The Very Basics
The computer's processor, also known as the central processing unit, or CPU, is the command center of any computer. It is the job of this unit to digest the sets of instructions known as "programs" and put them to use throughout the rest of the computer. The processor has taken many forms throughout the years, but today it is fairly universal that a modern processor will make use of the following method: fetch, decode, execute and write-back.
Every set of instructions that a CPU processes has to be retrieved from the computer's memory. The computer has several different types of memory, so in this first stage of the processor's implementation, it needs to pinpoint where to find the specific instructions it needs. This is done by a part of the processor called the program counter, which stores a number that bookmarks where the processor is in the program and where the next step can be located in the memory. Once the appropriate instructions can be accessed, the CPU is ready to move onto the next step.
Now that the processor has the instructions, it has to figure out what to do with them. The program has to be broken up into parts and translated to have meaning to the rest of the CPU. This is where the instruction set architecture (ISA) comes into play. This important part of the processor dictates how the unit understands lines of programming code, determining what kind of software the processor is capable of running. It is the ISA that breaks down the program and distributes it to the arithmetic/logic unit, or ALU, where it can be executed.
Now it is time for the instructions to be properly carried out. The ALU, contains the circuitry to perform all sorts of mathematical and logical functions. It can add, subtract, multiply, divide, and test for less than/greater than/equal to conditions. By these apparently simple means it is able to satisfy the demands of all forms of programs, everything from adding one to the time in a clock program to plotting the points of a 3-D model.
Now that the ALU has executed its function and come up with its results, it needs to store this information for later use. This could be saved to the main memory of the computer, or it could end up in one of the fast, temporary memory storage units in the CPU called registers. This new information could be put to use as the next part of the program is conjured up and the cycle begins anew.