Cache Policy

In the Sapphire RV64 SoC CPU, the data cache uses a write-back policy, which affects how write-to-memory are handled relative to the cache. Understanding write-back requires comparing it to the alternative, write-through, to highlight the trade-offs in performance, complexity, and memory traffic.

In a write-through cache, every write operation performed by the processor is immediately propagated to both the cache and the main memory. This ensures that the memory always contains the most up-to-date data, maintaining strong consistency between the cache and memory. The advantage of this approach is in its simplicity, where memory always reflects the latest write. Therefore, no special logic is required to track modified cache lines. However, the main disadvantage is its performance, where every stored operation generates a memory write, increasing bus traffic and slowing down the system when memory access latency is high. Even if the data is written repeatedly in the cache, each write still incurs a memory access.

In contrast, a write-back cache, as used in Sapphire RV64 SoC, optimizes performance by delaying memory updates. When the processor writes a cache line, the data is updated only in the cache, and a special flag, the 'dirty' bit, is set to indicate that the cache line contains modified data that has not yet been written to the main memory. Memory is updated only when the cache line is evicted to make space for a new line. This reduces the number of memory writes, particularly for frequently modified data, and decreases bus traffic, thereby improving overall system performance. The downside is added complexity: the cache controller must track which lines are dirty and ensure that they are written back correctly upon eviction. Additionally, software or peripheral devices that access memory directly must account for the possibility that the latest data resides in the cache rather than in main memory, which may require cache flushes or coherence mechanisms.

The cache line replacement policy is implemented using a pseudo-LRU algorithm, which tries to evict the least recently used cache lines when room is needed for new ones.

Functionally, write-back caching improves efficiency for workloads with frequent updates to the same memory locations, as multiple writes can be merged into a single memory transaction. Instruction fetches and read-only data are unaffected, so read-intensive workloads still benefit from caching. In Sapphire RV64 SoC, combining a write-back cache with hardware and software prefetching further enhances performance by reducing read and write latency and minimizing stalls caused by memory traffic.

In summary, the write-back policy used in Sapphire RV64 SoC allows the processor to defer writes to memory until a cache line is evicted, reducing memory traffic and improving performance, whereas a write-through cache immediately updates memory on every write, providing simplicity and strong consistency at the cost of additional memory accesses.