P-code machine

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)). This term is applied both generically to all such machines (such as the Java virtual machine (JVM) 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, among whose developers, the p in p-code was construed to mean pseudo more often than portable, thus 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. 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 a Java processor). Compared to direct translation into native machine code, a two-stage approach involving translation into p-code and execution by interpreting or just-in-time compilation (JIT) offers several advantages. It is much easier to write a small p-code interpreter for a new machine than it is to modify a compiler to generate native code for the same machine. Generating machine code is one of the more complicated parts of writing a compiler. By comparison, generating p-code is much easier because no machine-dependent behavior must be considered in generating the bytecode.

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