Michael B. Parker, MIT '89

MIT Address: (to 5/19)				Home Address: (5/20-6/2)
East Campus - Mun. 303				Lucy V. Parker
3 Ames Street					721 E. Walnut Ave.
Cambridge, MA 02139				Orange, CA 92667-6833
(617) 225-6303					(714) 639-9497


Jane Tazelaar, Editorial
Byte Magazine

One Phoenix Mill Lane
Peterborough, NH 03458
603-924-9281


Dear Jane,					Monday, 16 May 1988
---------

Hope things are going well with you.  I called Friday to update you
on my progress with the multitasking code and left a message with Ken
Sheldon.

A few weeks ago you asked me to send you both the final multitasking
package (for BIX distribution) and listing of the mpthd_switch() and
mpthd_xfer() functions (for printing in BYTE).

Right now is finals weeks at MIT.  It is the busiest time of the year
for me.  It is impossible for me to complete the entire package now.
However, I have finished the two procedures you requested and sent
them to you (see below).

In a few days, I will be returning home to California.  There, I will
finish the package and send it to you by the end of the month.

I hope this arrangement will work with your schedule.  If it doesn't,
please give me a call.  Well, either way it works out, I'd enjoy
hearing from you again!

Thank you for all your support and encouragement.  It is an honor to
be writing for your magazine.


		Sincerely,

				Mike Parker



Encl:	Code for mpthd_switch() and mptsk_xfer() functions.  Tab stops
are at every 8th column (standard ASCII).  All lines are less than 80 chars.

/****************************************************************************/
/* THE mpthd_switch() FUNCTION	- FOR SWITCHING BETWEEN THREADS OF EXECUTION*/
/****************************************************************************/
MPTHDID	mpthd_switch(newmpthd)
	MPTHDID	newmpthd;
{
	if (newmpthd->stackoverflow)	return((MPTHDID)0);
	if (newmpthd!=mpthd_me()) {
		mpsem_critsect(&(newmpthd->mpsem),{
			if (!setjmp(mpthd_me()->state)) {
				newmpthd->oldmpthd=	mpthd_me();
				mpthd_me()=		newmpthd;
				longjmp(mpthd_me()->state,1);
			}
			newmpthd=	mpthd_me()->oldmpthd;
		});
	}
	return(newmpthd);	/* RETURN OLD THREAD SWITCHED FROM */
}
/****************************************************************************/




/****************************************************************************/
/* THE mptsk_xfer() FUNCTION	- FOR TRANSFERING MSGS & CTRL BETWEEN TASKS */
/****************************************************************************/
MPMSGID	mptsk_xfer(MPBOXID mpbox, MPMSGID mpmsg, BOOL block, MPBOXID sched)
{
mpsig_critsect(do {  /* DISABLE INTERRUPTS */

/*SEND*/ if (mpmsg) {	/* IF SENDING A MESSAGE */
		MPTSKID	mptsk;
		mptsk=	mpbox_xfer(mpbox,	/* QUEUE MSG <XOR> GET TSK */
				mptsk_sendside, block?mpmsg:(MPMSGID)0);
		if (mptsk) {	/* A TASK WAS WAITING */
			/* TRANSFER MESSAGE */
 			mptsk_mpmsg(mptsk)= mpmsg;	mpmsg= 0;
			/* SCHEDULE BOTH TASKS */
			mpbox_xfer(mptsk_sched(mptsk),mptsk_recvside,mptsk);
			mpbox_xfer(mptsk_mysched(),mptsk_recvside,mptsk_me());
		} else {
			if (block) mpmsg=0;	/* MESSAGE SENT */
			break;			/* RETURN IMMEDIATELY */
		}

/*RECV*/} else {	/* IF RECEIVING A MESSAGE */
		mpmsg=	mpbox_xfer(mpbox,	/* GET MSG <XOR> QUEUE TSK */
				mptsk_recvside, block?mptsk_me():(MPMSGID)0);
		if (mpmsg)	break;	/* MESSAGE RECVED: RET IMMEDIATELY */
		if (!block)	break;	/* DON'T WAIT FOR MSG: RET IMMDLY */
	}
/*SCHD*/{	/* SCHEDULE A NEW TASK TO RUN */
		MPTSKID	mptsk;
		MPBOXID	cursched=	sched;	/* USE USER SCHED LIST */
		/* SEARCH FROM HIGHEST TO LOWEST PRIORITY 'TILL TASK FOUND */
		while (!(mptsk= mpbox_xfer(cursched,mptsk_recvside,
								(MPMSGID)0))){
			cursched=	mprng_next(cursched);
			if (cursched==sched)	/* IF END OF USER SCHED LIST*/
				cursched=  mptsk_topsched(); /* USE SYS LST */
		}
		mptsk_mympmsg()=mpmsg;	/* SAVE MESSAGE BETWEEN SWITCHES */
		mpthd_switch(str2fld(mptsk,mpthd));
		mpmsg=		mptsk_mympmsg();/* RETURN RESULT MESSAGE */
	}
} while (0););
return(mpmsg);
}
/****************************************************************************/



/****************************************************************************/
/* 	MACROS FOR TYPICAL USES OF THE mptsk_xfer() FUNCTION		    */
/****************************************************************************/
#define	mptsk_send(mpbox,mpmsg)	/* SEND mpmsg TO mpbox */		\
		mptsk_xfer(mpbox,mpmsg,BOOL_TRUE,mptsk_topsched())

#define	mptsk_csend(mpbox,mpmsg)/* SEND mpmsg TO mpbox IFF HAS A RECVR */ \
		mptsk_xfer(mpbox,mpmsg,BOOL_FALSE,mptsk_topsched())

#define	mptsk_recv(mpbox)	/* RECV A MPMSG FROM mpbox */		\
		mptsk_xfer(mpbox,(MPMSGID)0,BOOL_TRUE,mptsk_topsched())

#define	mptsk_crecv(mpbox)      /* RECV A MPMSG FROM mpbox IFF HAS A MSG */ \
		mptsk_xfer(mpbox,(MPMSGID)0,BOOL_FALSE,mptsk_topsched())

#define	mptsk_yield()		/* TEMPORARILY YIELD CTRL TO OTHER TASKS */ \
			mptsk_recv(mpbox_mysched())

#define	mptsk_yieldto(sched)	/* TEMPORARILY YIELD CTRL TO sched TSK LST */\
			mptsk_xfer(mpbox_mysched(),(MPMSGID)0,BOOL_TRUE,sched)
/****************************************************************************/