When it became clear that improvements in harddrive technology and the
increasing demands made by software would eventually strain the
capacity of the ATA interface, the drive industry, in the form of the
Small Form Factor (SFF) Committee, created a compatible extension of
ATA called ATA-2. This standard not only adds faster PIO
modes and DMA modes, but also improves upon Plug'n'Play and
compatibility with future revisions of the standard.
While there is also a new way of addressing sectors on the harddisk
(LBA), this is merely a simplification. Contrary to common myth LBA
proper has nothing to do with breaking the famous 504MB (528 million
bytes) barrier. In fact, even in the old ATA/IDE standard the capacity
limit is well over 100GB.
The latest revision of the standard is ATA-3. Key features include:
improved reliability, especially of PIO mode 4; a simple password-based
security scheme; more sophisticated power management; and Self
Monitoring Analysis and Report Technology (S.M.A.R.T.), allowing the
drive to warn you about certain types of impending failure.
ATA-3 does not define any faster modes. Even though some manufacturers
were once marketing "mode 5" equipment, there will never be a PIO mode
beyond ATA-2 PIO mode 4.
One of the disadvantages of ATA is that it was designed for harddisks
only. That was fine back when a high end PC shipped with just a floppy
drive and a 40MB harddisk, but today CD-ROM and tape drives are
commonplace devices that should preferably run off a single low-cost
interface. The ATA Packet Interface (ATAPI) is a standard designed
for devices such as CD-ROMs and tape drives that plug into an ordinary ATA
(IDE) port.
The principal advantage of ATAPI hardware is that it's cheap and works on
on every PC with an IDE or 'EIDE' adapter. ATAPI tape drives can enjoy
superior performance and reliability compared to the popular
QIC117 'floppy' tape devices.
Beware that although ATAPI devices plug into the IDE interface, they
differ considerably from an IDE harddisk. Caching controllers and other
intelligent interfaces will not work unless they're ATAPI aware. Booting
from an ATAPI CD-ROM is only possible with the latest BIOSes.
Work on the next standard, predictably called ATA-4, has already
started. First and foremost it is a much-needed attempt to merge ATA-3 and
ATAPI into one. Faster transfer modes are also on the agenda, of course:
it will incorporate Ultra-ATA's DMA/33, at least.
Regarding other features, the future for strong command overlap looks
rather bleak: the current proposal is a hideous animal and important
players like Microsoft do not plan on supporting it in its present
form. More limited forms of overlap stand a somewhat better chance of
surviving.
A rather different issue was the 504MB (or, equivalently, 528 million
bytes) capacity limit that becomes apparent when accessing IDE drives
through the BIOS. It is caused by the disk geometry (cylinders, heads,
sectors) supported by the combination of an IDE drive and the
BIOS' software interface ('int13'). Both IDE/ATA and the BIOS
are capable of supporting huge disks, but their combined limitations
conspire to restrict the useful capacity of the drive to 504MB. Since
only MSDOS still uses the BIOS for harddisk access, this is sometimes
erroneously thought to be an MSDOS limitation; other operating systems
experience the same restrictions at boot time though.
An Enhanced BIOS works around this problem by representing the
drive to the software using a different geometry than the native
geometry of the drive itself. This juggling act is called 'translation'.
For example, if your drive has 1500 cylinders and
16 heads, a translating BIOS will make software programs think that
the drive has 750 cylinders and 32 heads.
You do not need an "EIDE" adapter to use harddisks greater than
504MB.
The de facto standard is described in the Microsoft/IBM "INT 13
Extensions" document. Phoenix has presented a superset in their
"Phoenix Enhanced BIOS" specification. Phoenix, AMI, Award and MR
BIOSes are based on the Microsoft/IBM specification.
Last but not least, the usual limit of two devices was far too
restrictive if CD-ROMs and tape drives were to be connected to the IDE
interface in addition to the harddisk(s). Fortunately the solution was
already known in the form of a so-called secondary harddisk
interface. The possibility of such an interface has been there for a
long time, but support was lacking.
There is nothing special about a secondary channel; it is an ordinary
(E)IDE port that uses a different interrupt and I/O addresses to avoid
clashes with the ordinary (primary) one. A secondary interface
allows you to connect another ribbon cable with two more ATA devices
(harddisk, ATAPI CD-ROM or ATAPI tape). Today, many interfaces
combine both primary and secondary port on a single board to make a
dual-ported interface that handles up to four devices.
To use harddisks on the secondary port with DOS and Windows 3.x, you
will need BIOS (either system BIOS or adapter BIOS) or driver
support. You can recognize a BIOS with four drive support by the fact
that it allows for four sets of drive parameter in the BIOS setup.
There are two further (semi-)standard channels beyond the secondary
port: the tertiary and quaternary ones. Some soundcard IDE
interfaces can be configured as tertiary or quaternary. See section
10.4
for the I/O and IRQ assignments.
Software support for these is still rare.
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