#define NPROG 2000
Inst prog[NPROG]; /* the machine */
Inst *progp; /* next free spot for code generation */
Inst *pc; /* program counter during execution */
Inst *progbase = prog; /* start of current subprogram */
int returning; /* 1 if return stmt seen */
typedef struct Frame { /* proc/func call stack frame */
Symbol *sp; /* symbol table entry */
Inst *retpc; /* where to resume after return */
Datum *argn; /* n-th argument on stack */
int nargs; /* number of arguments */
} Frame;
#define NFRAME 100
Frame frame[NFRAME];
Frame *fp; /* frame pointer */
initcode() {
progp = progbase;
stackp = stack;
fp = frame;
returning = 0;
}
push(d)
Datum d;
{
if (stackp >= &stack[NSTACK])
execerror("stack too deep", (char*)0);
*stackp++ = d;
}
Datum pop() {
if (stackp == stack)
execerror("stack underflow", (char*)0);
return *--stackp;
}
constpush() {
Datum d;
d.val = ((Symbol*)*pc++)->u.val;
push(d);
}
varpush() {
Datum d;
d.sym = (Symbol*)(*pc++);
push(d);
}
whilecode() {
Datum d;
Inst *savepc = pc;
execute(savepc+2); /* condition */
d = pop();
while (d.val) {
execute(*((Inst**)(savepc))); /* body */
if (returning)
break;
execute(savepc+2); /* condition */
d = pop();
}
if (!returning)
pc = *((Inst**)(savepc+1)); /* next stmt */
}
ifcode() {
Datum d;
Inst *savepc = pc; /* then part */
execute(savepc+3); /* condition */
d = pop();
if (d.val)
execute(*((Inst**)(savepc)));
else if (*((Inst**)(savepc+1))) /* else part? */
execute(*((Inst**)(savepc+1)));
if (!returning)
pc = *((Inst**)(savepc+2)); /* next stmt */
}
define(sp) /* put func/proc in symbol table */
Symbol *sp;
{
sp->u.defn = (Inst)progbase; /* start of code */
progbase = progp; /* next code starts here */
}
call() /* call a function */
{
Symbol *sp = (Symbol*)pc[0]; /* symbol table entry */
/* for function */
if (fp++ >= &frame[NFRAME-1])
execerror(sp->name, "call nested too deeply");
fp->sp = sp;
fp->nargs = (int)pc[1];
fp->retpc = pc + 2;
fp->argn = stackp - 1; /* last argument */
execute(sp->u.defn);
returning = 0;
}
ret() /* common return from func or proc */
{
int i;
for (i = 0; i < fp->nargs; i++)
pop(); /* pop arguments */
pc = (Inst*)fp->retpc;
--fp;
returning = 1;
}
funcret() /* return from a function */
{
Datum d;
if (fp->sp->type == PROCEDURE)
execerror(fp->sp->name, "(proc) returns value");
