The system and boot partitions on IDE hard disks are also limited to 1024 cylinders if the controller does not translate the disk address. Secondary disks that do not contain the system or boot partition might not be limited to 1024 cylinders, depending on the translation scheme used by the controller and disk.
IDE disks use a different data structure for representing the number of cylinders, number of heads, and number of sectors per track than the Partition Table and BIOS INT 13 interface. According to the IDE specifications, the maximum number of cylinders is 65536, the maximum number of heads is 16, and the maximum number of sectors per track is 255. The maximum size is 136.9 GB. Because the BIOS/IDE interface has been constrained to use the lowest common denominator, the result is a limit of about 504 MB for an IDE disk on an Intel-based ISA bus computer.
There are new controllers and disk device drivers that use an Enhanced Disk Parameter Table to translate between the logical sector layout internal to the IDE disk and the logical sector layout addressable by the BIOS INT 13 interface. These disk device drivers and devices can support larger disks. These disks and controllers are referred to as enhanced IDE, or EIDE, devices.