Disk Drive Overlays (DDOs) are computer programs that alter apparent hard drive characteristics in order to permit modern drives that are largely incompatible with old BIOSes to be used. They are also known as Dynamic Drive Overlays.

Typically, a DDO will present itself to a BIOS as the Master Boot Record of a modest hard drive whose characteristics are acceptable to older BIOSes -- often as an 83MB Type 17 drive. The DDO will reside in the Master boot record and the following parts of cylinder 0 which is outside the file system of almost all PCs. When the BIOS loads and executes the MBR code. the DDO loads itself into memory. Once resident, the DDO traps all attempts to do BIOS disk IO and emulates a more modern BIOS that supports larger disk drives. A DDO can be used to allow a computer whose BIOS does not support CHS/LBA to use a drive larger than 504MB. DDOs are also used to allow BIOSes that do not support Extended Interrupt 13 to access drives larger than 8.3 GB. DDOs trap all attempts to access the disks including attempts to overwrite themselves. They maintain a virtual Master Boot Record that is alterable and is actually executed once the DDO is in control. Thus DDOs are compatible with programs like FDISK that overwrite the MBR and also with boot manager software.

Essentially, DDOs are tame, socially useful, boot sector computer viruses that do not infect other machines.

While DDOs can work, they present a few problems. In general, they must reside on the boot drive, and only work for that one drive. They require special provisions -- often obtuse e.g. press ctrl-Esc during a two second window -- in order to boot from a floppy if the hard drive is to be accessible from the floppy. They make disk cloning without using a second computer difficult or impossible. They typically allow only one hard drive to be overlaid. They may not be compatible with all operating systems. Diagnosing problems with a DDOed drive can be very confusing. Removing DDOs is not easy. It requires booting a floppy disk without DDO boot support or installing the DDO drive as a second drive. If the DDO is bypassed, the contents of the disk drive will generally be inaccessible.

With operating systems such as Linux that provide their own disk drivers, it is often possible to avoid DDOs by defining the bootable partition as being on a small partition within the disk area that the BIOS supports and mounting the remaining -- unsupported -- partition(s) after the OS is running.

DDOs can be either generic or drive specific. Some drive specific DDOs have traditionally been shipped with disk drives either on a support floppy or on a small partition preinstalled on the hard drive intended to be installed if the user desires to install them. Some DDOs include Drive Express, Drive Overlay, EZBios, EZDrive (Seagate), EZMaker, MaxBlast (Maxtor), OnTrack. There are probably others.


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