Hardware Reference
In-Depth Information
The black (opposite end) connector is known as the
master position
, which is where the primary
drive plugs in. The gray (middle) connector is for slave devices.
To use either the UDMA/33 or the UDMA/66 mode, your ATA interface, drive, BIOS, and cable
must be capable of supporting the mode you want to use. The operating system also must be capable
of handling direct memory access. Windows 95 OSR2 and later versions are ready out of the box, but
older versions of Windows 95 and NT (prior to Service Pack 3) require additional or updated
drivers to fully exploit these faster modes. Contact the motherboard or system vendor for the latest
drivers.
For reliability, Ultra-DMA modes incorporate an error-detection mechanism known as
cyclical
redundancy checking
(CRC). CRC is an algorithm that calculates a checksum used to detect errors in
a stream of data. Both the host (controller) and the drive calculate a CRC value for each Ultra-DMA
transfer. After the data is sent, the drive calculates a CRC value, and this is compared to the original
host CRC value. If a difference is reported, the host might be required to select a slower transfer
mode and retry the original request for data.
ATA/ATAPI-6 (ATA with Packet Interface-6)
ATA-6 includes Ultra-ATA/100 (also called Ultra-DMA or UDMA/100), which increases the Ultra-
ATA burst transfer rate by reducing setup times and increasing the clock rate. As with ATA-5, the
faster modes require the improved 80-conductor cable. Using the ATA/100 mode requires both a
drive and motherboard interface that supports that mode.
Work on ATA-6 began in 2000, and the standard was finished and officially published in 2002 as
“ANSI NCITS 361-2002, AT Attachment - 6 with Packet Interface.”
The major changes or additions in the standard include the following:
• Ultra-DMA (UDMA) Mode 5 added, which allows 100MBps (called UDMA/100, Ultra-
ATA/100, or just ATA/100) transfers.
• Sector count per command increased from 8 bits (256 sectors, or 131KB) to 16 bits (65,536
sectors, or 33.5MB), allowing larger files to be transferred more efficiently.
• LBA addressing extended from 228 to 248 (281,474,976,710,656) sectors, supporting drives
up to 144.12PB (petabytes = quadrillion bytes). This feature is often referred to as 48-bit LBA
or greater than 137GB support by vendors; Maxtor referred to this feature as Big Drive.
• CHS addressing was made obsolete; drives must use 28-bit or 48-bit LBA addressing only.
Besides adding the 100MBps UDMA Mode 5 transfer rate, ATA-6 extended drive capacity greatly,
and just in time. ATA-5 and earlier standards supported drives of up to only 137GB in capacity,
which became a limitation as larger drives were becoming available. Commercially available 3 1/2-
inch drives exceeding 137GB were introduced in 2001, but they were originally available only in
SCSI versions because SCSI doesn't have the same limitations as ATA. With ATA-6, the sector
addressing limit has been extended from 2
28
sectors to 2
48
sectors. What this means is that LBA
addressing previously could use only 28-bit numbers, but with ATA-6, LBA addressing can use
larger 48-bit numbers if necessary. With 512 bytes per sector, this raises the maximum supported
drive capacity to 144.12PB. That is equal to more than 144.12 quadrillion bytes! Note that the 48-bit
addressing is optional and necessary only for drives larger than 137GB. Drives 137GB or smaller
can use either 28-bit or 48-bit addressing.
