in January 1970 and shipments began early that year. DEC sold over 170,000 PDP-11s in the 1970s. Initially manufactured of small-scale transistor–transistor logic, a single-board large scale integration version of the processor was developed in 1975. A two- or three-chip processor, the J-11 was developed in 1979. The last models of the PDP-11 line were the single board PDP-11/94 and PDP-11/93 introduced in 1990. Innovative features Instruction set orthogonality The PDP-11 processor architecture has a mostly orthogonal instruction set. For example, instead of instructions such as load and store, the PDP-11 has a move instruction for which either operand (source and destination) can be memory or register. There are no specific input or output instructions; the PDP-11 uses memory-mapped I/O and so the same move instruction is used; orthogonality even enables moving data directly from an input device to an output device. More complex instructions such as add likewise can have memory, register, input, or output as source or destination. Most operands can apply any of eight addressing modes to eight registers. The addressing modes provide register, immediate, absolute, relative, deferred (indirect), and indexed addressing, and can specify autoincrementation and autodecrementation of a register by one (byte instructions) or two (word instructions). Use of relative addressing lets a machine-language program be position-independent. No dedicated I/O instructions Early models of the PDP-11 had no dedicated bus for input/output, but only a system bus called the Unibus, as input and output devices were mapped to memory addresses. An input/output device determined the memory addresses to which it would respond, and specified its own interrupt vector and interrupt priority. This flexible framework provided by the processor architecture made it unusually easy to invent new bus devices, including devices to control hardware that had not been contemplated when the processor was originally designed. DEC openly published the basic Unibus specifications, even offering prototyping bus interface circuit boards, and encouraging customers to develop their own Unibus-compatible hardware. The Unibus made the PDP-11 suitable for custom peripherals. One of the predecessors of Alcatel-Lucent, the Bell Telephone Manufacturing Company, developed the BTMC DPS-1500 packet-switching (X.25) network and used PDP-11s in the regional and national network management system, with the Unibus directly connected to the DPS-1500 hardware. Higher-performance members of the PDP-11 family departed from the single-bus approach. The PDP-11/45 had a dedicated data path within the CPU, connecting semiconductor memory to the processor, with core memory and