NAME
       GetUnit - allocate the disk for a driver

   SYNOPSIS
       lastDriver = GetUnit( unitPointer ), DRResource
       D0                     A1             A6

        struct discresourceunit *getunit(struct discresourceunit *);

   FUNCTION
        This routine allocates the disk to a driver.  It is either
        immediately available, or the request is saved until the disk
        is available.  When it is available, your unitPointer is
        sent back to you (via ReplyMsg).  You may then reattempt the
        GetUnit.

        Allocating the disk allows you to use the disk's resources.
        remember however that there are four units to the disk; you are
        only one of them.  Please be polite to the other units (by never
        selecting them, and by not leaving interrupts enabled, etc.).

        When you are done, please leave the disk in the following state:
            dmacon dma bit ON
            dsklen dma bit OFF (write a #DSKDMAOFF to dsklen)
            adkcon disk bits -- any way you want
            intena:disk sync and disk block interrupts -- Both DISABLED
            cia resource index interrupt -- disabled
            8520 outputs -- doesn't matter, because all bits will be
                set to inactive by the resource.
            8520 data direction regs -- restore to original state.

        NOTE: GetUnit() does NOT turn on the interrupts for you.
              You must use ableicr (for the index interrupt) or intena
              (for the diskbyte and diskblock interrupts) to turn them
              on.  You should turn them off before calling giveunit,
              as stated above.

   INPUTS
        unitPtr - a pointer you your disk resource unit structure.
                Note that the message filed of the structure MUST
                be a valid message, ready to be replied to.  Make sure
                ln_Name points to a null-terminated string, preferably
                one that identifies your program.

                You need to set up the three interrupt structures,
                in particular the IS_DATA and IS_CODE fields.  Set them
                to NULL if you don't need that interrupt.  Also, set
                the ln_Type of the interrupt structure to NT_INTERRUPT.
                WARNING: don't turn on a disk resource interrupt unless
                the IS_CODE for that interrupt points to executable code!

                IS_CODE will be called with IS_DATA in A1 when the
                interrupt occurs.  Preserve all regs but D0/D1/A0/A1.
                Do not make assumptions about A0.

   RESULTS
        lastDriver - if the disk is not busy, then the last unit
                to use the disk is returned.  This may be used to
                see if a driver needs to reset device registers.
                (If you were the last user, then no one has changed
                any of the registers.  If someone else has used it,
                then any allowable changes may have been made).  If the
                disk is busy, then a null is returned.

   EXCEPTIONS

   SEE ALSO
        giveunit()

   BUGS