user manual

38 www.xilinx.com MicroBlaze Processor Reference Guide
1-800-255-7778 UG081 (v6.0) June 1, 2006
Chapter 1: MicroBlaze Architecture
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For example: in a MicroBlaze configured with C_ICACHE_BASEADDR= 0x00300000,
C_ICACHE_HIGHADDR=0x0030ffff, C_CACHE_BYTE_SIZE=4096, and
C_ICACHE_LINELEN=8; the cacheablememoryof 64kB uses16 bits ofbyteaddress,and
the 4kB cache uses12 bits of byte address, thus the required address tagwidth is: 16-12=4
bits. The total number of block RAM primitives required in this configuration is: 2
RAMB16 for storing the 1024 instruction words, and 1 RAMB16 for 128 cache line entries,
each consisting of: 4 bits of tag, 8 word-valid bits, 1 line-valid bit. In total 3 RAMB16
primitives.
Instruction Cache Operation
For every instruction fetched, the instruction cache detects if the instruction address
belongs to the cacheable segment. If the address is non-cacheable, the cache controller
ignores the instruction and lets the OPB or LMB complete the request. If the address is
cacheable, a lookup is performed on the tag memory to check if the requested address is
currently cached. The lookup is successful if: the word and line valid bits are set, and the
tag address matches the instruction address tag segment. On a cache miss, the cache
controllerwillrequestthenewinstructionovertheinstructionCacheLink(IXCL)interface,
and wait for the memory controller to return the associated cache line.
Instruction Cache Software Support
MSR Bit
The ICE bit in the MSR provides software control to enable and disable caches.
Thecontentsofthecachearepreservedby default when the cache is disabled. Theusercan
invalidate cache lines using the WIC instruction or using the hardware debug logic of
MicroBlaze.
WIC Instruction
The optional WIC instruction (C_ALLOW_ICACHE_WR=1) is used to invalidate cache
linesinthe instruction cache froman application. For adetailed description, pleasereferto
Chapter 4, “MicroBlaze Instruction Set Architecture”. The cache must be disabled
(MSR[ICE]=0) when the instruction is executed.
Data Cache
Overview
MicroBlaze may be used with an optional data cache for improved performance. The
cached memory range must not include addresses in the LMB address range.
The data cache has the following features
• Direct mapped (1-way associative)
• Write-through
• User selectable cacheable memory address range
• Configurable cache size and tag size
• Caching over CacheLink (XCL) interface
• Option to use 4 or 8 word cache-lines