L1caches

L1Caches, or Level 1 caches, are the smallest and fastest memory stores in a modern central processing unit. Located on the same silicon as the processor cores, they sit between the execution units and the higher levels of the memory hierarchy to supply data and instructions with minimal delay. Most CPUs implement a split L1 cache, consisting of separate L1 data caches and L1 instruction caches, though some designs use a unified L1 cache for both data and instructions.

Sizes and organization: L1 caches are intentionally small to maximize speed and energy efficiency. Typical per-core sizes range from about 16 to 64 kilobytes for each cache, with data and instruction caches totaling around 32 to 128 kilobytes per core in many designs. Cache lines are usually 64 bytes in modern processors, with some using 32 or other sizes. The caches are highly associative, commonly 2- to 8-way, to balance hit rate and implementation complexity. In some architectures, the L1 caches can be unified rather than split.

Performance characteristics: L1 caches have the lowest latency among the memory hierarchy and are usually private to a core. Access latencies are measured in a few cycles for both data and instructions, and bandwidth is high to feed the ALUs. Misses in L1 trigger access to higher caches (L2, L3) or main memory, incurring a substantial penalty compared with an L1 hit.

Write policy and coherence: Data L1 caches are typically write-back and write-allocate; writes are kept in the L1 until eviction. Instruction caches are usually read-only. L1 caches are private per core, and coherence across cores is maintained by protocols (such as MESI) coordinating with L2 and L3 caches. Some architectures use inclusive or exclusive relationships between L1 and L2.

Impact and context: Because of their speed, L1 caches strongly influence processor performance and are a major focus for software and hardware optimization. Their small size means they cannot hold the entire working set, so efficient use of locality and prefetching remains important.