Intel
®
81341 and 81342—Application DMA Unit
Intel
®
81341 and 81342 I/O Processors
Developer’s Manual
December 2007
472
Order Number: 315037-002US
5.3.3
P+Q Chain Descriptor Format
The P+Q chain descriptor can be used to perform an P+Q-transfer while performing a
Galois field multiply function on the all of the source data streams; this operation is
referred to as a P+Q-transfer. See
Section 5.7.3, “XOR Operation with P+Q RAID-6” on
for more details.
To perform a transfer, one or more chain descriptors must first be written to memory.
shows the format of an individual P+Q chain descriptor. Every P+Q chain
descriptor requires a minimum of eight contiguous words in the local memory and is
required to be aligned on a 32-byte address boundary. While a minimum of eight words
are required, a maximum of 38 words may be used to perform the P+Q function on up
to 16 source data streams.
Note:
The minimum size (number of source = 1) P+Q chain descriptor is only useful for
performing a Galois field multiply on a single source data stream and writing it to the Q
Destination Address (e.g., ‘P Transfer Disable’ bit in the ADCRx is set).
Warning:
The hardware requires that bits 3 down to 0 of the Q_Destination Address (Word 2 and
5) and the P_Destination Address (Word 4 and 5) are programmed to the same four-bit
value. Furthermore, the hardware requires this even if the
P Transfer Disable
bit is
set to disable P transfer to the P_Destination Address. For example, in this case the
P_Destination Address may contain an invalid address such that bits 3 down to 0 of
that address equals to bits 3 down to 0 of the Q_Destination Address.