Page:Scheme - An interpreter for extended lambda calculus.djvu/38

Sussman and Steele

December 22, 1975

37

Implementation of the Interpreter

(DEFPROP LABELS LABELS AINT)

(DEFUN LABELS ()
       (SETQ **TEM** (MAPCAR '(LAMBDA (DEF)
                                      (LIST (CAR DEF)
                                            (LIST 'BETA (CADR DEF) NIL)))
                             (CADR **EXP**)))
       (MAPC '(LAMBDA (VC) (RPLACA (CDDADR VC) **TEM**)) **TEM**)
       (SETQ **ENV** (NCONC **TEM** **ENV**)
             **EXP** (CADDR **EXP**)
             **PC** 'AEVAL))

We now come to the multiprocess primitives.

CREATE!PROCESS temporarily creates a new set of machine registers (by the lambda-binding mechanism of the host language), establishes the new process in those registers, swaps it out, and returns the new process id; returning causes the old machine registers to be restored.

(DEFUN CREATE!PROCESS (EXP)
       ((LAMBDA (**PROCESS** **EXP** **ENV** **UNEVLIS** **EVLIS** **PC** **CLINK** **VAL**)
                (SWAPOUTPROCESS)
                **PROCESS**)
        (GENSYM)
        EXP
        **ENV**
        NIL
        NIL
        'AEVAL
        (LIST NIL NIL NIL NIL 'TERMINATE NIL)
        NIL))

(DEFUN START!PROCESS (P)
       (COND ((OR (NOT (ATOM P)) (NOT (GET P 'CLINK)))
              (ERROR '|BAD PROCESS -- START!PROCESS| **EXP** 'FAIL-ACT)))
       (OR (EQ P **PROCESS**) (MEMQ P **QUEUE**)
           (SETQ **QUEUE** (NCONC **QUEUE** (LIST P))))
       P)

(DEFUN STOP!PROCESS (P)
       (COND ((MEMQ P **QUEUE**)
              (SETQ **QUEUE** (DELQ P **QUEUE**)))
             ((EQ P **PROCESS**) (TERMINATE)))
       P)

TERMINATE is an internal microcode routine which terminates the current process. If the current process is the only one, then all processes have been stopped, and so a new SCHEME top level is created; otherwise TERMINATE pulls the next process off the scheduler queue and swaps it in. Note that we cannot use SWAPINPROCESS because a RESTORE will happen in EVLIS as soon as TERMINATE completes (this is a very deep global property of the interpreter, and a fine