# 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). 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.
Programs that have been translated to P-code can either be interpreted by a software program that emulates the behaviour 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).

## P-code versus machine code

While a typical compiler model is aimed at translating a program code into machine code, the idea of a P-code machine follows a two-stage approach involving translation into P-code and execution by interpreting or just-in-time compilation (JIT) through the P-code machine.
This separation makes it possible to detach the development of a P-code interpreter from the underlying machine code compiler, which has to consider machine-dependent behaviour in generating its bytecode. This way a P-code interpreter can also be implemented quicker, and the ability to interpret the code at runtime allows for additional run-time checks which might not be similarly available in native code. Further, as P-code is based on an ideal virtual machine, a P-code program can often be smaller than the same program translated to machine code. Conversely, the two-step interpretation of a P-code-based program leads to a slower execution speed, though this can sometimes be addressed with just-in-time compilation, and its simpler structure is easier to reverse-engineer than native code.

## Implementations of P-code

In the early 1980s, at least two operating systems achieved machine independence through extensive use of P-code. The Business Operating System (BOS) was a cross-platform operating system designed to run P-code programs exclusively. The UCSD p-System, developed at The University of California, San Diego, was a self-compiling and self-hosting operating system based on P-code optimized for generation by the Pascal language.
In the 1990s, translation into p-code became a popular strategy for implementations of languages such as Python, Microsoft P-Code in Visual Basic and Java bytecode in Java.
The language Go uses a generic, portable assembly as a form of p-code, implemented by Ken Thompson as an extension of the work on Plan 9 from Bell Labs. Unlike Common Language Runtime (CLR) bytecode or JVM bytecode, there is no stable specification and the Go build tools do not emit a bytecode format to be used at a later time. The Go assembler uses the generic assembly language as an intermediate representation and the Go executables are machine-specific statically linked binaries.

## UCSD P-Machine
### Architecture

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. Like Pascal, the P-code is strongly typed, supporting Boolean (b), character (c), integer (i), real (r), set (s), and pointer (a) data types natively.
Some simple instructions:

### Environment

Similar to a real target CPU, the P-System has only one stack shared by procedure stack frames (providing return address, etc.) and the arguments to local instructions. Three of the machine's registers point into the stack (which grows upwards):
* SP points to the top of the stack (the stack pointer).
* MP marks the beginning of the active stack frame (the mark pointer).
* EP points to the highest stack location used in the current procedure (the extreme pointer).
Also present is a constant area, and, below that, the heap growing down towards the stack. The NP (the new pointer) register points to the top (lowest used address) of the heap. When EP gets greater than NP, the machine's memory is exhausted.
The fifth register, PC, points at the current instruction in the code area.

### Calling conventions

Stack frames look like this:
The procedure calling sequence works as follows: The call is introduced with
where n specifies the difference in nesting levels (remember that Pascal supports nested procedures). This instruction will mark the stack, i.e. reserve the first five cells of the above stack frame, and initialize previous EP, dynamic, and static link. The caller then computes and pushes any parameters for the procedure, and then issues
to call a user procedure (n being the number of parameters, p the procedure's address). This will save the PC in the return address cell, and set the procedure's address as the new PC.
User procedures begin with the two instructions
The first sets SP to MP + i, the second sets EP to SP + j. So i essentially specifies the space reserved for locals (plus the number of parameters plus 5), and j gives the number of entries needed locally for the stack. Memory exhaustion is checked at this point.
Returning to the caller is accomplished via
with C giving the return type (i, r, c, b, a as above, and p for no return value). The return value has to be stored in the appropriate cell previously. On all types except p, returning will leave this value on the stack.
Instead of calling a user procedure (cup), standard procedure q can be called with
These standard procedures are Pascal procedures like readln (csp rln), sin (csp sin), etc. Peculiarly eof is a p-Code instruction instead.

## Example machine

Niklaus Wirth specified a simple p-code machine in the 1976 book Algorithms + Data Structures = Programs. The machine had 3 registers - a program counter p, a base register b and a top-of-stack register t. There were 8 instructions:
* lit 0, a : load constant a
* opr 0, a : execute operation a (13 operations: RETURN, 5 mathematical functions, and 7 comparison functions)
* lod l, a : load variable l, a
* sto l, a : store variable l, a
* cal l, a : call procedure a at level l
* int 0, a : increment t-register by a
* jmp 0, a : jump to a
* jpc 0, a : jump conditional to a
This is the code for the machine, written in Pascal:
This machine was used to run Wirth's PL/0, a Pascal subset compiler used to teach compiler development.

## Microsoft P-code

P-code is a name for several of Microsoft's proprietary intermediate languages. They provided an alternate binary format to machine code. At various times, Microsoft has said P-code is an abbreviation for either packed code or pseudo code.
Microsoft P-code was used in Visual C++ and Visual Basic. Like other P-code implementations, Microsoft P-code enabled a more compact executable at the expense of slower execution.