L1i

L1I stands for Level 1 Instruction cache. It is a small, fast cache memory integrated into a CPU core that stores recently fetched instructions to speed up the instruction fetch stage. In most designs, L1I is private to the core and operates alongside the Level 1 data cache (L1D) as part of the processor’s memory hierarchy.

Organization and access: L1I is typically implemented with fast SRAM and configured as direct-mapped or low-associative. Cache sizes are small, often a few kilobytes, with line sizes commonly 32 or 64 bytes. The L1I cooperates with the instruction fetch unit, an ITLB (instruction translation lookaside buffer), and a branch predictor to supply instructions with low latency.

Performance and behavior: When the next instructions are in the L1I, fetch latency is minimized and throughput increases. If an L1I miss occurs, the processor may fetch from L2/L3 caches or main memory, incurring higher latency. Miss penalties are influenced by the accuracy of the branch predictor and by the instruction fetch unit design.

Variants: Some CPU designs use separate L1I and L1D caches, while others employ a unified L1 cache serving both instruction and data. The exact size, associativity, and line size vary widely across architectures such as Intel, AMD, and ARM.

History and impact: Dedicated L1 caches became common in superscalar processors during the 1980s and 1990s and remain a standard feature in modern CPUs. The L1I’s effectiveness depends on instruction locality and pipeline architecture, making it a key factor in overall CPU performance.

See also: Instruction cache, Level 1 cache, CPU cache, memory hierarchy.