ATA Features

The ATA standards have gone a long way toward eliminating incompatibilities and problems with interfacing IDE drives to ISA/PCI bus systems. The ATA specifications define the signals on the 40-pin connector, the functions and timings of these signals, cable specifications, and so on. The following sections cover ATA commands; the host protected area used for system recovery software; and ATAPI interfacing for optical, removable-media, and tape drives.

ATA Commands

One of the best features of the ATA interface is the enhanced command set. The ATA interface was modeled after the WD1003 controller that IBM used in the original AT system. All ATA drives must support the original Western Digital (WD) command set (eight commands), with no exceptions, which is why ATA drives are so easy to install in systems today. All IBM-compatible systems have built-in ROM BIOS support for the WD1003, so they essentially support ATA as well.

In addition to supporting all the WD1003 commands, the ATA specification added numerous other commands to enhance performance and capabilities. These commands are an optional part of the ATA interface, but several of them are used in most drives available today and are very important to the performance and use of ATA drives in general.

Perhaps the most important is the Identify Drive command. This command causes the drive to transmit a 512-byte block of data that provides all details about the drive. Through this command, any program (including the system BIOS) can find out exactly which type of drive is connected, including the drive manufacturer, the model number, the operating parameters, and even the serial number of the drive. Many modern BIOSes use this information to automatically receive and enter the drive's parameters into CMOS memory, eliminating the need for the user to enter these parameters manually during system configuration. This arrangement helps prevent mistakes that can later lead to data loss when the user no longer remembers what parameters he or she used during setup.

The Identify Drive data can tell you many things about your drive, including the following:

• The number of logical block addresses available, using LBA mode

• The number of physical cylinders, heads, and sectors available in P-CHS mode

• The number of logical cylinders, heads, and sectors in the current translation L-CHS mode

• The transfer modes (and speeds) supported

• The manufacturer and model number

• The internal firmware revision

• The serial number

• The buffer type/size, indicating sector buffering or caching capabilities

Two other important commands are Read Multiple and Write Multiple. These commands permit multiple-sector data transfers and, when combined with block-mode PIO capabilities in the system, can result in incredible data transfer rates that are many times faster than single-sector PIO transfers. Some older systems require you to select the correct number of sectors supported by the drive, but most recent systems automatically determine this information for you. Note that some BIOSs refer to Read Multiple as "block mode."

Many other enhanced commands are available, and there is even room for a given drive manufacturer to implement vendor-unique commands. Certain vendors often create such commands for their own unique features. Often, vendor-unique commands control features such as low-level formatting and defect management. This is why low-level format (LLF) programs can be so specific to a particular manufacturer's ATA drives and why many manufacturers make their own LLF programs available.

The Host Protected Area

Most PCs sold today include some form of automated product recovery or restoration feature that allows a user to easily restore the operating system and other software on the system to the state it was in when the system was new. Originally, this was accomplished via one or more product recovery CDs containing automated scripts that reinstalled all the software that came preinstalled on the system when it was new.

Unfortunately, the CDs could be lost or damaged, they were often problematic to use, and including them by default cost manufacturers a lot of money. This prompted PC manufacturers to move the recovery software to a hidden partition of the boot hard drive. Although this wastes some space on the drive, the recovery software normally fits on from one to four CDs, which occupies 1GB3GB of drive space. With 60GB or larger drives, this amounts to 5% or less of the total space. Still, storing this information on the hidden partition was less than satisfactory because the partition could easily be damaged or overwritten by partitioning software or other utilities, so there was no way to make it secure.

In 1996, Gateway proposed a change to the ATA-4 standard under development that would allow a space called the host protected area (HPA) to be reserved on a drive. This change was ratified, and the HPA feature set was incorporated into the ATA-4 specification that was published in 1998. A separate BIOS firmware interface specification called Protected Area Run Time Interface Extension Services (PARTIES) was initiated in 1999 that defined services an operating system could use to access the HPA. The PARTIES standard was completed and published in 2001 as NCITS 346-2001, "Protected Area Run Time Interface Extension Services."

The HPA works by using the optional ATA SET MAX ADDRESS command to make the drive appear to the system as a slightly smaller drive. Anything from the new max address (that is, the newly reported end of the drive) to the true end of the drive is considered the HPA and is accessible only using PARTIES commands. This is more secure than having a hidden partition because any data past the end of the drive simply cannot be seen by any normal application, or even a partitioning utility such as PartitionMagic or Partition Commander. Still, if you want to remove the HPA, you can use some options in the BIOS Setup or separate commands to reset the max address, thus exposing the HPA. At that point, you can run a utility such as PartitionMagic or Partition Commander to resize the adjacent partition to include the extra space that was formerly hidden and unavailable.

Most new systems using Phoenix FirstBIOS come with their recovery software and diagnostics in the HPA because this is part of the new Phoenix FirstBIOS core managed environment (CME), which is used by a large number of OEMs (including IBM) on most desktop and laptop systems starting in 2003.

ATAPI

The ATAPI standard is designed to provide the commands necessary for devices such as CD-ROM and DVD drives, removable media drives such as SuperDisk and Zip drives, and tape drives that plug in to an ordinary ATA (IDE) connector. The principal advantage of ATAPI hardware is that it's cheap and works with your current adapter. All modern ATA CD-ROM drives support the ATAPI protocols, and generally the terms ATA and ATAPI are synonymous. In other words, an ATAPI CD-ROM is an ATA CD-ROM and vice versa.

Caution

Starting in 1998, most systems began supporting the Phoenix El Torito specification, which enables booting from ATAPI CD or DVD drives. Systems without El Torito support in the BIOS can't boot from an ATAPI CD or DVD drive. Even with ATAPI support in the BIOS, you must still load a driver to use ATAPI under DOS or Windows; an ATAPI driver is supplied with all recent Windows and Linux versions. Most recent server operating systems, including Windows 2000 Server and Windows Server 2003, include bootable CDs for easy installation.

You should normally keep ATA devices you will be accessing simultaneously on separate channels. Because ATA does not typically support overlapping access, when one drive is being accessed on a given channel, the other drive on the same channel can't be accessed. By keeping the CD-ROM or DVD drive and hard disk on separate channels, you can more effectively overlap accessing between them.

This is easy to do if your server uses SCSI or SATA hard disks for storage and has a single CD or DVD drive. The CD or DVD drive is the only ATA device in the system, so it cannot interfere with hard disk storage. However, this can be trickier on a server that uses ATA drives for both hard disk storage and optical drives. In such cases, you need to be sure to use one ATA host adapter for the hard disk and a separate one for the CD or DVD drive. If you are using PATA drives in a RAID array, they are connected to a special ATA host adapter designated as a RAID adapter. You can use the non-RAID ATA adapter for the CD or DVD drive.

If you must put a PATA hard disk and CD or DVD drive on the same cable, you should set PATA hard drives as masters (Device 0) and parallel ATAPI drives as slaves (Device 1).