Exploring PDP-1 Lisp (1960)
A detailed exploration of the PDP-1 Lisp implementation from 1960, covering its syntax, memory layout, and runtime environment. The article examines how early Lisp ran on the PDP-1, including its unique features and constraints compared to later Lisp dialects.
Background
The PDP-1 was an early minicomputer (1959) from Digital Equipment Corporation (DEC) — a machine about the size of a large desk with only 4096 18-bit words of memory. Lisp, created by John McCarthy in 1958, is one of the oldest high-level programming languages, best known for pioneering symbolic computation and automatic memory management (garbage collection). This article explores a 1960 implementation of Lisp for the PDP-1, which predates the more famous Lisp machines of the 1970s and 1980s. Lisp 1.5 (1962) was the first widely distributed Lisp standard; the PDP-1 Lisp described here is an even earlier, experimental version that ran on extremely limited hardware. Understanding this implementation matters because it shows how early computer scientists made sophisticated language features (recursion, list processing, garbage collection) work on machines with thousands of times less memory than a modern smartphone.