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A level 1 cache (L1 cache) is a memory cache that is directly built into the microprocessor, which is used for storing the microprocessor’s recently accessed information, thus it is also called the primary cache. It is also referred to as the internal cache or system cache.
L1 cache is the fastest cache memory, since it is already built within the chip with a zero wait-state interface, making it the most expensive cache among the CPU caches. However, it has limited size. It is used to store data that was accessed by the processor recently, critical files that need to be executed immediately and it is the first cache to be accessed and processed when the processor itself performs a computer instruction.
In more recent microprocessors, the L1 cache is divided equally into two: a cache that is used to keep program data and another cache that is used to keep instructions for the microprocessor. Some older microprocessors, on the other hand, make use of the undivided L1 cache and uses it to store both program data and microprocessor instructions.
It is implemented with the use of static random access memory (SRAM), which comes in different sizes depending on the grade of the processor. This SRAM makes use of two transistors per bit. The two transistors form a circuit known as a 'flip-flop’ since it has two states it can flip between; the second transistor manages the output of the first transistor. For as long as power is supplied to the circuit, it can hold data without external assistance.
All L1 cache designs follow the same process; the control logic of the L1 cache stores frequently used data in the cache and only updates external memory when the CPU hands over control to other bus masters when peripheral devices are doing direct memory access.