Windows XP ARC Naming Conventions

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There is a simple rule-of-thumb applied here -- primary partitions (on an individual hard disk) are always numbered before extended partitions.

Partition numbering begins at "1" and you can have up to four primary partitions or three primary partitions with one extended partition. The extended partition can be segmented into logical drives, each with its own partition number. Examine the following drive:

The primary partition is numbered first (1), and then the extended partition with logical drives. But what would happen if you added another primary partition?

The original primary partition retains its number. The new primary partition takes the highest number not being used by a primary partition. In this instance, that number is 2. The logical drives are all renumbered, starting with the first number not being used by ANY primary partition.

The way partitions are numbered affect the ARC paths in the "boot.ini" file with Microsoft Windows XP on a machine with an "x86" processor or in the list of "Boot Selections" on a RISC system.

Understanding Arc Naming Convention
An ARC path was the way that NT had to communicate with a RISC system. The RISC processor needed two distinct disk partitions for NT. The firmware started up and looked at the system partition for the operating system initialize (kernel). The system partition then called up the boot partition (the NT files). The firmware authors the boot selections menu. Boot selections were stored as hardware environmental variables in NVRAM.

In an x86 system, it works the same way, except that an x86 processor does not need two distinct partitions for XP. Frequently, both the system and boot partitions are on the same disk partition. The partition numbering is stored in ARC paths in the "boot.ini" file.

It is important to remember on an x86 system that the system partition contains what we perceive as the "boot files". The system partition calls up the boot partition, which contains the operating system files. The ARC paths in the Boot.ini point to the "boot partition", or the separate operating system files.

An ARC path in the Boot.ini file is always written one of two ways:
multi(x) disk(x) rdisk(x) partition(x)

scsi(x) disk(x) rdisk(x) partition(x)
"Multi" means that the disk is attached to either an IDE controller or a SCSI adapter with the BIOS enabled. "SCSI" means that the disk is attached to a SCSI adapter with the BIOS not enabled. If there is only one controller (or adapter) on the motherboard, the number contained in the parentheses is "0". Each subsequent controller/adapter is incremented by one. Therefore, the second controller would be multi/scsi (1), etc.

"Disk" relates to the SCSI bus number on an adapter where the BIOS is not enabled. Bus numbering starts at "0"; the first bus is "0", the second, "1", and so on. For "multi", this number is ALWAYS "0", because this section of the ARC path is not valid.

"Rdisk" refers to the ordinal number of the hard disk on an IDE controller or a SCSI adapter with the BIOS enabled. The first hard disk is numbered "0", the second "1", and so forth. For "scsi", this number is ALWAYS "0", because this section of the ARC path is not valid.

"Partition" refers to the partition referenced on the disk. Partition numbering starts at "1". As previously mentioned, primary partitions always are numbered first, with the extended partition (containing logical
drives) following.

In this case, the System partition is on the primary partition, and the Boot partition is located on the extended partition, on the first logical drive. If this were the first hard drive connected to the only IDE controller, the ARC path in the Boot.ini would be:
multi(0) disk(0) rdisk(0) partition(2)

If you break it down:
multi(0) - it is an IDE controller [multi] and it is the only controller [number starts at 0]
disk(0) - because it is an IDE controller, this is null
rdisk(0) - it is the first hard drive connected [numbering starts at 0]

partition(2) - the boot partition [operating system files] are on the second partition [the extended partition, first logical drive]. Numbering begins at "1", and the primary partition is counted first.

Now examine the second diagram, when another primary partition is added:

The ARC path would now read:
multi(0) disk(0) rdisk(0) partition(3)
The original primary partition retained its number (1), the new primary partition received the next highest number (2), and the logical drives were all renumbered. The boot partition was on the extended partition. The Boot.ini file should be edited and the ARC path modified to read as above.

Deborah Timmons is a Microsoft Certified Trainer and Microsoft Certified Systems Engineer. She came into the Microsoft technical field after six years in the adaptive technology field, providing technology and training for persons with disabilities. She is the President and co-owner of Integrator Systems Inc.

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