PDP-11

PDP-11/40

J11 microprocessor

PDP-11 was a computer manufactured by Digital Equipment Corp. in the 1970s and 1980s, belonging to the PDP series. It was the first minicomputer to interconnect all system elements — processor, memory, and peripherals — to a single, bidirectional, asynchronous communication bus. This device, called UNIBUS, allowed devices to send, receive, or exchange data without having to pass through memory. The PDP-11 was one of the best-selling minicomputer series of its time and was one of the first computers to run the Unix system, developed at Bell Laboratories.

It evolved with technology, going from having the CPU made with TTL MSI circuits to using microprocessors, like the LSI-11. The latest versions, PDP-11/73 onwards, incorporated J-11, with duplicate registers, three stacks (stack) (User, Kernel and Supervisor), virtual memory (22 bits), separate cache and spaces for instructions and data. It was a very ambitious chip but it never lived up to expectations, due to arguments and confrontations between DEC and Harris, the chip manufacturer.

During the cold war, the PDP-11 architecture was cloned without permission from the manufacturer so that programs running on machines of this series could be run unchanged on clones made in Eastern Europe.

Technical qualities

• Eight 16-bit records:

• The first six (%0 to %5) of general purpose.
• The seventh (%6) is Stack Pointer (punter of the stack).
• The eighth (%7) is the Program Counter (programme counter).

• It has a highly orthogonal set of instructions, with eight direction modes.
• It's a two-way processor.
• It has four flags: ZNVC

• Z=1 means the last value that has come out of the ALU is 0.
• N=1 means the last value that has come out of the ALU is negative.
• V=1 means that in the last operation of the ALU there has been a overflow.
• C is equivalent to the output of the last operation of the ALU.