Hardware Reference
In-Depth Information
other becomes the slave. Most cables implement this by removing the metal insulation displacement
bit from the pin-28 hole, which can be difficult to see at a glance. Other cables have a section of pin
28 visibly cut from the cable somewhere along the ribbon. Because this is such a minor modification
to the cable and can be difficult to see, cable select cables typically have the connectors labeled
master, slave, and system, indicating that the cable controls these options rather than the drive. All
80-conductor Ultra-ATA cables are designed to use cable select.
With cable select, you simply set the CS jumper on all drives and then plug the drive you want to be
the master into the connector labeled master on the cable and the drive you want to be the slave into
the connector labeled slave.
The only downside I see to using cable select is that it can restrict how the cable is routed or where
you mount the drive that is to be master versus slave because they must be plugged into specific cable
connector positions.
PATA PIO Transfer Modes
ATA-2 and ATA-3 defined the first of several higher-performance modes for transferring data over
the PATA interface, to and from the drive. These faster modes were the main part of the newer
specifications and were the main reason they were initially developed. The following section
discusses these modes.
The PIO (programmed I/O) mode determines how fast data is transferred to and from the drive using
PIO transfers. In the slowest possible mode—PIO Mode 0—the data cycle time can't exceed 600
nanoseconds (ns). In a single cycle, 16 bits are transferred into or out of the drive, making the
theoretical transfer rate of PIO Mode 0 (600ns cycle time) 3.3MBps, whereas PIO Mode 4 (120ns
cycle time) achieves a 16.6MBps transfer rate.
Most motherboards with ATA-2 or greater support have dual ATA connectors on the motherboard.
Most of the motherboard chipsets include the ATA interface in their South Bridge components, which
in most systems is tied into the PCI bus.
Older 486 and some early Pentium boards have only the primary connector running through the
system's PCI local bus. The secondary connector on those boards usually runs through the ISA bus
and therefore supports up to Mode 2 operation only.
When interrogated with an
Identify Drive
command, a hard disk returns, among other things,
information about the PIO and DMA modes it is capable of using. Most BIOSs automatically set the
correct mode to match the capabilities of the drive. If you set a mode faster than the drive can handle,
data corruption results.
ATA-2 and newer drives also perform Block Mode PIO, which means they use the Read/Write
Multiple commands that greatly reduce the number of interrupts sent to the host processor. This
lowers the overhead, and the resulting transfers are even faster.
PATA DMA Transfer Modes
ATA drives support two types of transfers: programmed input/output (PIO), and direct memory
access (DMA) transfers. DMA means that the data is transferred directly between drive and memory
without using the CPU as an intermediary, as opposed to PIO. This offloads much of the work of
transferring data from the processor, in effect allowing the processor to do other things while the
transfer is taking place. DMA transfers are much faster than PIO transfers and are supported by all
modern ATA devices.
