P-code Machine - Example Machine

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, with one (opr) having multiple forms.

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

const levmax=3; amax=2047; type fct=(lit,opr,lod,sto,cal,int,jmp,jpc); instruction=packed record f:fct; l:0..levmax; a:0..amax; end; procedure interpret; const stacksize = 500; var p, b, t: integer; {program-, base-, topstack-registers} i: instruction; {instruction register} s: array 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; l := l - 1 end; base := b1 end {base}; begin writeln(' start pl/0'); t := 0; b := 1; p := 0; s := 0; s := 0; s := 0; repeat i := code; p := p + 1; with i do case f of lit: begin t := t + 1; s := a end; opr: case a of {operator} 0: begin {return} t := b - 1; p := s; b := s; end; 1: s := -s; 2: begin t := t - 1; s := s + s end; 3: begin t := t - 1; s := s - s end; 4: begin t := t - 1; s := s * s end; 5: begin t := t - 1; s := s div s end; 6: s := ord(odd(s)); 8: begin t := t - 1; s := ord(s = s) end; 9: begin t := t - 1; s := ord(s <> s) end; 10: begin t := t - 1; s := ord(s < s) end; 11: begin t := t - 1; s := ord(s >= s) end; 12: begin t := t - 1; s := ord(s > s) end; 13: begin t := t - 1; s := ord(s <= s) end; end; lod: begin t := t + 1; s := s end; sto: begin s := s; writeln(s); t := t - 1 end; cal: begin {generate new block mark} s := base(l); s := b; s := p; b := t + 1; p := a end; int: t := t + a; jmp: p := a; jpc: begin if s = 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, which was a Pascal subset compiler used to teach compiler development.