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MS OS Partition Information

Date: March 20, 2003

OS	Boots from	Partition Types	Boot Code Boundary	OS Space Required
DOS 6.2	Primary	FAT	2 GB	8 MB
Windows 95a	Primary	FAT	2 GB	90 MB
Windows 95b (OSR2)	Primary	FAT or FAT32	8 GB	90 MB
Windows 98	Primary	FAT or FAT32	8 GB	175 MB
Windows 98SE	Primary	FAT or FAT32	8 GB**	190 MB
Windows ME	Primary	FAT or FAT32	8 GB**	300 MB
Windows NT	Primary*	FAT or NTFS	2 GB	120 MB
Windows 2000	Primary*	FAT, FAT32 or NTFS	8 GB**	650 MB
Windows XP	Primary*	FAT, FAT32 or NTFS	8 GB**	>2 GB
		More about types...
* WNT, W2K, WXP must boot from a primary partition on the first hard drive (the system drive); however, only a few files must reside on that partition. The remaining system files may reside on another partition (the boot drive.) To find an operating system, the PC will look in the partition table to determine the active partition, and it will look on that partition to find an operating system. Hence, the first "drive" accessed is the system drive for any operating system. WNT, W2K, and WXP may continue booting the operating system from the system drive if the files required for booting the operating system are found on that drive. In this case, the system drive is also the boot drive. However, these OS's, from information found on the system drive, can be instructed to look on another partition/drive to find the files needed to complete the OS boot process, and hence, in this case, that drive is known as the boot drive. The remaining OS boot files can be on another primary partition, or on a logical drive in an extended partition, on the first or other hard drive.
** The boot code boundary limit can be exceeded on a hard drive with an LBA-compatible MBR.
See Partitioning 101 by K9MkII (Bill) for descriptive information about what a partition table does for the OS, where it is located, and why it is needed as well as information about entries in boot.ini..

A partition table must have at least one primary partition. The primary partition has a defined location in hardware at a specific block of a disk. A partition table permits 4 primary partitions, or 3 primary partitions and 1 extended partition at maximum. Only 1 extended partition can be established. Multiple logical drives can be established in a partition. Only one primary partition, at a time, can be designated active.

A partition can only be created in "free space", aka unpartitioned space. Partitions under 2 GB are partitioned as FAT 16 by default. The minimum size FAT16 partition is 32 MB. If you choose to enable Large Disk Support under Win 95b (OSR2), Win 98, Win 98SE, or WinME, then partitions larger than 512 MB will be formatted as FAT32.

During OS installation, you may want to hide other partitions from the install process if you are installing multiple operating systems. Microsoft Operating systems see the active partition as Drive C. DOS based operating systems (Windows 3.1, Windows 9X) can only recognize one primary partition per disk drive, and cannot recognize more than 23 logical drives. Microsoft operating systems cannot boot from a logical drive.

Most OS's support the concept of an extended partition. The entry in the partition table for an extended partition is a special type. There are two of these. Microsoft added a second type for "Large Disk Support." An extended partition is not located at a block of a disk specified by a standard. The entry in the primary partition table for an extended partition contains a pointer to an unspecified logical partition table (LPT). The construction for an LPT is similar to the MBR. Each LPT has two entries, the start of the logical partition associated with the LPT and a pointer to the next LPT. Logical partitions are daisy-chained and all elements of the daisy chain are contained within the extended partition. A extended partition creates slices within a primary partition.

Partitions should end on cylinder boundaries (except for the first partition.) The order of entries in the partition table should match the order of partition tables on a disk. CHS values should match absolute sector values if under 8 GB and should be set to max values otherwise. The MBR is on the disk's first sector at CHS 0/0/1 and the first partition usually starts at CHS 0/1/1 (sector 63), so the rest of the sectors on the first track are normally vacant.

Each partition table entry is 16 bytes. For CHS drives the start and end location of a partition is stored and for LBS, the start and size in sectors is stored. Also the partition type and the "active" partition flag is stored there. Versions of FDISK that do not support large drives will incorrectly compute LBA values. There is an FDISK update for Windows 98 available from Microsoft. See Microsoft KB 245213.