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Like many other P-code machines, the UCSD P-Machine is a stack machine, which means that most instructions take their operands from a stack, and place results back on the stack. Thus, the add instruction replaces the two topmost elements of the stack with their sum. A few instructions take an immediate argument.
Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software interpreter.Unlike human-readable [1] source code, bytecodes are compact numeric codes, constants, and references (normally numeric addresses) that encode the result of compiler parsing and performing semantic analysis of things like type, scope, and nesting depths of ...
The UCSD p-code was optimized for execution of the Pascal programming language. Each hardware platform then only needed a p-code interpreter program written for it to port the entire p-System and all the tools to run on it. Later versions also included additional languages that compiled to the p-code base.
This performance advantage was eroded by the later availability of p-code to native machine code translators, and mainstream 16-bit microprocessors such as the Intel 8086 and Motorola 68000. When details of the MicroEngine were first released, the system accumulated a very large number of pre-orders (for the time).
P-code is an alternative term for bytecode, machine-independent code that achieves independence by targeting a p-code machine, a virtual machine designed for running p-code rather than the intention to emulate any specific hardware architecture.
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Joel McCormack is an American computer scientist who designed the NCR Corporation version of the p-code machine, which is a kind of stack machine popular in the 1970s as the preferred way to implement new computing architectures and languages such as Pascal and BCPL.
Machine code is generally different from bytecode (also known as p-code), which is either executed by an interpreter or itself compiled into machine code for faster (direct) execution. An exception is when a processor is designed to use a particular bytecode directly as its machine code, such as is the case with Java processors .