PDP-8 (minicomputer) | 1965

PDP-8

 The PDP-8 is a 12-bit minicomputer series produced by Digital Equipment Corporation (DEC). It was the first commercially successful minicomputer, with over 50,000 units sold during its lifetime. The basic design of the PDP-8 followed the innovative LINC computer, but it used a simpler instruction set, which was an extension of the PDP-5's instruction set. Similar computers from DEC included the PDP-12, which modernized the PDP-8 and LINC concepts, and the industrial control system-oriented PDP-14.

The earliest model of the PDP-8, called "Straight-8," was first introduced on March 22, 1965, with a price of $18,500, which would be approximately $178,900 in today's money. This model was built using diode-transistor logic (DTL) chip cards and was about the size of a small home refrigerator. The PDP-8 was the first computer to be sold for less than $20,000, and it became the best-selling computer of its time. In 1966, the PDP-8/S model was released. It came in both desktop and rack-mounted versions, and used a 1-bit serial arithmetic logic unit (ALU) that made it smaller and cheaper, though slower than the original PDP-8. The base PDP-8/S was sold for under $10,000, marking the first time a computer was priced below $10,000.

Subsequent models such as the PDP-8/I, PDP-8/L, PDP-8/E, PDP-8/F, PDP-8/M, and PDP-8/A were faster and implemented a fully parallel architecture, but they were designed using much cheaper transistor-transistor logic (TTL). These later PDP-8 models were widely used and well known. The PDP-8/E was especially popular, receiving positive feedback for supporting a variety of I/O devices. The last commercially available PDP-8 model, the "CMOS-8," was released in 1979, but it failed due to its lack of price competitiveness. It was based on Intersil’s 6100 microprocessor and featured low power consumption thanks to CMOS technology, though it was used in some military embedded systems.

The PDP-8 is known for its low cost, simplicity, expandability, and cost-effective design. Its historical significance lies in the fact that it made computers accessible to a much wider audience through its affordable price and mass production, leading to new applications for computers. The simple instruction set of the PDP-8 later influenced the development of RISC (Reduced Instruction Set Computing) architectures, which became important in computer design.

The PDP-8 used a 12-bit word size for arithmetic operations. Unsigned integers could range from 0 to 4095, while signed integers could range from -2048 to +2047. However, software allowed for various levels of precision. For example, a floating-point interpreter enabled the use of 36-bit floating-point representations. Compared to other large computers of its era, the PDP-8 offered relatively inexpensive interfacing with peripheral devices.

The memory address space of the PDP-8 was 12 bits, allowing it to store 4,096 12-bit words, which is equivalent to approximately 6 KiB by modern standards. Additional memory expansion devices allowed for more memory by swapping memory banks. The memory used magnetic core technology, with a cycle time of 1.5 microseconds (0.667 MHz), and memory reference instructions typically executed at 0.333 MIPS. The PDP-8/E model's pocket reference card from 1974 listed the basic instruction time as 1.2 microseconds, with memory reference instructions taking 2.6 microseconds.

The PDP-8 was designed with communications and text processing in mind, making it well-suited for the 6-bit character codes widely used at the time. Its 12-bit words could efficiently store two 6-bit characters. One of the early uses of the PDP-8 was for typesetting, utilizing the 6-bit teletypesetting code (TTS).

The PDP-8’s instruction set used 3-bit operation codes (opcodes) and offered only eight instructions. Programmers could use many additional instruction mnemonics, which were translated into OPR (operational) or IOT (input/output) instructions by the assembler. The PDP-8 had only three visible registers: a 12-bit accumulator (AC), a program counter (PC), and a link register (L). Additional registers, such as the memory buffer register and memory address register, were hidden from the programmer and used for various purposes at different times. For example, the memory buffer register could be used as an operand in arithmetic operations, part of the instruction register, or for writing data back to memory.

I/O processing in the PDP-8 was handled through a single interrupt mechanism, and devices communicated via I/O buses and I/O instructions. Both serial and parallel I/O devices were supported, and DMA (Direct Memory Access) channels were used to connect high-speed devices. Common peripheral devices included printers, teletypes, paper tape readers, and punched card readers.

Mathematical operations were handled by software, although an optional Extended Arithmetic Element (EAE) was available for faster arithmetic. The EAE added the MQ register to handle multiplication and division instructions and was optional in the original PDP-8, 8/I, and 8/E models but became a standard feature in the Intersil 6100 microprocessor version.

The PDP-8 was optimized for simplicity, with unnecessary features removed and logic shared where possible to reduce costs. Instructions provided automatic increment, automatic clear, and indirect addressing modes to improve software speed, reduce memory usage, and utilize cheaper memory instead of expensive registers.

Early PDP-8 models were relatively cheap compared to other commercial computers but used expensive production methods based on prototypes, with the use of thousands of very small standardized logic modules and complex wire-wrapped backplane technology, with gold connections.

The later PDP-8/S models divided logic voltages into two categories and used a serial bit-path data structure for arithmetic operations. This model's CPU contained around 519 logic gates, which helped achieve a smaller case size and lower cost. The PDP-8/E, a more capable and faster model, was redesigned to offer more value and used an OMNIBUS instead of wire-wrapped backplane technology, providing greater expandability.

It is estimated that over 300,000 units of the PDP-8 series were sold, with several different models released.

Compared to modern computer architectures, the PDP-8's instruction set is much simpler, making it easy to emulate. Enthusiasts have recreated entire PDP-8 systems using a single FPGA device.

There are several PDP-8 software simulators available online, and some PDP-8 models have been re-implemented as open-source hardware. The best simulators can accurately run DEC's operating systems and diagnostic software. These simulators often support the full range of peripherals, including later models like the PDP-8/S, and use far fewer resources than modern personal computers.

One of the commercial versions of the PDP-8/S virtual machine ran on a Kaypro 386 (80386-based computer) and was written by David Beecher in C and assembly. This virtual machine was developed to replace a failed PDP-8/S computer controlling fuel-handling machinery in reactor #85 at the Ft. St. Vrain nuclear power plant in Colorado. The system, reviewed by Rockwell International, operated flawlessly for 2.5 years while removing fuel from the reactor and dismantling the plant. The virtual PDP-8/S also simulated the paper tape loader and front panel functions.