PDP 8

Introduction

Digital Equipment Corporation created the PDP-8, a 12-bit minicomputer (DEC). With over 50,000 units sold over the model’s lifespan, it was the first commercially successful minicomputer. Although it features a smaller instruction set that is an extended version of the PDP-5 instruction set, its fundamental design is similar to the ground-breaking LINC. The PDP-14 industrial controller system and the PDP-12, a modified version of the PDP-8 and LINC ideas, are similar devices made by DEC.

The PDP-8 offers value by combining low cost, ease of use, expandability, and thorough engineering. The PDP-8’s low cost and huge production made a computer accessible to many new customers for many new purposes, which has the greatest historical relevance. It continues to be important as a classic illustration of value-engineered computer design.

Memory

The greatest number of words that the original PDP-8 can address indirectly through a word pointer is 4,096, which corresponds to the 4,096 distinct values that a 12-bit word may have. 4,096 12-bit words, or 6 kB, correspond to 6,144 bytes in current terms. It became beneficial to increase this limit as programmes got more intricate and memory became less expensive.

New hardware that was not part of the original design appended high-order bits to the program’s produced effective addresses in order to retain compatibility with older applications. The accessible memory is increased by a factor of 8 by the Memory Extension Controller, making a total of 32,768 words available. This increase was deemed enough because a full 32K of memory would cost the same as the CPU at the time that core memory cost roughly 50 cents per word.

Virtualisation

The Memory Extension Controller was improved to support machine virtualization on the PDP-8/E and subsequent generations. Under the supervision of a virtual machine manager, a programme created to utilise all of a PDP-8’s resources can live with other similar programmes on the same PDP-8. All I/O instructions, including those that used the Memory Extension Controller, can be made by the manager to set off a trap (an interrupt handled by the manager). The manager can then map memory references, data or instruction fields, and I/O to different devices in this manner. The manager provides each original application with full access to a “virtual machine.”

Instructions set

The operation code is represented by the three high-order bits (labelled bits 0 through 3) of the 12-bit instruction word. Bits 5 through 11 give a 7-bit address for the six memory-related operations. If bit 4 is set, it indicates that the address should be completed using the 5 high-order bits of the programme counter (PC) register and that the addressed location should be inside the same 128 words as the instruction. If bit 4 is set to zero, the first 128 words of memory are utilised to address the place if bit 4 is clear. Bit 3 designates indirection; if set, the address collected so far refers to a 12-bit value in memory that provides the instruction’s real effective address.