user manual

MicroBlaze Processor Reference Guide www.xilinx.com 39
UG081 (v6.0) June 1, 2006 1-800-255-7778
Data Cache
R
• Cache on and off controlled using a bit in the MSR
• Optional WDC instruction to invalidate data cache lines
General Data Cache Functionality
When the data cache is used, the memory address space in split into two segments: a
cacheablesegmentand a non-cacheablesegment.The cacheable areaisdeterminedby two
parameters: C_DCACHE_BASEADDR and C_DCACHE_HIGHADDR. All addresses
within this range correspond to the cacheable address space. All other addresses are non-
cacheable.
The cacheable data address consists of two parts: the cache address, and the tag address.
The MicroBlaze data cache can be configured from 2kB to 64 kB. This corresponds to a
cache address of between 11 and 16 bits. The tag address together with the cache address
should match the full address of cacheable memory.
For example: in a MicroBlaze configured with C_ICACHE_BASEADDR= 0x00400000,
C_ICACHE_HIGHADDR=0x00403fff, C_CACHE_BYTE_SIZE=2048, and
C_ICACHE_LINELEN=4; the cacheablememoryof 16kB uses14 bits ofbyteaddress,and
the 2 kB cache uses 11 bits of byte address, thus the required address tag width is: 14-11=3
bits. The total number of block RAM primitives required in this configuration is: 1
RAMB16 for storing the 512 instruction words, and 1 RAMB16 for 128 cache line entries,
each consisting of: 3 bits of tag, 4 word-valid bits, 1 line-valid bit. In total 2 RAMB16
primitives.
Data Cache Operation
The MicroBlaze data cache implements a write-through protocol. A store to an address
withinthe cacheable rangewill, providedthatthe cacheis enabled, generatean equivalent
byte, halfword, or word write over the data CacheLink (DXCL) to external memory. The
write will also update the cached data if the target address word is in the cache (i.e. the
write is a cache-hit). A write cache-miss does not load the associated cache line into the
cache.
Figure 1-10: Data Cache Organization
Data Address Bits
0 3031
Cache Word Address
Tag Address
-
-
Tag
Data
BRAM
BRAM
Addr
Addr
=
Tag
Valid
Cache_Hit
Cache_data
Load_Instruction