MPC8280 datasheet(527/1386 Pages) FREESCALE

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MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

12-1

Chapter 12

Secondary (L2) Cache Support

The MPC8280 has features to support an externally controlled secondary (L2) cache such as the Freescale

MPC2605 integrated secondary cache for microprocessors that implement the PowerPC architecture. This

chapter describes the MPC8280’s L2 cache interface—configurations, operation, programmable

parameters, system requirements, and timing.

12.1

L2 Cache Configurations

The MPC8280 supports three L2 cache configurations—copy-back mode, write-through mode, and

ECC/parity mode. The following sections describe the L2 cache modes.

12.1.1

Copy-Back Mode

The use of a copy-back L2 cache offers several advantages over direct access to the memory system. In

copy-back mode, cacheable write operations are performed to the L2 cache without updating main

memory. Since every cacheable 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. In copy-back mode, cacheable read operations that hit in the L2 cache are serviced from the L2

cache without requiring a memory transaction and its associated latency. Copy-back mode offers the

greatest performance of all the L2 cache modes.

Copy-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 main memory. During L2 cache line replacement,

L2 cache blocks that have been modified (dirty) are written back to memory; unmodified (not dirty) L2

cache blocks are invalidated and overwritten without being written back to memory.

Copy-back mode requires that the L2 cache is able to initiate copy-backs to main memory. To do this, the

L2 cache must act as a bus master and implement the bus arbitration signals BR, BG, and DBG. The

MPC8280 can also support additional bus masters (60x or MPC8280-type) in copy-back mode.

MPC8280 Datasheet(PDF) 527 Page - Freescale Semiconductor, Inc