Figure shows three levels of cache and introduces the nomenclature we will use in the remainder of the document. L1d is the level 1 data cache, L1i the level 1 instruction cache, etc. Note that this is a schematic; the data flow in reality need not pass through any of the higher-level caches on the way from the core to the main memory. CPU designers have a lot of freedom designing the interfaces of the caches. For programmers these design choices are invisible.
In addition we have processors which have multiple cores and each core can have multiple “threads”. The difference between a core and a thread is that separate cores have separate copies of (almost {Early multi-core processors even had separate 2nd level caches and no 3rd level cache.}) all the hardware resources. The cores can run completely independently unless they are using the same resources—e.g., the connections to the outside—at the same time. Threads, on the other hand, share almost all of the processor's resources. Intel's implementation of threads has only separate registers for the threads and even that is limited, some registers are shared.
