Hardware Reference
In-Depth Information
Older 486 and some early Pentium boards have only the primary connector running
throughthesystem'sPCIlocalbus.Thesecondaryconnectoronthoseboardsusuallyruns
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 enhanced
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.DMAmeansthatthedataistransferreddirectlybetween
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.
There are two distinct types of direct memory access: singleword (8-bit) and multiword
(16-bit). Singleword DMA modes were removed from the ATA-3 and later specifications
and are obsolete. DMA modes are also sometimes called
busmaster
ATA modes because
they use a host adapter that supports busmastering. Ordinary DMA relies on the legacy
DMA controller on the motherboard to perform the complex task of arbitration, grabbing
thesystembusandtransferringthedata.InthecaseofbusmasteringDMA,allthisisdone
by a higher-speed logic chip in the host adapter interface (which is also on the mother-
board).
Systems using the Intel PIIX (PCI IDE ISA eXcelerator) and later South Bridge chips (or
equivalent) can support busmaster ATA. The singleword and multiword busmaster ATA
modes and transfer rates are shown in
Tables 7.6
and
7.7
, respectively.
Table 7.6 Singleword (8-Bit) DMA Modes and Transfer Rates
