p-code machine

In

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

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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 the 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 Just-in-time compilation. P-code is often also easier to reverse-engineer than native code.

Implementations of p-code

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In the early 1980s, at least two

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:

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

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

) 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

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Stack frames look like this:

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

The procedure calling sequence works as follows: The call is introduced with

 mst n

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 initialise previous EP, dynamic, and static link. The caller then computes and pushes any parameters for the procedure, and then issues

 cup n, p

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

 ent 1, i
 ent 2, j

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

 retC

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

 csp q

These standard procedures are Pascal procedures like readln() (csp rln), sin() (csp sin), etc. Peculiarly eof() is a p-Code instruction instead.

Example machine

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Niklaus Wirth specified a simple p-code machine in the 1976 book

top-of-stack register

t. There were 8 instructions:

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};