P-code machine

In computer programming, a p-code machine, or portable code machine is a virtual machine designed to execute p-code (the assembly language of a hypothetical CPU). This term is applied both generically to all such machines (such as the Java Virtual Machine and MATLAB precompiled code), and to specific implementations, the most famous being the p-Machine of the Pascal-P system, particularly the UCSD Pascal implementation.

Although the concept was first implemented circa 1966 (as O-code for BCPL and P – a code for the Euler Language), the term p-code first appeared in the early 1970s. Two early compilers generating p-code were the Pascal-P compiler in 1973, by Nori, Ammann, Jensen, Hageli, and Jacobi, and the Pascal-S compiler in 1975, by Niklaus Wirth.

Programs that have been translated to p-code can either be interpreted by a software program that emulates the behavior of the hypothetical CPU, or translated into the machine code of the CPU on which the program is to run and then executed. If there is sufficient commercial interest, a hardware implementation of the CPU specification may be built (e.g., the Pascal MicroEngine or a version of the Java processor).

Compared to direct translation into native machine code, a two-stage approach involving translation into p-code and execution by an interpreter or just-in-time compiler offers several advantages.

One of the significant disadvantages of p-code is execution speed, which can sometimes be remedied through the use of a JIT compiler. P-code is often also easier to reverse-engineer than native code.

...
Wikipedia