Technotes

Technote DV 06

February 1986

Changes since February 1986: Corrected the C sample code specifically for "do...while" loop to continue while the driverName did not match that of the name of the driver for a unit entry.

You should already be familiar with The Device Manager chapter of Inside Macintosh before reading this technical note.

The Pascal code at the end of this note demonstrates how to obtain the reference number of a driver that has been installed in the Unit Table. The reference number may then be used in subsequent calls to the Device Manager such as Open, Control and Prime.

One thing to note is that the dRAMBased bit really only tells you whether dCtlDriver is a pointer or a handle, not necessarily whether the driver is in ROM or RAM. SCSI drivers, for instance, are in RAM but not relocatable; their DCE entries contain pointers to them.


From MPW Pascal:

	PROCEDURE GetDrvrRefNum(driverName: Str255; VAR drvrRefNum: INTEGER);

	   TYPE
	      WordPtr      = ^INTEGER;

	   CONST
	      UTableBase   = $11C;     {low memory globals}
	      UnitNtryCnt  = $1D2;

	      dRAMBased    = 6;        {bit in dCtlFlags that indicates ROM/RAM}
	      drvrName     = $12;      {length byte and name of driver [string]}

	   VAR
	      negCount    : INTEGER;
	      DCEH        : DCtlHandle;
	      drivePtr    : Ptr;
	      s           : Str255;

	   BEGIN
	      UprString(driverName, FALSE); {force same case for compare}

	      negCount := - WordPtr(UnitNtryCnt)^; {get -(table size)}
	      {Check to see that driver is installed, obtain refNum.}
	      {Assumes that an Open was done previously -- probably by an INIT.}
	      {Driver doesn't have to be open now, though.}

	      drvrRefNum := - 12 + 1;  {we'll start with driver refnum = -12,
	                                right after .ATP entry}

	      {Look through unit table until the driver found or reach the end.}

	      REPEAT
	         drvrRefNum := drvrRefNum - 1; {bump to next refnum}
	         DCEH := GetDCtlEntry(drvrRefNum); {get handle to DCE}

	         s := '';              {no driver, no name}

	         IF DCEH <> NIL THEN
	            WITH DCEH^^ DO BEGIN 	{this is safe -- no chance of heap
						moving before dCtlFlags/dCtlDriver
						references}

		 IF (dCtlDriver <> NIL) THEN BEGIN
	                IF BTST(dCtlFlags, dRAMBased) THEN
	                   drivePtr := Handle(dCtlDriver)^ {zee deréference}
	                ELSE
	                   drivePtr := Ptr(dCtlDriver);

	                IF drivePtr <> NIL THEN BEGIN
	                   s := StringPtr(ORD4(drivePtr) + drvrName)^;
	                   UprString(s,FALSE); {force same case for compare}
	                END;
		 END;             {IF}
	            END;               {WITH}
	      UNTIL (s = driverName) OR (drvrRefNum = negCount);

	      {Loop until we find it or we've just looked at the last slot.}

	      IF s <> driverName THEN drvrRefNum := 0; {can't find driver}
	   END;

short       GetDrvrRefNum(driverName)
char		*driverName[256];

{  /* GetDrvrRefNum */

	#define		UnitNtryCnt	0x1d2

	/*bit in dCtlFlags that indicates ROM/RAM*/
	#define	      dRAMBased     6        
	/*length byte and name of driver [string]*/
	#define	      drvrName      0x12      

	short			negCount,dRef;
	DCtlHandle		DCEH;
	char			*drivePtr,*s;

	negCount = -*(short *)(UnitNtryCnt); /*get -(table size)*/

		
	/*Check to see that driver is installed, obtain refNum.*/
	/*Assumes that an Open was done previously -- probably by an INIT.*/
	/*Driver doesn't have to be open now, though.*/
		
	dRef = -12 + 1;  /*we'll start with driver refnum == -12,
						right after .ATP entry*/
		
	/*Look through unit table until we find driver or reach the end.*/
		
	do {
		dRef -= 1; /*bump to next refnum*/
		DCEH = GetDCtlEntry(dRef); /*get handle to DCE*/
			
		s = "";

		if ((DCEH != nil) && ( (**DCEH).dCtlDriver != nil) )
		{
			if (((**DCEH).dCtlFlags >> dRAMBased) & 1)
							/* test dRamBased bit */
				drivePtr = *(Handle) (**DCEH).dCtlDriver;
							/*zee deréference*/
			else
				drivePtr = (**DCEH).dCtlDriver;
			
			if (drivePtr != nil)  
				s = drivePtr + drvrName;
		}
	} while (!(EqualString(s,driverName,0,0)) && (dRef != negCount));
	/*Loop until we find it or we've just looked at the last slot.*/
		
	if (EqualString(s,driverName,0,0))  
		return dRef;
	else
		return 0; /*can't find driver*/
}/* GetDrvrRefNum */

That's all there is to locating a driver and picking up the reference number.

Further Reference:

*	The Device Manager