sampled and changed simultaneously.

Machine timing is performed by a delay line chain. Auxiliary delay line chains time the step counter instructions (multiply, divide, etc.). The machine is thus internally synchronous with step counter instructions being asynchronous. The machine is asynchronous for in-out operations, that is, the completion of an in-out operation initiates the following instruction.

MECHANICAL DESCRIPTION

The PDP-3 consists of two mechanical assemblies, the Console and the Equipment Frame. Fig. 3 is a photograph of PDP-1 which is an 18 bit version of PDP-3.

Console

The Console is a desk approximately seven feet long. It contains the controls and indicators necessary for operation and maintenance of the machine. A cable connects the Console to the Equipment Frame.

Equipment Frame

The Equipment Frame is approximately six feet high and two feet deep. The length is a function of the amount of optional features included. The Central Processor requires a length of five and one half feet. The power cabinet is twenty-two inches long. A memory cabinet is thirty-two inches long and will hold three memory modules (12,288 words per cabinet). Memory cabinets may be added at any time.

Magnetic tape units require twenty-two inches per transport. A tape unit cabinet may be connected as an extension of the Equipment Frame or may be a free-standing frame.

ENVIRONMENTAL REQUIREMENTS

The PDP-3 requires no special room preparation. The computer will operate properly over the normal range of room temperature.

The Central Processor and memory together require thirty amperes of 110 volts single phase 60 cycle ac. Each inactive tape transport requires two amperes and the one active transport requires 10 amperes.