The UCSD p-System, Apple Pascal, and a dream of cross-platform compatibility

The UCSD p-System, Apple Pascal, and a dream of cross-platform compatibility never quite realized I’ve been thinking about the UCSD P-System a lot lately, and I thought I’d write about it a bit, and about bytecode systems more-generally. And along the way, I hope to write some interesting code. What’s the p-System? Way back in 1974-1978, computer scientists at the University of California, San Diego campus developed a new portable operating system, compiler, and tools to run on both the PDP-11 minicomputers and the increasingly-common microcomputers. They called this UCSD Pascal, and it was available for a wide variety of systems, in just a few years. The secret to its wide portability was that the system sat “on top of” a very small kernel of machine-dependent code, which implemented a “virtual machine” called the “p-machine”, a kind of imaginary CPU specifically designed to be a good target for a Pascal Compiler. Because this was a relatively-small amount of code, it was possible ... Read full article.