Understanding the BIOS 1,024 Cylinder Limit

For understanding the BIOS 1,024 Cylinder Limit, You can safely use PartitionMagic or VolumeManager to 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, the products are careful to observe the BIOS 1,024 cylinder limit on systems where it applies.

The BIOS 1,024 cylinder limit pertains to your system only if:

  • You have a hard disk with a capacity of more than 504 MB.
  • The hard disk was manufactured before (approximately) 1994.
  • The BIOS in your system does not have the INT 13 BIOS extensions built in.

You may also encounter problems if your hard disk is larger than 8 GB.

If you have such a system and you use only DOS, neither the DOS FDISK utility nor any PowerQuest product allows you to 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

however,

you can use PartitionMagic without difficulty. The only instance where you may encounter problems is if all the following criteria apply:

  • you use both DOS and another OS;
  • the other OS is able to see and use disk space past the first 1,024 cylinders of 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.

TIP: 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.

Understanding the 2 GB Boot Code Boundary

An OS’s boot code is stored in both the master boot record (MBR) and the partition boot record, enabling the OS to boot properly. In some OSs, however, this boot code is written in such a way that it inadvertently imposes a limit on the location of both the partition boot record and the files needed to boot the OS.

DOS (versions 6.x and earlier) and Windows NT (versions 4.0 and earlier), and Windows 2000 are both affected by this boot code limitation. When booting these two operating systems, the cylinder-head-sector (CHS) address of the beginning boot code sector must be calculated in order to retrieve the sector’s information and load and execute the next part of the boot process. The CHS value for the needed sector is calculated as follows:

Sector Number / Sectors Per Track

Because of the way the boot code is written, the product of this calculation must fit in a 16-bit register. The largest value a 16-bit register can contain is 64K. If the number is larger than 64K, the number is truncated, resulting in an incorrect value that skews the remaining calculations. The boot process fails to load and execute the needed sector, thus preventing the OS from booting.

Most current hard disks have 63 sectors per track, creating a 64K boot code boundary at 2 GB.

If your disk is older or uses drive overlay software, this boundary may be lower.

If a partition begins or extends beyond this boundary, the CHS value of the partition’s boot code sector cannot be correctly calculated; therefore, the partition and its OS cannot boot.

This same limit applies to the DOS IO.SYS file and the Windows NT file,

NTLOADER.EXE. If either of these files are install or move beyond the 64K boot code boundary, the corresponding OS fails to boot.

Image result for BIOS 1,024 Cylinder

IMPORTANT! To boot properly, DOS requires that the first three sectors of IO.SYS are below the 2 GBboot code boundary.

Both the IO.SYS and NTLOADER.EXE files are usually locates near the beginning of the partition in which they are install. When you resize partitions using PartitionMagic, Drive Image, and VolumeManager,

this area of the partition may be vacate to make room for a larger FAT or other file system structures. As a result, IO.SYS or NTLOADER.EXE may be moved beyond the 64K boundary, thus preventing the OS from booting.

Changing the BIOS LBA Mode Setting

WARNING! Never change the LBA mode in your system BIOS once data is present on any hard disk. Changing this setting may cause data corruption and loss.

Most modern system BIOS designs support LBA or Logical Block Addressing. The LBA mode setting, whether enabled or disabled on your system, determines how your computer translates logical cylinder-head-sector (CHS) addresses. If you change this setting, the resulting shift in CHS values may corrupt all the files and partitions on your hard disks.

If you must change the LBA mode setting in your system BIOS, first back up all data on your hard disks. Contact the BIOS and/or disk manufacturer’s technical support departments to ensure you understand how to proceed safely.

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