The get-logical-device function is supported only under MS-DOS versions 3.2 and later and only on block devices; in character-device drivers it should do nothing except set the done bit in the status word. This function is called only if bit 6 is set in the device attribute word of the device header. It corresponds to the get-logical-device-map service supplied to application programs through Int 21H Function 44H Subfunction 0EH.
The get-logical-device function returns a code for the last drive letter used to reference the device; if only one drive letter is assigned to the device, the returned unit code should be zero. Thus, this function can be used to determine whether more than one drive letter is assigned to the same physical device.
The get-logical-device function is called with
RH + 1 BYTE Unit code
RH + 2 BYTE Command code = 23 (17H)
It returns
RH + 1 BYTE Last unit referenced, or zero
RH + 3 WORD Status