
MOTOROLA
Chapter 6. Memory Interface
6-23
For 60x processor single beat writes to system memory, the MPC106 latches the data,
performs a double-word read from system memory (checking and correcting any ECC
errors), and merges the write data from the processor with the data read from memory. The
MPC106 then generates a new ECC code for the merged double word and writes the data
and ECC code to memory. This read-modify-write process adds six clock cycles to a
single-beat write operation. If page mode retention is enabled (PGMAX
≠
0), the MPC106
keeps the memory in page mode for the read-modify-write sequence.
For 60x processor burst writes to system memory, the MPC106 latches the data in the
internal copy-back buffer and flushes the buffer to memory at the earliest opportunity. The
MPC106 generates the ECC codes when the flush occurs. Note that the MPC106 does not
check the data being overwritten in memory.
For PCI writes to system memory with ECC enabled, the MPC106 latches the data in the
internal PCI to memory write buffer (PCMWB). If the PCI master writes complete double
words to system memory, the MPC106 generates the ECC codes when the PCMWB is
flushed to memory. However, if the PCI master writes 32-, 16-, or 8-bit data that cannot be
gathered into a complete double word in the PCMWB, a read-modify-write operation is
required. The MPC106 performs a double word read from system memory (checking and
correcting any ECC errors), and then merges the write data from the PCI master with the
data read from memory. The MPC106 then generates a new ECC code for the merged
double word and writes the data and ECC code to memory. If page mode retention is
enabled (PGMAX
≠
0), the MPC106 keeps the memory in page mode for the
read-modify-write sequence.