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Tracing the History of the Computer - DEC PDP-11




The PDP-11, sold by Digital Equipment Corporation in the 1970s and 1980s, was one the most successful computers of all times, beginning life as a minicomputer and ending up as a micro or supermicro/supermini.

The PDP-11, which was a 16-bit minicomputer, was the successor to DEC's PDP-8 computer in the PDP series of computers. It had several uniquely innovative features, and was easier to program than its predecessors. While well-liked by programmers, it was eventually superseded by personal computers, including the IBM PC and Apple II.

Unique Features of the PDP-11 Series

Instruction Set

Programmers liked the PDP-11 design because it had a highly-orthogonal instruction set which allowed a programmer to separately memorize all of the operations and the methods of accessing operands. They could then predict that any access method (or "addressing mode") would work with any operation; they did not have to learn a list of exceptions or special cases in which an operation had a special or restricted set of addressing modes.

In some logical sense, the set of addressing modes provided one "basis", and the set of operations provided another. Each two-operand instruction was separated into two six-bit operand identifiers (each consisting of a three-bit register number, and a three-bit addressing mode) and a four-bit op-code; single-operand instructions had one six-bit operand identifier, and a ten-bit op-code. All op-codes operated with any operand identifier address mode (or combination of them, for the two-operand instructions). Of the 8 registers (numbered 0 through 7), 7 were general-purpose and could be used for most purposes, although register 6 was specially recognized by the hardware as the stack pointer for some instructions; register 7 was the program counter. This latter innovation, together with some of the addressing modes, provided constants, absolute addresses, and relative (position independent) addressing.


32-bit words were typically stored in a middle-endian format. Due to the popularity of the PDP-11, this format is still sometimes referred to as pdp-endian.

No Dedicated Bus

In the most radical departure from other, earlier computers, the PDP-11 had no dedicated bus for input/output; it had only a memory bus. All input and output devices were mapped to addresses in memory, so in addition, no special I/O instructions were needed. The interrupt system was intentionally designed to be as simple as possible, while assuring that no event in an interrupt sequence could be missed. A device would request an interrupt by asserting a common input into one of four priority lines; the processor would respond over an interrupt daisy chain grant line, one for each priority level. (A daisy chain is a sequence of logic gates arranged in series to order events. Generally the first logic gate has first access to the grant. The daisy chain order established the order of the devices at that priority level.)

In the case of the PDP-11 design, this meant that the interrupt grant order was determined by how close the physical hardware was to the CPU on the bus. When the CPU responded, the device would place its vector address on the bus; this was the address of a 4-byte block of memory. The CPU would then load the status register and program counter from the vector table; the new contents of the status register would generally temporarily disable interrupts. The address in the program counter would be the starting address of the code to run for the interrupt. The interrupt code would then service the device, and in the process, write to the interrupting device to re-enable the interrupt signal. Finally, a special RTI (return from interrupt) instruction would return the CPU to where it was before the interrupt (which might have been in a lower-priority interrupt). Note that this process prevents loss of interrupts; at every stage, if the interrupt is not serviced, it remains in place, to be sensed on the next cycle. If a sequence is erroneously started, the CPU would time out, generating a special spurious interrupt; the spurious interrupt would warn users of bad hardware.

Designed for Mass Production

Finally, the PDP-11 was designed to be produced in a factory by semiskilled labor. All of the dimensions of its pieces were relatively noncritical. All parts of the computer chassis were constructed from injection-molded plastic, or bent steel rod (lighter than sheet metal). It used a push-bonded backplane. That is, the printed circuit board plugged into a backplane connector. The backplane connector had terminals that could be connected by pushing wires into them. The terminal would cut the insulation around the wire and bite into the wire to form a gas-tight (i.e. corrosion-proof, therefore reliable) connection. The connector blocks were very similar to telephone connection blocks. The case was injection-molded plastic that snapped over the steel-rod chassis.

The LSI-11

The LSI-11 was the first PDP-11 model produced using large-scale integration; the entire CPU was contained on 4 LSI chips. It used a bus which was a close variant of the Unibus called the Q-Bus; it differed from the Unibus primarily in that addresses and data were multiplexed onto a shared set of wires, as opposed to having separate sets of wires, as in the Unibus. It also differed slightly in how it addressed I/O devices and it eventually allowed a 22-bit physical address (whereas the Unibus only allowed an 18-bit physical address) and block-mode operations (which the Unibus did not support).

The CPU's microcode includes a debugger that directly communicated to a standard RS-232 terminal. This was innovative because the microcode is the part of the irreducible guts of the computer, a critical part of the control unit. If it doesn't work, there is no computer. The debugger provided a way to examine the computer's registers, memory and input and output devices. Thus, if the CPU worked at all, it was possible to examine and correct the computer's internal state. The built-in debugger avoided the expense and inconvenience of a front panel with an array of switches and lights, which was then the typical way to enter digital data into a near-dead computer.

The microcode also included a generic bootstrap, to which all DEC disk drives were compatible.

These two innovations meant that most of the time, the computer just worked. If it did not boot from its big disk, it would boot from its floppy. If the hardware worked at all, it talked to you through a terminal in a familiar way.

The Decline of the PDP-11

Although the basic architecture was extremely good, and the PDP-11 line was continually updated to use newer technologies, it finally died off for one principal reason: the 16-bit virtual address space was simply too small. When large VLSI memory chips became very cheap, the PDP-11 was just not capable of using large amounts of memory easily.

DEC's own successor to the PDP-11, the VAX (for "Virtual Address Extension (to the PDP-11)") addressed all of these issues, but was aimed at the high-end market.

As engineers migrated to architectures which supported a larger address space, 32-bit computing began to be supported on microprocessor chips such as the Motorola 68000 and Intel 386 families; eventually the economics of large-scale production of those chips made them so cheap there was no cost advantage for the PDP-11.


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