In computer programming, a P-code machine (portable code machine) is a virtual machine designed to execute P-code, the assembly language or machine code of a hypothetical central processing unit (CPU). The term P-code machine is applied generically to all such machines (such as the Java virtual machine (JVM) and MATLAB pre-compiled code), as well as specific implementations using those machines. One of the most notable uses of P-Code machines is the P-Machine of the Pascal-P system. The developers of the UCSD Pascal implementation within this system construed the P in P-code to mean pseudo more often than portable; they adopted a unique label for pseudo-code meaning instructions for a pseudo-machine.
Although the concept was first implemented circa 1966 as O-code for the Basic Combined Programming Language (BCPL) and P code for the language Euler, the term P-code first appeared in the early 1970s. Two early compilers generating P-code were the Pascal-P compiler in 1973, by Kesav V. Nori, Urs Ammann, Kathleen Jensen, Hans-Heinrich Nägeli, and Christian Jacobi, and the Pascal-S compiler in 1975, by Niklaus Wirth.