Central processing unit - Wikipedia
It performs the basic arithmetical, logical, and input/output operations of a computer system. The CPU is like the brains of the computer - every. The speed of the computer processor is a major factor in The CPU is also one of the most expensive components on the motherboard. The CPU interacts closely with primary storage, or main memory, referring to it for both Now let us consider the components of the central processing unit. In fact, a logic unit can usually discern six logical relationships: equal to, less than.
A computer program is normally stored in the memory unit of the Central Processing Unit. These 3 units are the primary components of a CPU.
Various functions of CPU and operations are generally performed by these 3 units are described below. The primary job of the memory unit is to store data or instructions and intermediate results.
Memory unit supplies data to the other units of a CPU. In Computer Organization, memory can be divided into two major parts primary memory and secondary memory. Speed and power and performance of a memory depends on the size and type of the memory. When an instruction is processed by the central processing unit, the main memory or the RAM Random Access Memory stores the final result before it is sent to the output device. All inputs and outputs are intermediate and are transmitted through the main memory.
It is the unit which controls all the operations of the different units but does not carry out any actual data processing operation.
Keeping these items in memory when the program is not running is not feasible for three reasons: Most types of memory only store items while the computer is turned on; data is destroyed when the machine is turned off. If more than one program is running at once often the case on large computers and sometimes on small computersa single program can not lay exclusive claim to memory.
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There may not be room in memory to hold the processed data. How do data and instructions get from an input device into memory?
The control unit sends them. After being processed, the information is sent to memory, where it is hold until it is ready to he released to an output unit. The chief characteristic of memory is that it allows very fast access to instructions and data, no matter where the items are within it.
We will discuss the physical components of memory-memory chips-later in this chapter. To see how registers, memory, and second storage all work together, let us use the analogy of making a salad. In our kitchen we have: The process of making the salad is then: The refrigerator is the equivalent of secondary disk storage.
It can store high volumes of veggies for long periods of time. The counter top is the equivalent of the computer's motherboard - everything is done on the counter inside the computer.
The cutting board is the ALU - the work gets done there. The recipe is the control unit - it tells you what to do on the cutting board ALU. Space on the counter top is the equivalent of RAM memory - all veggies must be brought from the fridge and placed on the counter top for fast access. Note that the counter top RAM is faster to access than the fridge diskbut can not hold as much, and can not hold it for long periods of time.
The corners of the cutting board where we temporarily store partially chopped veggies are equivalent to the registers.
The corners of the cutting board are very fast to access for chopping, but can not hold much. The salad bowl is like a temporary register, it is for storing the salad waiting to take back to the fridge putting data back on a disk or for taking to the dinner table outputting the data to an output device.
Now for a more technical example. Suppose the program calculates the salary of an employee. In more complex CPUs, multiple instructions can be fetched, decoded and executed simultaneously. This section describes what is generally referred to as the " classic RISC pipeline ", which is quite common among the simple CPUs used in many electronic devices often called microcontroller. It largely ignores the important role of CPU cacheand therefore the access stage of the pipeline.
Some instructions manipulate the program counter rather than producing result data directly; such instructions are generally called "jumps" and facilitate program behavior like loopsconditional program execution through the use of a conditional jumpand existence of functions. These flags can be used to influence how a program behaves, since they often indicate the outcome of various operations.
For example, in such processors a "compare" instruction evaluates two values and sets or clears bits in the flags register to indicate which one is greater or whether they are equal; one of these flags could then be used by a later jump instruction to determine program flow. Fetch[ edit ] The first step, fetch, involves retrieving an instruction which is represented by a number or sequence of numbers from program memory. The instruction's location address in program memory is determined by a program counter PCwhich stores a number that identifies the address of the next instruction to be fetched.
After an instruction is fetched, the PC is incremented by the length of the instruction so that it will contain the address of the next instruction in the sequence. This issue is largely addressed in modern processors by caches and pipeline architectures see below.
In the decode step, performed by the circuitry known as the instruction decoder, the instruction is converted into signals that control other parts of the CPU.
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Those operands may be specified as a constant value called an immediate valueor as the location of a value that may be a processor register or a memory address, as determined by some addressing mode.
In some CPU designs the instruction decoder is implemented as a hardwired, unchangeable circuit. In others, a microprogram is used to translate instructions into sets of CPU configuration signals that are applied sequentially over multiple clock pulses. In some cases the memory that stores the microprogram is rewritable, making it possible to change the way in which the CPU decodes instructions.
Execute[ edit ] After the fetch and decode steps, the execute step is performed. Depending on the CPU architecture, this may consist of a single action or a sequence of actions. During each action, various parts of the CPU are electrically connected so they can perform all or part of the desired operation and then the action is completed, typically in response to a clock pulse.
Very often the results are written to an internal CPU register for quick access by subsequent instructions.
Central processing unit
In other cases results may be written to slower, but less expensive and higher capacity main memory. For example, if an addition instruction is to be executed, the arithmetic logic unit ALU inputs are connected to a pair of operand sources numbers to be summedthe ALU is configured to perform an addition operation so that the sum of its operand inputs will appear at its output, and the ALU output is connected to storage e. When the clock pulse occurs, the sum will be transferred to storage and, if the resulting sum is too large i.