Understanding the BIOS 1024 Cylinder Limit

The BIOS 1024 cylinder limitation exists because the start and end cylinder values in the partition table (and some BIOSs) have a maximum value of 1024. Because some operating systems such as DOS 6.22 use the CHS (Cylinder, Head, and Sector) values to address sectors on the disk, they cannot access sectors beyond the 1024 cylinder. When you start your computer the BIOS boots the operating system using the CHS values to locate the first sector of the bootable partition. If the partition starts past the 1024 cylinder, the BIOS may not be able to boot it because it cannot address a cylinder number higher than 1024.

With PartitionMagic, you can safely partition any drive, regardless of the number of cylinders on the drive. In fact, to prevent you from performing partition operations that might cause problems, PartitionMagic is careful to observe the BIOS 1,024 cylinder limit on computer systems where one or more of the following applies:

?The hard disk has a capacity 504 MB or more and the BIOS translation mode is set to Normal or CHS.
?The hard disk has a capacity 504 MB or more and the BIOS was manufactured prior to 1994 (approximately).
?The hard disk has a capacity 8 GB or more.

If you have such a computer system and you use only DOS, neither the DOS FDISK utility nor PartitionMagic lets you see cylinders beyond the 1,024th cylinder or include them in any partition. Space beyond the 1,024th cylinder always remains invisible.
Even if the BIOS 1,024 cylinder limit applies to your system, you can use PartitionMagic without difficulty. The only instance where you may encounter a problem is if all the following criteria apply:

?You use DOS and another OS.
?The other OS can "see" and use disk space past the first 1,024 cylinders on the disk.
?You use PartitionMagic or the FDISK utility of the other OS to create a partition extending beyond the 1,024th cylinder.
?You then run the DOS PartitionMagic executable.

When you run the DOS PartitionMagic executable, you may or may not be able to see the newly created partition that extends beyond the 1,024 cylinder limit. Even if you can see the partition, you cannot use the DOS PartitionMagic executable to perform any operations on that partition. This restriction applies to both primary and extended partitions that contain space beyond the 1,024th cylinder. If an extended partition exceeds the cylinder limit, you cannot perform operations on any one of the contained logical partitions, even if the logical partition itself does not extend past the 1,024th cylinder.

The disk map in the PartitionMagic main window displays an arrow indicator at the 1024 cylinder boundary (and the 2 GB boot boundary), so you can see where your partitions are located relative to the 1024 cylinder limit. Be sure that all OS partitions on a disk start prior to cylinder 1024. This ensures that you can boot the OS. Also, use caution when moving a bootable partition; if the partition is moved beyond cylinder 1024 it may no longer be bootable. To fix this problem you can move the partition below the 1024 cylinder marker.

If you use DOS, but your system has the INT 13 BIOS extensions, PartitionMagic lets you see cylinders past the normal 1,024 limit and perform operations on partitions extending beyond this boundary.

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