NAME
        AllocPotBits -- allocate bits in the potgo register.

   SYNOPSIS
        allocated = AllocPotBits(bits)
        D0                       D0

        uword allocpotbits( uword );

   FUNCTION
        The AllocPotBits routine allocates bits in the hardware potgo
        register that the application wishes to manipulate via
        writepotgo.  the request may be for more than one bit.  a
        user trying to allocate bits may find that they are
        unavailable because they are already allocated, or because
        the start bit itself (bit 0) has been allocated, or if
        requesting the start bit, because input bits have been
        allocated.  A user can block itself from allocation: i.e.
        it should FreePotgoBits the bits it has and re-AllocPotBits if
        it is trying to change an allocation involving the start bit.

   INPUTS
        bits - a description of the hardware bits that the application
                wishes to manipulate, loosely based on the register
                description itself:
            START (bit 0) - set if you wish to use start (i.e. start
                    the proportional controller counters) with the
                    input ports you allocate (below).  You must
                    allocate all the DATxx ports you want to apply
                    START to in this same call, with the OUTxx bit
                    clear.
            DATLX (bit 8) - set if you wish to use the port associated
                    with the left (0) controller, pin 5.
            OUTLX (bit 9) - set if you promise to use the LX port in
                    output mode only.  The port is not set to output
                    for you at this time -- this bit set indicates
                    that you don't mind if STARTs are initiated at any
                    time by others, since ports that are enabled for
                    output are unaffected by START.
            DATLY (bit 10) - as DATLX but for the left (0) controller,
                    pin 9.
            OUTLY (bit 11) - as OUTLX but for LY.
            DATRX (bit 12) - the right (1) controller, pin 5.
            OUTRX (bit 13) - OUT for RX.
            DATRY (bit 14) - the right (1) controller, pin 9.
            OUTRY (bit 15) - OUT for RY.

   RESULTS
        allocated - the START and DATxx bits of those requested that
                were granted.  The OUTxx bits are don't cares.