System Description PDP-6

This document describes the PDP-6 Programmed Data Processor, a flexible, general-purpose digital computing system designed for scientific data processing by Digital Equipment Corporation. The PDP-6 features modular construction, allowing for expandable configurations with various processors, memory modules (including different speeds and 36-bit word lengths), and up to 128 input/output devices. It includes built-in hardware for multiprogramming and time-sharing, such as memory protection and program relocation registers, and supports asynchronous operations.

The system's architecture is based on two main busses: a processor-memory bus for direct memory access (262,144 words) and memory cycle overlap, and a processor-input/output bus with a seven-channel priority interrupt system. Key components include the Type 166 Arithmetic Processor, a 36-bit processor with 16 accumulators, 15 index registers, built-in fast floating-point arithmetic (14 µsec average multiply), and a comprehensive instruction set of 363 operations, including byte manipulation. The Type 167 Drum Processor facilitates high-speed data transfers between core and mass memory. Memory modules offer various capacities (e.g., 8,192 to 16,384 words) and speeds (2 µsec cycle time for core, 0.4 µsec for fast memory), with memory overlap significantly improving system efficiency by allowing processors to operate without waiting for memory write cycles.

The programming environment is extensive, comprising a Monitor, Symbolic Assembler and Macro Processor (MACRO-6), FORTRAN II Compiler, Debugging Aids, and a program library. This system supports on-line program preparation and debugging, and utilizes memory protection to secure user storage areas. Various input/output equipment is available, including Micro Tape, Magnetic Tape, Card Punch/Reader, Teleprinters, Displays, and Analog-to-Digital Converters. Instruction execution times vary depending on the memory subsystem (fast vs. core memory) and addressing modes, with fast memory significantly reducing times.