SPRUJB6B November 2024 – May 2025 AM2612
Critical Word First is a CPU cache concept which states that although the CPU wishes to fetch a particular cache line, the CPU prefers to see an offset into the cache line prior to seeing the beginning of the cache line. This is also known as wrapping burst, where a burst request to a cache line is not aligned to the start of the cache line. RL2 supports critical word first access from R5F CPU. For example, if the CPU jumps to the last word of a cache line not currently in the L1, the CPU requests a cache line burst, but the starting address of the burst is the last bus aligned word address. Say the cache line size is 32 bytes, and the bus width is 64 bits wide. If the CPU jumps to an address of 0x1c offset into a particular non-allocated cache line, the CPU requests address 0x18 in a wrapping burst of 32 bytes, resulting in offset addresses 0x18, 0x00, 0x08, 0x10 being the burst order requested. This is known as critical word first or wrapping burst request. That is the offset 0x18 is the critical word the CPU want to see in the cache line.