4.3.3. DCache organization

The DCache is organized as four segments, each containing 64 lines, and each line containing eight words. The position of the line within the segment is a number from 0 to 63. This is called the index. A line in the cache can be uniquely identified by its segment and index. The index is independent of the MVA. The segment is selected by bits [6:5] of the MVA.

Bits [4:2] of the MVA specify which word within a cache line is accessed. For halfword operations, bit [1] of the MVA specifies which halfword is accessed within the word. For byte operations, bits [1:0] specify which byte within the word is accessed.

Bits [31:7] of the MVA of each cache line are called the TAG. The MVA TAG is stored in the cache, along with the eight words of data, when the line is loaded by a linefill.

Cache lookups compare bits [31:7] of the MVA of the access with the stored TAG to determine whether the access is a hit or miss. The cache is therefore said to be virtually addressed.

The DCache logical model is the same as for the ICache. See Figure 4.1.

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