CPU / PROCESSOR

CPU - the brains of the operation

CPU (Central Processing Unit), or sometimes simply processor, is the component in a digital computer that interprets instructions and processes data contained in software. CPUs provide the fundamental digital computer trait of programmability, and are one of the core components found in almost all modern microcomputers, along with primary storage and input/output facilities. A CPU that is manufactured using integrated circuits, is known as a microprocessor. Since the mid-1970s, single-chip microprocessors have almost totally replaced all other types of CPUs, and today the term "CPU" almost always applies to some type of microprocessor.

CPU or the phrase "central processing unit" is, in general terms, a description of a certain class of logic machines that can execute complex computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage. However, the term itself and its acronym have been in use in the computer industry at least since the early 1960s . The design and implementation of CPUs have changed dramatically , but their fundamental operation has remained much the same.

Early CPUs were custom-designed as a part of a larger, usually one-of-a-kind, computer. However, this costly method of designing custom CPUs for a particular application has largely given way to the development of inexpensive and standardized classes of processors that are suited for one or many purposes. This standardization trend generally began in the era of discrete transistor mainframes and minicomputers and has rapidly accelerated with the popularization of the integrated circuit (IC). The IC has allowed increasingly complex CPUs to be designed and manufactured in very small spaces .

Both the miniaturization and standardization of CPUs have increased the presence of these digital devices in modern life far beyond the limited application of dedicated computing machines. Modern microprocessors appear in everything from automobiles to cell phones to children's toys.

Registers,Addressws, and Data Buses

Inside the CPU are two types of buses.The address bus defines the location in memory in which data is to be retrieved from. The total width of the address bus is very important becuse it determines the maximum number of available addresses , and therefore the total amount of memory of cpu -and entire system - can directly manipulate .Each position in memory has to have one specific label ,just like you have the address for your house.

Data Buses

The data bus is a similar bundle of wires used to transfer data to and from the CPU.The wider the data bus ,the more information that can be moved at a time and(if the software is capable) and the more complex the information can be.When you hear the term"32-bit CPU" it is usually referring to tha data bus width of the CPU, because it gives a rough idea of CPU's possible performance.

When discussing the data bus ,there are two numbers to keep in mind;one is the width of the external data path. This is the number of bits that can be moved between the CPU and other component of the computer such as RAM.The width of the internal data pathrefers to data that is acually moved just within the CPU itself. In some cases, the width of these two buses is the same ;in others they're different.

Registers

An other characteristic of how CPU can handle data is its register size. This refers to the width of the igh-speed data storage area insidethe CPU. The earliest PCs had 16-bit data registers,while the more recent Pentium microprocessor has a 64-bit register.The width and number of registers determines the number of bits the CPU can work on at one time.

Early microprocessors had registers that were only four or eight bits wide.Modern CPUs have registers 32 or 64 bits wide. With a wider rigester, a CPU can manipulate more data with every operation. If used properly, a 64-bit register can operate twice as fast as those has a 32-bit register. Having more rigisters inside the microprocessor does not itself improve speed, since the microprocessor can only fetch data from one register at a time . The additional registers do however offer advantages to programmers if they use them efficiently, since there are more places to store data for execution.

CPU operation

The fundamental operation of most CPUs, regardless of the physical form they take, is to execute a sequence of stored instructions called a program. The program is represented by a series of numbers that are kept in some kind of computer memory. There are four steps that nearly all CPUs use in their operation: fetch, decode, execute, and writeback.

The first step, fetch, involves retrieving an instruction (which is represented by a number or sequence of numbers) from program 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. Often the instruction to be fetched must be retrieved from relatively slow memory, 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.

In the decode step, 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 ISA (Instruction Set Architecture). 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 required for that instruction, such as operands for an addition operation. Such operands may be given as a constant value (called an immediate value), or as a place to locate a value: a register or a memory address, as determined by some addressing mode. In older designs the portions of the CPU responsible for instruction decoding were unchangeable hardware devices. However, in more abstract and complicated CPUs and ISAs, a microprogram is often used to assist in translating instructions into various configuration signals for the CPU. This microprogram is sometimes rewritable so that it can be modified to change the way the CPU decodes instructions even after it has been manufactured.

After the fetch and decode steps, the execute step is performed. During this step, various portions of the CPU are connected so they can perform the desired operation. 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). If the addition operation produces a result too large for the CPU to handle, an arithmetic overflow flag in a flags register may also be set .

The final step, writeback, 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. 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.

After the execution of the instruction and writeback of the resulting data, the entire process repeats, with the next instruction cycle normally fetching the next-in-sequence instruction because of the incremented value in the program counter. 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 than the one described here, multiple instructions can be fetched, decoded, and executed simultaneously. This section describes what is generally referred to as the "Classic RISC pipeline," which in fact is quite common among the simple CPUs used in many electronic devices (often called microcontrollers).

Installing the CPU

It's much easier to install your processor while the motherboard is outside of the case, as it gives you much more free space in which to work. First, you'll want to make sure that your CPU has either the supplied heatsink and or fan attached (which should have already been done by the OEM). Next, simply plug in your Processor to the CPU connector, usually located in the upper left hand corner of your Mainboard. Depending on what type of CPU you are installing, be it Slot 1, Slot A, or Socket 370, there is only one correct way to install the CPU, gently slide the CPU into position, and check to make sure you have it firmly in place.