The BIOS Int 13h CHS barrier

   BIOS Int 13h   255 Heads   Drive CHS  

Maximum Cylinders   1024  1024  65536
Maximum Heads   256  255  16
Max. Sectors/track   63  63  256

Maximum Capacity   7.88GB  7.84GB  128GB

As the BIOS Int 13h column shows:
1024 Cylinders x 256 Heads x 63 Sectors x 512 Bytes/Sector
= 8455716864 Bytes
So the theoretical BIOS Int 13h limit is 7.88GB (8.46 billion Bytes).

However, because DOS and other older operating systems cannot handle a 256th Head, this becomes:
1024 Cylinders x 255 Heads x 63 Sectors x 512 Bytes/Sector
= 8422686720 Bytes
Thus the "real world" BIOS Int 13h limit of 7.84GB (8.42 billion Bytes).

    7.88GB is the limitation for how much drive space the BIOS Int 13h system call can access, using a CHS address (L-CHS). This is a BIOS specification limitation, and not a hardware or ATA (P-CHS) limitation. And while the BIOS can access the first 7.84 to 7.88GB on a larger capacity hard drive, all space beyond that should be accessed using LBA addressing. This requires a BIOS supporting the Extended Int 13h functions, and an operating system that can use LBA addressing; or an operating system that can bypass the BIOS by directly accessing a drive.


NOTE: Some SCSI controllers (Adaptec?) Will add Enhanced or Extended Int 13h functions to a system BIOS that does not provide them. And while this is intended for the SCSI drives, these are the same functions that are used by EIDE or ATA drives. (Just be aware that this additional BIOS code may not be loaded if there is no appropriate SCSI device connected to the controller when the system boots.) Otherwise, a product like the Micro Firmware ATA Pro BIOS add-in cards can be used to provide Extended Int 13h functions to a BIOS that does not have them.


    For a multi-boot computer, using a large hard drive, any "legacy" operating system that uses CHS drive addressing as part of the boot process, may require that all files to load the operating system are kept below this BIOS Int 13h (7.84GB or 7.88GB) limit. See the 1024 Cylinder limit in the XOSL HOWTOs and FAQs for more details about that.

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Page Content Updated: 12 June 2005