The Intel 80486 has an on-chip, unified cache. It contains 8...

The Intel 80486 has an on-chip, unified cache. It contains 8 Kbytes and has a fourway set-associative organization and a block length of four 32-bit words. The cache is organized into 128 sets. There is a single "line valid bit" and three bits, $\mathrm{B} 0, \mathrm{~B} 1$, and $\mathrm{B} 2$ (the "LRU" bits), per line. On a cache miss, the 80486 reads a 16-byte line from main memory in a bus memory read burst. Draw a simplified diagram of the cache and show how the different fields of the address are interpreted.

Key Concepts

Cache Line (Block) and Transfer Mechanism

A cache line, or block, is the smallest unit of data that can be transferred between the cache and the main memory. When a cache miss occurs, the entire block is fetched from the main memory; this block is then stored in the appropriate set of the cache. The block transfer mechanism, such as a bus memory read burst, is designed to quickly move a contiguous sequence of bytes from memory to cache, optimizing overall data transfer efficiency.

Least Recently Used (LRU) Replacement Policy

The LRU replacement policy is used in set-associative caches to decide which cache line to evict when a new block is loaded into a full set. By tracking the usage history (often with specific bits per line), the cache can approximate which block has been least recently used, thereby making an informed decision to replace it. This strategy helps in reducing cache misses by retaining blocks that are more likely to be reused.

Set-Associative Cache

Set-associative caches divide the cache into several sets, each containing multiple cache lines (or ways). A given memory block can be stored in any line within a specific set. This organization balances the advantages of direct-mapped caches (simplicity and speed) with those of fully associative caches (flexibility and reduced conflict misses).

Unified Cache

A unified cache is one that stores both instructions and data in the same cache memory, eliminating the need for separate caches. This design simplifies the cache structure and can lead to more efficient use of the available cache space, as both instructions and data compete for the same cache resources.

Address Field Partitioning

In cache design, the memory address is divided into several fields such as the tag, the index, and the block (or byte) offset. The index selects which cache set to look at, the tag is used to verify that the data in the set corresponds to the requested memory block, and the offset identifies the exact data within the block. This partitioning is crucial for mapping a main memory address to its corresponding cache location.

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