PDP11

The PDP-11 is a series of 16-bit minicomputers produced by Digital Equipment Corporation (DEC) beginning in 1970. It became one of the most influential families of its era, renowned for its simple, orthogonal instruction set, flexible addressing modes, and a broad software ecosystem, including an early version of the UNIX operating system.

Hardware and architecture: The core is register-based, with eight 16-bit general-purpose registers (R0–R7). R7 serves as the program counter and R6 as the stack pointer. Instructions are 16-bit and support several addressing modes, ranging from direct register operations to indirect and deferred forms. The processor status word contains condition codes and interrupt priority. The architecture typically uses a 16-bit address space, and I/O was interfaced through DEC's UNIBUS (and later Q-Bus) systems.

Models and operating systems: The PDP-11 family encompassed a wide range of machines, including the 11/20, 11/30, 11/40, 11/44, 11/60, and 11/70, with varying performance and features such as optional paging. It supported several operating systems, such as RT-11 for real-time tasks, RSX-11 and RSTS/E; and most famously, UNIX, which was developed on the PDP-11 and helped popularize both the OS and the hardware.

Impact and legacy: The PDP-11's design influenced later DEC architectures, notably the development of the VAX line. It achieved long production life and widespread deployment in universities, laboratories, and industry, and left a lasting imprint on computer engineering and operating-system design.