![](http://datasheet.mmic.net.cn/330000/INTEL82801_datasheet_16416407/INTEL82801_119.png)
82801AA and 82801AB Datasheet
5-69
Functional Description
When the Bus Master IDE controller is reading data from the memory regions, bit 1 of the Base
Address is masked and byte enables are asserted for all read transfers. The controller reads to a
boundary of 64 bytes, regardless of byte count field of the PRD. Only the byte count value is
transferred to the drive however. When writing data, bit 1 of the Base Address is not masked and if
set, causes the lower Word byte enables to be deasserted for the first DWord transfer. The write to
PCI typically consists of a 32-byte cache line. If valid data ends prior to end of the cache line, the
byte enables are deasserted for invalid data.
The total sum of the byte counts in every PRD of the descriptor table must be equal to or greater
than the size of the disk transfer request. If greater than the disk transfer request, the driver must
terminate the bus master transaction (by setting bit 0 in the Bus Master IDE Command Register to
0) when the drive issues an interrupt to signal transfer completion.
5.14.2.2
Line Buffer
A single line buffer exists for the ICH Bus master IDE interface. This buffer is not shared with any
other function. The buffer is maintained in either the read state or the write state. Memory writes
are typically 4-DWord bursts and invalid DWords have C/BE[3:0]#=0Fh. The line buffer allows
burst data transfers to proceed at peak transfer rates.
The Bus Master IDE Active bit in Bus Master IDE Status register is reset automatically when the
controller has transferred all data associated with a Descriptor Table (as determined by EOT bit in
last PRD). The IDE Interrupt Status bit is set when the IDE device generates an interrupt. These
events may occur prior to line buffer emptying for memory writes. If either of these conditions
exist, all PCI Master non-memory read accesses to ICH are retried until all data in the line buffers
has been transferred to memory.
5.14.2.3
Bus Master IDE Timings
The timing modes used for Bus Master IDE transfers are identical to those for PIO transfers. The
DMA Timing Enable Only bits in IDE Timing register can be used to program fast timing mode for
DMA transactions only. This is useful for IDE devices whose DMA transfer timings are faster that
its PIO transfer timings. The IDE device DMA request signal is sampled on the same PCI clock
that DIOR# or DIOW# is deasserted. If inactive, the DMA Acknowledge signal is deasserted on
the next PCI clock and no more transfers take place until DMA request is asserted again.
Figure 5-12. Physical Region Descriptor Table Entry
051910_3.drw
Byte 3
Byte 2
Byte 1
Byte 0
EOT
Reserved
Byte Count [15:1]
Memory Region Physical Base Address [31:1]
Memory
Region
Main Memory
0
0