GetDCtlEntry
short refNum ; device driver reference number
GetDCtlEntry returns a handle leading to a device control entry.
refNum the device driver reference number of the Device Control Entry to
obtain..
Returns: a handle to a Device Control Entry; maintained in the unit table.
Notes: The unit table is a list where the location of each Device Control Entry is
kept. It is a 128-byte nonrelocatable block with 32, 4-byte entries.
Each 4-byte entry is assigned a unit number ranging from 0 to 31 and
each entry contains a handle that identifies the location of the individual
driver's device control entry.
Unit numbers are used to locate device control entries within the unit
table. The unit number of a device driver can be computed from its
reference number using the equation
unit number = -1*( refNum+1)