From http://support.microsoft.com/?kbid=102873:
multi(X)disk(Y)rdisk(Z)partition(W)\<winnt_dir>
MULTI(X) Syntax
The MULTI(X) syntax of the ARC path is only used on x86-based computers. In Windows NT version 3.1 this path is only valid for IDE and ESDI drives; in Windows NT version 3.5, 3.51 and 4.0 it is valid for SCSI drives as well.
The MULTI() syntax indicates to Windows NT that it should rely on the computers BIOS to load system files. This means that the operating system will be using interrupt (INT) 13 BIOS calls to find and load NTOSKRNL.EXE and any other files needed to boot Windows NT.
The X, Y, Z, and W parameters have the following meaning:
* X is the ordinal number of the adapter and should always be 0 (see the text below for the reason).
* Y is always 0 (zero) if the ARC path starts with MULTI(), because MULTI() invokes the INT 13 call as described above and therefore does not need the DISK() parameter information.
* Z is the ordinal for the disk on the adapter and is usually a number between 0 and 3.
* W is the partition number. All partitions receive a number except for type 5 (MS-DOS Extended) and type 0 (unused) partitions, with primary partitions being numbered first and then logical drives. NOTE: The first valid number for W is 1, as opposed to X, Y, and Z which start at 0 (zero).
Theoretically, this syntax could be used to start Windows NT on any drive in the system. However, this would require that all drives are correctly identified through the standard INT 13 interface; since support for this varies from disk controller to disk controller and most system BIOS only identify a single disk controller through INT 13, in practice it is only safe to use this syntax to start Windows NT from the first two drives connected to the primary disk controller, or the first four drives in the case of a dual-channel EIDE controller.
In a pure IDE system, the MULTI() syntax will work for up to the four drives maximum on the primary and secondary channels of a dual-channel controller.