All modern multi-core CPUs have at least a three-level cache (refer to Why do we need multiple levels of cache memory?).
L1 is the fastest and smallest, L2 has slightly more latency but is larger, and L3 holds data that is shared among all cores in the processor (and is even larger and even slower). All's well and good.
Googling the phrase "L4 Cache" however do not yield empty results. There are apparently some CPUs that do include an L4 cache (apparently the Intel Broadwell i7-5775C does have 128MB eDRAM implemented as L4 cache).
Does anyone know what the L4 cache is for? I cannot find anywhere documentation regarding its purpose and function.
Best Answer
The level 4 cache (L4 cache) is a way to link the Level 3 cache which can be accessed by the CPU and the L4 cache which can be access by both the CPU and GPU.
Source: Wikipedia - CPU cache
This L4 cache remained in Intel's microarchitecture throughout the lifespan of Haswell and Broadwell.
In Skylake, the eDRAM arrangement changes:
The article additional goes on to indicate that:
Anandtech - Intel Skylake Mobile Desktop Launch Architecture