DRAM as an L4 Cache: How Tag Comparison is Performed and Comparison Times

DRAM, or Dynamic Random Access Memory, is a main memory type used in computers to store data. However, its usage as an L4 (fourth-level) cache in CPU architectures is not widespread. This article explores whether the tag comparison step is performed by the CPU or specialized circuitry, and how long it typically takes to compare, for example, 4 tags in a 4-way cache set.

Overview of Cache Hierarchy and Tag Comparison

The design of a cache hierarchy typically involves multiple levels of caching, from L1 to L3. A fourth-level cache (L4) is less common due to practical and economic constraints. DRAM could, in theory, serve as an L4 cache, but the question arises: is tag comparison performed by the CPU or specialized circuitry?

To answer this question accurately, it is important to understand the specific architecture of the processor in question. The role of cache, whether it is an L3, L4, or any other level, is usually handled by dedicated logic, not as part of the main flow of instruction execution.

cache Logic vs. Main Memory

If the CPU uses DRAM as cache, the source of the data held in cache becomes a critical point. Conventional DRAM connected to the CPU's memory bus is not cache in the traditional sense. By definition, the CPU is unaware that it is dealing with cached data, making it unsuitable for an L4 cache in program execution.

For DRAM to be effectively used as cache, some additional logic must be implemented. This logic could be programmed instructions running on the CPU, but this approach would not be practical because the data source would be the same DRAM. There would be no performance benefit since cache access would be as expensive as direct data access.

Implementing cache logic using DRAM, especially for an L4 cache, would typically hold data from slower storage devices such as hard disks or SSDs. This setup could provide performance benefits due to the slower access times of these storage mediums.

Historical Examples and Current Practices

Historically, Samsung offered a feature called Magician, which allowed DRAM to be used as a software cache for their early SSDs. However, this feature has been largely replaced by placing DRAM directly on the SSD controller and placing the cache logic there, as DRAM is much faster than NAND flash.

Architecture and Performance Considerations

At the architectural level, the L1 instruction and data caches are typically very tightly coupled to the CPU core. They operate in parallel with virtual-to-physical address translation. L2 and L3 caches (and potentially L4) are usually shared and less integrated with the CPU core. The addresses for these caches arrive as physical addresses, and tag comparisons are done by logic close to the tag arrays.

Most set-associative cache designs compare all ways in parallel and do so in one cycle. The exact cycle time depends on the cache level (L2, L3, etc.) and may differ from the core clock cycle time. A comparator for each cache way is relatively inexpensive compared to the overall cost of the cache architecture.

Conclusion

While DRAM can theoretically serve as an L4 cache, its performance benefits are limited unless the data is held from slower storage devices. The tag comparison for such a cache would generally be handled by specialized circuitry rather than the CPU. Understanding the specific architecture of the system is crucial when discussing cache performance and design.