5-2
MPC106 PCIB/MC User's Manual
MOTOROLA
Chapter 2, “Signal Descriptions,” contains the signal definitions for the L2 cache interface
and Chapter 3, “Device Programming,” details the configurable parameters that are used to
initialize the L2 cache interface. In addition, Chapter 8, “Internal Control,” provides
information about the internal buffers that permit the MPC106 to coordinate memory
accesses between the L2 cache, the 60x processor(s), and devices on the PCI bus.
5.1 L2 Cache Configurations
The following sections describe the various L2 cache configurations that the MPC106
supports.
5.1.1 Write-Back Cache Operation
The use of a write-back L2 cache offers several advantages over direct access to the
memory system. Since every L1 write operation does not go to main memory but to the L2
cache which can be accessed more quickly, write operation latency is reduced along with
contention for the memory system. Subsequent read accesses from the processor that hit in
the L2 cache are also expedited in comparison to the memory system. Write-back L2 cache
blocks implement a dirty bit in their tag RAM, which indicates whether the contents of the
L2 cache block have been modified from that in the memory system. L2 cache blocks that
have been modified (dirty bit set) will be written back to memory on L2 cache line
replacement, while unmodified L2 cache blocks will be invalidated and overwritten without
being cast out to memory.
5.1.2 Write-Through Cache Operation
Write-through L2 caches reduce read latency in the same way write-back L2 caches do, but
write operations from the primary (L1) cache are written to both the L2 cache and the
memory system, thereby exhibiting the same latency as an ordinary memory write. A write-
through L2 cache keeps memory coherent with the contents of the L2 cache, and removes
the need for maintenance of a dirty bit in the tag RAM.