In computer programming, a p-code machine (portable code machine^[1]) 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 pre-compiled 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,^[2] 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,^[3] 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).

Benefits and weaknesses of implementing p-code

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. This makes it useful for getting a compiler up and running quickly.
• Since p-code is based on an ideal virtual machine, a p-code program is often much smaller than the same program translated to machine code.
• When the p-code is interpreted, the interpreter can apply additional run-time checks that are difficult to implement with native code.

One of the significant disadvantages of p-code is execution speed, which can sometimes be remedied via JIT compiling. P-code is often also 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.^[4]

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 Go executables are machine-specific statically linked binaries.^[5]

UCSD p-Machine

Insn.   Stack   Stack   Description
        before  after
 
adi     i1 i2   i1+i2   add two integers
adr     r1 r2   r1+r2   add two reals
inn     i1 s1   b1      set membership; b1 = whether i1 is a member of s1
ldi     i1 i1   i1      load integer constant
mov     a1 a2   a2      move
not     b1 b1   -b1     boolean negation

EP ->
      local stack
SP -> ...
      locals
      ...
      parameters
      ...
      return address (previous PC)
      previous EP
      dynamic link (previous MP)
      static link (MP of surrounding procedure)
MP -> function return value

 mst n

 cup n, p

 ent 1, i
 ent 2, j

 retC

 csp q

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:

1. lit 0, a : load constant a
2. opr 0, a : execute operation a (13 operations: RETURN, 5 math functions, and 7 comparison functions)
3. lod l, a : load variable l, a
4. sto l, a : store variable l, a
5. cal l, a : call procedure a at level l
6. int 0, a : increment t-register by a
7. jmp 0, a : jump to a
8. jpc 0, a : jump conditional to a^[6]

This is the code for the machine, written in Pascal:

const
	amax=2047;      {maximum address}
	levmax=3;       {maximum depth of block nesting}
	cxmax=200;      {size of code array}

type 
	fct=(lit,opr,lod,sto,cal,int,jmp,jpc);
	instruction=packed record 
		f:fct;
		l:0..levmax;
		a:0..amax;
	end;

var
	code: array [0..cxmax] of instruction;

procedure interpret;

  const stacksize = 500;

  var
    p, b, t: integer; {program-, base-, topstack-registers}
    i: instruction; {instruction register}
    s: array [1..stacksize] of integer; {datastore}

  function base(l: integer): integer;
    var b1: integer;
  begin
    b1 := b; {find base l levels down}
    while l > 0 do begin
      b1 := s[b1];
      l := l - 1
    end;
    base := b1
  end {base};

begin
  writeln(' start pl/0');
  t := 0; b := 1; p := 0;
  s[1] := 0; s[2] := 0; s[3] := 0;
  repeat
    i := code[p]; p := p + 1;
    with i do
      case f of
        lit: begin t := t + 1; s[t] := a end;
        opr: 
          case a of {operator}
            0: 
              begin {return}
                t := b - 1; p := s[t + 3]; b := s[t + 2];
              end;
            1: s[t] := -s[t];
            2: begin t := t - 1; s[t] := s[t] + s[t + 1] end;
            3: begin t := t - 1; s[t] := s[t] - s[t + 1] end;
            4: begin t := t - 1; s[t] := s[t] * s[t + 1] end;
            5: begin t := t - 1; s[t] := s[t] div s[t + 1] end;
            6: s[t] := ord(odd(s[t]));
            8: begin t := t - 1; s[t] := ord(s[t] = s[t + 1]) end;
            9: begin t := t - 1; s[t] := ord(s[t] <> s[t + 1]) end;
            10: begin t := t - 1; s[t] := ord(s[t] < s[t + 1]) end;
            11: begin t := t - 1; s[t] := ord(s[t] >= s[t + 1]) end;
            12: begin t := t - 1; s[t] := ord(s[t] > s[t + 1]) end;
            13: begin t := t - 1; s[t] := ord(s[t] <= s[t + 1]) end;
          end;
        lod: begin t := t + 1; s[t] := s[base(l) + a] end;
        sto: begin s[base(l)+a] := s[t]; writeln(s[t]); t := t - 1 end;
        cal: 
          begin {generate new block mark}
            s[t + 1] := base(l); s[t + 2] := b; s[t + 3] := p;
            b := t + 1; p := a
          end;
        int: t := t + a;
        jmp: p := a;
        jpc: begin if s[t] = 0 then p := a; t := t - 1 end
      end {with, case}
  until p = 0;
  writeln(' end pl/0');
end {interpret};

This machine was used to run Wirth's PL/0, a Pascal subset compiler used to teach compiler development.^[7]

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 have said p-code is an abbreviation for either packed code^[8] or pseudo code.^[9]

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.

Other implementations

See also

• Bytecode
• Intermediate representation
• Joel McCormack, designer of the NCR Corporation version of the p-code machine
• Runtime system
• Token threading
• City & Guilds Mnemonic Code
• Platform-independent model – Software engineering model

References

Further reading

• Pemberton, Steven; Daniels, Martin. Pascal Implementation: The P4 Compiler and Interpreter. John Wiley. ISBN 0-13-653031-1.
• Pemberton, Steven, ed. (2011-04-13). "Pascal Implementation: A Book and Sources". (NB. Has Pascal sources of the P4 compiler and interpreter, usage instructions.)
• Pemberton, Steven, ed. (2011-04-13). "pcode of the Pascal Compiler as compiled by itself". (NB. Has the p-code of the P4 compiler, generated by itself.)
• "The Jefferson Computer Museum's page on the UCSD p-System".
• "Open Source implementation"., including packaging and pre-compiled binaries; a friendly fork of the Klebsch. "Klebsch implementation".
• Terry, Pat (2005). Compiling with C# and Java. p. 624. ISBN 0-321-26360-X.
• Wirth, Niklaus (1975). Algorithms + Data Structures = Programs. ISBN 0-13-022418-9.
• Wirth, Niklaus (1996). Compiler Construction. ISBN 0-201-40353-6.
• Liffick, Blaise W., ed. (1979). The Byte Book of Pascal. ISBN 0-07-037823-1.
• Barron, David William, ed. (1981). Pascal: The Language and its Implementation. ISBN 0-471-27835-1. (NB. Especially see the articles Pascal-P Implementation Notes and Pascal-S: A Subset and its Implementation.)

External links

• VB P-code Information by Mr Silver at the Wayback Machine (archived December 22, 2015)