CPU – Central Processing Unit, the computer brain, it executes or processes instructions received from software (code) and commands. The term existed since the early 1960's, although examples of CPU computers existed since the mid 1940's.
In the early days of computers, engineers hard wired the machines to perform different tasks. Fixed program computers, like ENIAC the first computer, required teams of engineers and scientists who prepared them for each computation they performed. Its creators originally planned ENIAC as a stored-program computer with a CPU, but they scrapped the CPU in favor of completing ENIAC sooner.
Prior to ENIAC's completion, John von Neumann wrote a paper about EDVAC, a computer with a CPU that executed commands stored in memory modules. Modern CPU computers owe their existence to Von Neumann's EDVAC.
CPUs are digital devices, meaning they use discreet states as opposed to analog devices using a continuous range of values. To achieve these discreet states CPUs use switching devices. Early CPUs used mechanical switching devices. Later, devices such as vacuum tubes and electrical relays replaced the mechanical switches. Vacuum tubes had the advantage of speed, but took time to heat up and eventually stopped working. Even though vacuum tube CPUs failed constantly – on average after only eight hours of use – speed trumped reliability.
CPUs moved to transistors in the 1950's and 1960's. Transistor based CPUs were faster, consumed less power, and failed far less often. As transistors became smaller and smaller so that integrated circuits contained more and more of them, CPU clock rates jumped from the 100's of kHz or single digit MHz to 10's of MHz. Still, CPUs required many integrated circuit chips.
But because smaller and smaller integrated circuits run into problems such as electromigration – ions in the conductive substance move – researchers constantly search for new ways to design CPUs. Parallel processing, multiple CPUs sharing processing tasks, is widely available in modern computers. Researchers have also made progress with quantum computing in which quantum phenomenon operates on data.
CPUs whether mechanical, vacuum tube, electronic switch, transistor or microchip operate in the same simple four step process:
- Fetch – The CPU retrieves an instruction from program memory.
- Decode – The CPU decodes the instruction, meaning the CPU breaks the instruction down into parts understood by various portions of the CPU.
- Execute – The CPU (the appropriate portions of the CPU) execute the tasks.
- Writeback – The CPU records the results of the operations to some form of memory.
The CPU fetches instructions according to the program counter. The PC keeps track of the CPU's place in the program. Some program instructions are jumps, telling the CPU to locate an instruction at a non-sequential address.
Some sophisticated CPUs fetch and execute multiple instructions at the same time. Many CPUs employ a cache, their own fast memory. The CPU copies frequently used data to this cache in order to process it more quickly.
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