UCSD Pascal was a specific implementation of the programming language Pascal which used the p-Code machine architecture.

Notable was the introduction of separately compilable Units and a String type. Both of which influenced the design of the Ada programming language.

The UCSD Pascal compiler was distributed as part of a portable operating system, the p-System.

There were four versions of UCSD p-Code engine (p-Code incompatible) each with several revisions of the p-System (and UCSD Pascal); represented with the leading Roman Numeral; operating system revisions were enumerated as the "dot" number following the p-Code Roman Numeral. vis: II.3 represented the third revision of the p-System running on the second revision of the p-Machine.

• Version I

Original version, never officially distributed outside of the University of California, San Diego. However the Pascal sources for both Versions I.3 and I.5 were freely exchanged between interested users. Specifically the patch revision I.5a was known to be one of the most stable.

• Version II

Widely distributed, available on many early microcomputers.

• Version III

Custom version written for Western Digital to run on their Pascal Micro-Engine microcomputer.

• Version IV

Commercial version, developed and sold by Sof-tech. Did not sell well due to combination of their pricing structure, performance problems due to p-Code interpreter, and competition with native operating systems (which it often ran on top of). After Sof-tech dropped the product it was picked up by Pecan Systems (a relatively small company formed of p-System users and fans). Sales revived somewhat, due mostly to Pecan's reasonable pricing structure, but the p-System and UCSD Pascal gradually lost the market to native operating systems and compilers.