|Main Page > Motley > MicroHouse PC Hardware Library Volume I: Hard Drives|
|Previous||Table of Contents||Next|
To protect the skew factors and defect information on intelligent IDE drives, all you have to do is run them in translation mode. In translation mode, this information cannot be overwritten.
Intelligent Zoned Recording IDE
The last and most sophisticated IDE drives combine intelligence with zoned recording. With zoned recording, the drive has a variable number sectors per track in several zones across the surface of the drive. Because the PC BIOS can handle only a fixed number of sectors on all tracks, these drives always must run in translation mode. Because these drives are always in translation mode, you cannot alter the factory-set interleave and skew factors or wipe out the factory defect information.
You still can low-level format these drives, however, and use such a format to map or spare additional defective sectors that crop up during the life of the drive.
ATA-2 (Enhanced IDE)
ATA-2 is an extension of the original ATA (IDE) specification. The most important additions are performance enhancing features such as fast PIO and DMA modes. ATA-2 also features improvements in the Identify Drive command allowing a drive to tell the software exactly what its characteristics are; this is essential for both Plug and Play and compatibility with future revisions of the standard.
ATA-2 is often called Enhanced IDE (or EIDE). EIDE is technically a marketing program from Western Digital. Fast-ATA and Fast-ATA-2 are similar Seagate-inspired marketing programs, which are also endorsed by Quantum. As far as the hard disk and BIOS are concerned, these are all different terms for basically the same thing.
There are four main areas where ATA-2 and EIDE have improved the original ATA/IDE interface:
The following section describes these improvements.
Increased Drive Capacity
ATA-2/EIDE allows for increased drive capacity over the original ATA/IDE specification. This is done through an Enhanced BIOS, which makes it possible to use hard disks exceeding the 504M barrier. The origin of this limit is the disk geometry (cylinders, heads, sectors) supported by the combination of an IDE drive and the BIOS' software interface. Both IDE and the BIOS are capable of supporting huge disks, but their combined limitations conspire to restrict the useful capacity to 504M.
An Enhanced BIOS circumvents this by using a different geometry when talking to the drive than when talking to the software. What happens in between is called translation. For example, if your drive has 2,000 cylinders and 16 heads, a translating BIOS will make programs think that the drive has 1,000 cylinders and 32 heads.
You can usually tell if your BIOS is enhanced by the ability to specify more than 1,024 cylinders in the BIOS setup, although this is not conclusive. If you see drive-related settings like LBA, ECHS, or even Large, these are tell-tale signs of a BIOS with translation support. Most BIOSs with a date of 1994 or later are enhanced. If your system currently does not have an Enhanced BIOS, you may be able to get an upgrade.
There are roughly three ways today's BIOSs can handle translation: standard CHS addressing, Extended CHS addressing, and LBA addressing. They are summarized in the following table:
|BIOS Mode||Operating System to BIOS||BIOS to Drive Ports|
|Standard||Logical CHS Parameters||Logical CHS Parameters|
|Extended CHS||Translated CHS Parameters||Logical CHS Parameters|
|LBA||Translated CHS Parameters||LBA Parameters|
In Standard CHS, there is only one possible translation step internal to the drive. The drive's actual, physical geometry is completely invisible from the outside with all zoned recorded ATA drives today. The cylinders, heads, and sectors printed on the label for use in the BIOS setup are purely logical geometry, and do not represent the actual physical parameters. Standard CHS addressing is limited to 16 heads and 1,024 cylinders, which gives a limit of 504MB.
This is often called "Normal" in the BIOS setup, and causes the BIOS to behave like an old-fashioned one without translation. Use this setting if your drive has fewer than 1,024 cylinders or if you want to use the drive with a non-DOS operating system that doesn't understand translation.
In Extended CHS, a translated logical geometry is used to communicate between the drive and the BIOS, while a different translated geometry is used to communicate between the BIOS and everything else. In other words, there are normally two translation steps. The drive still translates internally, but has logical parameters that exceed the 1,024 cylinder limitation of the standard BIOS. In this case, the drive's cylinder count is usually divided by 2, and the head count is multiplied by 2 to get the translated values from those actually stored in the CMOS Setup. This type of setting breaks the 504/528MB barrier.
This is often called "Large" or "ECHS" in the BIOS setup, and tells the BIOS to use Extended CHS translation. It uses a different geometry (cylinders/heads/sectors) when talking to the drive than when talking to the BIOS. This type of translation should be used with drives that have more than 1,024 cylinders but that do not support LBA (Logical Block Addressing). Note that the geometry entered in your BIOS setup is the logical geometry, not the translated one.
LBA is a means of linearly addressing sector's addresses, beginning at Cylinder 0, Head 0, Sector 1 as LBA 0, and proceeding on to the last physical sector on the drive. This is new in ATA-2, but has always been the one and only addressing mode in SCSI.
With LBA, each sector on the drive is numbered starting from 0. The number is a 28-bit binary number internally, which translates to a sector number of from 0 to 268,435,456. Because each sector represents 512 bytes, this results in a maximum drive capacity of exactly 128GB, or 137 billion bytes. Unfortunately, the operating system still needs to see a translated CHS, so the BIOS determines how many sectors there are, and comes up with Translated CHS to match. The BIOS CHS limits are 1,024 cylinders, 256 heads, and 63 sectors per track, which limits total drive capacity to just under 8GB.
|Previous||Table of Contents||Next|