User`s guide

3.4.2 memData Bus
With a memData bus of 128 bits, four 21071-BA chips are required, that is:
• 21071-BA0 connects to longword 0 (memdata<31:0>)
• 21071-BA1 connects to longword 1 (memdata<63:32>)
• 21071-BA2 connects to longword 2 (memdata<95:64>)
• 21071-BA3 connects to longword 3 (memdata<127:64>)
3.4.3 epiData Bus
Each 21071-BA has 32 epiData bus pins. The epiData pins of the 21071-BA
chips are tied together to form a 32-bit epiData bus.
3.4.4 Memory Read Buffer
The memory read buffer stores data that is read from memory before it is
returned to the CPU on the sysBus or to DMA in the DMA read buffer.
Each 21071-BA stores four longwords of data and corresponding parity bits
in the memory read buffer. The 4-chip system contains an additional eight
longwords of storage; however, the extra storage is not usable.
3.4.5 I/O Read Buffer and Merge Buffer
On CPU-initiated memory transactions, the buffer performs the merge buffer
functions. On CPU-initiated I/O read transactions addressed to or through
the 21071-DA, the buffer acts as the I/O read buffer. Loading of data into the
buffer is controlled by the 21071-CA and 21071-DA.
Each 21071-BA contains four longwords of data and corresponding parity
bits. The parity bits are meaningful only for merge data. The parity bits are
UNPREDICTABLE for I/O read data.
3.4.6 I/O Write Buffer and DMA Read Buffer
This buffer can store up to four entries of data, where each entry has four
longwords for each 21071-BA. Data from this buffer is sent out on the epiData
bus. In the 21072-AA implementation, two entries of this buffer are allocated
for I/O write data storage, and two entries are allocated for DMA read data
storage. The four 21071-BA system uses only two of the four longwords of each
entry. However, the extra storage is not accessible. Loading of each entry can
be controlled separately, allowing maximum flexibility in allocating the buffer
entries to the 21071-DA. Loading of the buffer is handled by the 21071-CA,
with the address provided by the 21071-DA on iolinesel<1:0>. The 21071-DA
controls unloading of the buffer.
Functional Description 3–21