How does a computer CPU work ?

BACKGROUND: How a computer ‘does things’

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Firstly here’s a quick and simple backgrounder as to how a computer works. A computer does things by basically executing sets of instructions. These sets of instructions are called programsExample program instructions to continually accept 2 user number inputs, Add them & print it out :-
10 READ : Input #1
20 READ : Input #2
30 ADD: Input#1 + Input#2
40 PRINT : Result
50 GOTO : Line 10. Programs run on top of the machines’ Operating System, like Microsoft Windows. It is the Operating System that loads a program and runs the instructions contained in it. The Operating Sytem acts like a ‘middle-man’ interacting between the program and the computers hardware, like the mouse or screen or memory or disk. The mouse might be moved by the user and the Operating System will then tell the running program what happened in case it wants to do anything about it. Likewise when a program wants to say send something to the printer attached to the PC it will tell the Opearing System it wants to talk to the printer.

A program like Microsoft Word takes inputs like your mouse clicks and executes the appropriate set of instructions. : load a file; save a file; format text etc. In the course of doing this many low level operations are performed : move a number from memory location X to location Y; get the value from memory at location A; get the value from memory location B; add them together and store it in memory location C. This all happens extremely quickly and seamlessly from your point of view. It’s within this maelstrom of low level activity that we want to look at what the CPU is and what it is doing that enables all this to happen.

See panel below for a list of the computers key components

So … What is a CPU?

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The Central Processing Unit, or CPU, is considered to be the brain of a computer. Without this vital component, the computer wouldn’t be able do any of the functions it is capable of doing. Basically, it is a chip that handles the moving and processing of data and the execution of programming instructions. It’s responsible for receiving and processing the input from the keyboard, the mouse, the web-cam and any other number of peripheral devices. All hardware components and programs installed on the system must go through the CPU before their function can be carried out.

PARTS OF THE CPU

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PUTTING IT ALL TOGETHER : How does it all work?

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All CPUs use the same basic set of operations when running a program instructions:

1. Fetch

The control unit fetches (gets) the instruction from memory. The location in program memory is determined by a program counter (PC), which stores a number that identifies the current position in the program. In other words, the program counter keeps track of the CPU’s place in the current program. After an instruction is fetched, the PC is incremented by the length of the instruction word in terms of memory units. This can be thought of like a finger advancing to the next instruction in a list of instructions (the program). Often the instruction to be fetched must be retrieved from relatively slow memory (RAM), causing the CPU to stall while waiting for the instruction to be returned. This issue is largely addressed in modern processors by caches and pipeline architectures. The instruction that the CPU fetches from memory is used to determine what the CPU is to do.

2. Decode

The control unit then decodes the instruction (decides what it means). The instruction is broken up into parts that have significance to other portions of the CPU. The way in which the numerical instruction value is interpreted is defined by the CPU’s instruction set architecture (ISA).Often, one group of numbers in the instruction, called the opcode, indicates which operation to perform. The remaining parts of the number usually provide information, ie that data, required for that instruction. The control unit then passes this information to the Arithmetic Logic Unit (ALU)

3. Execute

The ALU is given control and performs the actual operation on the data. If, for instance, an addition operation was requested, an arithmetic logic unit (ALU) will be connected to a set of inputs and a set of outputs. The inputs provide the numbers to be added, and the outputs will contain the final sum. The ALU contains the circuitry to perform simple arithmetic and logical operations on the inputs (like addition and bitwise operations).

4. Writeback

In the final step, writeback, the ALU simply “writes back” the results of the execute step to some form of memory. Very often the results are written to some internal CPU register for quick access by subsequent instructions. In other cases results may be written to slower, but cheaper and larger, main memory (RAM). Some types of instructions manipulate the program counter rather than directly produce result data. These are generally called “jumps” and facilitate behavior like loops, conditional program execution (through the use of a conditional jump), and functions in programs. Many instructions will also change the state of digits in a “flags” register. These flags can be used to influence how a program behaves, since they often indicate the outcome of various operations. For example, one type of “compare” instruction considers two values and sets a number in the flags register according to which one is greater. This flag could then be used by a later jump instruction to determine program flow.

…and repeat it all again…

After the execution of the instruction and writeback of the resulting data, the entire process repeats, with the next instruction in the program. If the completed instruction was a jump, the program counter will be modified to contain the address of the instruction that was jumped to, and program execution continues normally. In more complex CPUs, multiple instructions can be fetched, decoded, and executed simultaneously.