Portable Standard Lisp

Like most older Lisps, in the first step, PSL compiles Lisp code to LAP code, which is another cross-platform language. However, where older lisps mostly compiled LAP directly to assembly language or some architecture dependent intermediate, PSL compiles the LAP to C code, which would run in a virtual machine language; so programs written in it are as portable as C in principle, which is very portable. The compiler was written in PSL or a more primitive dialect named System Lisp or SYSLISP as "... an experiment in writing a production-quality Lisp in Lisp itself as much as possible, with only minor amounts of code written by hand in assembly language or other systems languages." so the whole ensemble could bootstrap itself, and improvements to the compiler improved the compiler. Some later releases had a compatibility package for Common Lisp, but this is not sustained in the modern versions.

Criticism

Portable Standard Lisp has fewer features than other Lisps, such as Common Lisp, and some people found it unpleasant to use. Richard P. Gabriel wrote in his popular essay Lisp: Good News, Bad News, How to Win Big, "the third most standard Lisp was Portable Standard Lisp, which ran on many machines, but very few people wanted to use it;".