Intel
®
81341 and 81342—Application DMA Unit
Intel
®
81341 and 81342 I/O Processors
Developer’s Manual
December 2007
500
Order Number: 315037-002US
5.7.5
P+Q Update Operation
The P+Q Update operation generates new versions of the P and Q check data blocks
when a single block write occurs to a data storage subsystem that is protected by P+Q
Figure 65, “The P+Q Update Algorithm” on page 501
describes the P+Q Update
operation implementation. In this illustrative example, an old data block (SAR0), a new
data block (SAR1), an old P check data block (SAR2), and an old Q check data block
(SAR3) are used to generate a new P check data block and a new Q check data block.
The intermediate result is kept in the ADMA P and Q result buffers before being written
back to local memory. The source data is located at addresses 0 A000 0400H, 0 A000
0800H, 0 A000 0C00H and 0 A000 1000H respectively.
All data transfers needed for this operation are controlled by chain descriptors located
in local memory. The Application DMA as a master on the internal bus and through its’
dedicated DDR SDRAM MCU port initiates the data transfer. The algorithm is
implemented such that as data is read from local memory, the ADMA executes the XOR
operation to generate new P, and applies the GF Multiply function to the incoming
source data followed by the XOR operation to generate new Q.
Only a single descriptor is required and generates both check values. Each descriptor is
processed as illustrated in
Note:
Optionally, the user may disable the generation of new P while generating new Q.