When using the instruction cache, the memory address space splits into two
segments: a cacheable segment and a non-cacheable segment. The cacheable segment is
determined by two parameters: C_ICACHE_BASEADDR and
C_ICACHE_HIGHADDR. All addresses within this range
correspond to the cacheable address segment. All other addresses are non-cacheable.
The cacheable segment size must be 2N, where N is a
positive integer. The range specified by C_ICACHE_BASEADDR and C_ICACHE_HIGHADDR
must comprise a complete power-of-two range, such that range = 2N, and the N least significant bits of C_ICACHE_BASEADDR must be zero.
The cacheable instruction address consists of two parts: the cache address, and the tag address. The instruction cache can be configured from 64 bytes to 64 kB. This corresponds to a cache address of between 6 and 16 bits. The tag address together with the cache address need to match the full address of cacheable memory.
When selecting cache sizes below 2 kB, distributed RAM is used to implement the
tag RAM and instruction RAM. Distributed RAM is always used to implement the tag RAM,
when setting the parameter C_ICACHE_FORCE_TAG_LUTRAM to
1. This parameter is only available with cache size 8 kB and less for 4 word cache
lines, with 16 kB and less for 8 word cache lines, and with 32 kB and less for 16 word
cache-lines.
For example, in a 32-bit MicroBlaze V configured
with C_ICACHE_BASEADDR= 0x0030_0000, C_ICACHE_HIGHADDR=0x0030_ffff, C_ICACHE_BYTE_SIZE= 4096, C_ICACHE_LINE_LEN= 8, and C_ICACHE_FORCE_TAG_LUTRAM= 0, the cacheable memory of 64 kB uses 16 bits
of byte address, and the 4 kB cache uses 12 bits of byte address. The required address
tag width is therefore 16-12 = 4 bits. The total number of block RAM primitives required
in this configuration is: two RAMB16 for storing the 1024 instruction words, and one
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.
The following figure shows the organization of the instruction cache.
