PDP1120

The PDP-11/20, released by Digital Equipment Corporation (DEC) in 1970, was a foundational model in the PDP-11 family of minicomputers. It was designed as a cost-effective and relatively simpler machine compared to its more powerful siblings, targeting a broader market that included educational institutions, small businesses, and research labs.

The PDP-11/20 featured a 16-bit architecture with a 16-bit data bus and a 16-bit address bus, allowing for direct addressing of 64 KB of memory. It utilized a microprogrammed control unit and a set of 8 general-purpose registers, which were a significant advancement for their time, contributing to the machine's programming flexibility. Its instruction set was orthogonal, meaning instructions could operate on various data types and addressing modes, simplifying software development.

Commonly used with the RT-11 operating system, the PDP-11/20 was highly influential in the development of computing. Its architecture laid the groundwork for many subsequent PDP-11 models and had a lasting impact on computer design principles, including the concept of memory-mapped I/O. The PDP-11/20's affordability and programmability made it a popular choice for teaching computer architecture and programming, cementing its legacy in the history of computing.