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→Basic CPU Features
* '''Register width''' - [[Register|Registers]] are generally fixed-width, with 8, 16, 32, and 64 bit widths common (though other values are sometimes seen). Some CPUs have multiple registers of different sizes, or can access smaller subsets of larger registers (e.g., accessing the first 8 bits of a 64-bit register when needed), or can access a register as two smaller registers or one larger register (e.g., the 16-bit accumulator register on the 6809E processor can also be accessed as two 8-bit registers). Processors with wider registers are generally considered more powerful, though this may actually be a drawback in some specific situations.
* The '''number of registers''' varies from three or four to many dozen. Some processors are equipped with multiple sets of registers, and can rapidly switch between the register sets on demand (e.g., Intel "Hyperthreading" technology), which simplifies and speeds up process or thread switches. Since registers are often significantly faster than RAM, a larger register set is generally considered better, except that it will take longer to save a larger register set when switching processes. The full set of registers available on a CPU is known as the ''register file''.
* The ''work'' of a CPU is performed by '''Execution Units''', which perform operations such as loading and storing data from/to memory (load/store unit), performing integer math (integer unit), executing [[Bitwise Operations|bitwise operations]], and performing floating-point math (floating-point unit, or FPU). The length of time taken to perform an operation varies according to the sophistication of the execution unit and the complexity of the operation. For example, a multiplication can be performed in many different ways, ranging from repeated addition (very slow, but requiring very little hardware logic) to table lookup (very fast, but requiring a lot of silicon), with most operations implementations falling somewhere in the middle. A multiplication will almost always take longer to perform than an addition, and may vary according to carry and overflow sub-operations required. The use of multiple units permits faster operations to be completed on some execution units while other (slower) operations are taking place on other execution units.
* As instructions are performed, special results are recorded as '''[[Flags|flags]]''' within the CPU. For example, adding or multiplying two numbers will set a "Carry" flag when the result overflows the word width. Other flags may indicate zero or negative result values. These flags can then be used in later operations -- for example, a branch may be taken if the carry flag is set. The number of flags, their specific meanings, and the circumstances under which they are set (to binary "1") and cleared (to binary "0") vary from architecture to architecture.
* '''Cache''' is high-speed memory placed between RAM and the CPU. This memory is faster than main RAM but much smaller; it improves performance by enabling the CPU to continue to write data quickly and continue without waiting for the data to be written out to main RAM. It also provides fast access to instructions or data that are accessed repeatedly, such as when a small loop is being executed. The performance difference between a loop that fits into cache and a loop that does not fit into cache can be substantial. Cache memory is arranged in "lines" which are typically a multiple of the word size; requesting a memory address that is not in cache results in a "cache miss" which causes a stall while the cache contents are retrieved from main memory. Cache design varies in many details, especially in write behaviour -- the cache can simply carry a write through to main memory (write-through), or it can hold the data in cache and write it back at a later time (write-back). There may be multiple levels of cache of varying sizes.
