Computer CPU and Function

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What is CPU | CPU Definition | CPU Meaning

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What is CPU: It's meaning is Central Processing Unit.  Sometimes referred to simply as the central processor or Nerve Centre or heart, but more commonly called processor, the CPU is where most calculations take place. The CPU is the brains of the computer.

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CPU Definition

Central processing unit (CPU) is the central component of the Computer System. Sometimes it is called as microprocessor or processor. It is the brain that runs the show inside the Computer. All functions and processes that is done on a computer is performed directly or indirectly by the processor. 

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Obviously, computer processor is one of the most important element of the Computer system. CPU is consist of transistors,that receives inputs and produces output.Transistors perform logical operations which is called processing. It is also, scientifically, not only one of the most amazing parts of the PC, but one of the most amazing devices in the world of technology.

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In terms of computing power, the computer processor is the most important element of a computer system. It add and compare its data in cpu chip. A CPU of all computers, whether micro, mini or mainframe must have three parts.

Parts of CPU

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Arithmetic Logic Unit (ALU): It is the part of computer processor (CPU) can be used to perform arithmetic and logic operations. An arithmetic-logic unit (ALU) is further divided into two parts, (AU) arithmetic unit  and a (LU) logic unit.

Control Unit (CU): Decodes the program instruction. CPU chip used in a computer is partially made out of Silica. on other words silicon chip used for data processing are called Micro Processor. 

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Registers: It is temporary storage areas of the computer processor. It is managed by control unit(CU).Registers holding data,instruction and address that are needed by program while running.

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The processor plays a significant role in the following important aspects of your computer system; 

Performance: The processor is probably the most important single determinant of system performance in the Pc. While other components also playa key role in determining performance, the processor's capabilities dictate the maximum performance of a system. The other devices only allow the processor to reach its full potential.  

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Software Support: Newer, faster processors enable the use of the latest software. In addition, new processors such as the Pentium with MMX Technology, enable the use of specialized software not usable on earlier machines.

Reliability and Stability: The quality of the processor is one factor that determines how reliably your system will run. While most processors are very dependable, some are not. This also depends to some extent on the age of the processor and how much energy it consumes. 

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Energy Consumption and Cooling: Originally processors consumed relatively little power compared to other system devices. Newer processors can consume a great deal of power. Power consumption has an impact on everything from cooling method selection to overall system reliability.

Motherboard Support: The processor that decides to use in your system will be a major determining factor in what sort of chipset we must use, and hence what motherboard you buy. The motherboard in turn dictates many facets of. The system's capabilities and performance. 

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Von-Neumann Architectures

Von Neumann Architecture also known as the Von Neumann model, the computerconsisted of a CPU, memory and I/O devices. The program is stored in the memory. The CPU fetches an instruction from the memory at a time and executes it.

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Thus, the instructions are executed sequentially which is a slow process. Neumann m/c are called control flow computer because instruction are executed sequentially as controlled by a program counter. To increase the speed, parallel processing of computer have been developed in which serial CPU’s are connected in parallel to solve a problem. Even in parallel computers, the basic building blocks are Neumann processors.

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The von Neumann architecture is a design model for a stored-program digital computer that uses a processing unit and a single separate storage structure to hold both instructions and data. It is named after mathematician and early computer scientist John von Neumann. Such a computer implements a universal Turing machine, and the common "referential model" of specifying sequential architectures, in contrast with parallel architectures. 

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One shared memory for instructions (program) and data with one data bus and one address bus between processor and memory. Instructions and data have to be fetched in sequential order (known as the Von Neuman Bottleneck), limiting the operation bandwidth. Its design is simpler than that of the Harvard architecture. It is mostly used to interface to external memory.

Direct memory access (DMA)

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 DMA stands for "Direct Memory Access" and is a method of transferring data from the computer's RAM to another part of the computer without processing it using the CPU. While most data that is input or output from your computer is processed by the CPU, some data does not require processing, or can be processed by another device.

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In these situations, DMA can save processing time and is a more efficient way to move data from the computer's memory to other devices. In order for devices to use direct memory access, they must be assigned to a DMA channel. Each type of port on a computer has a set of DMA channels that can be assigned to each connected device. For example, a PCI controller and a hard drive controller each have their own set of DMA channels. 

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For example, a sound card may need to access data stored in the computer's RAM, but since it can process the data itself, it may use DMA to bypass the CPU. Video cards that support DMA can also access the system memory and process graphics without needing the CPU. Ultra DMA hard drives use DMA to transfer data faster than previous hard drives that required the data to first be run through the CPU. 

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An alternative to DMA is the Programmed Input/Output (PIO) interface in which all data transmitted between devices goes through the processor. A newer protocol for the ATAIIDE interface is Ultra DMA, which provides a burst data transfer rate up to 33 mbps. Hard drives that come with Ultra DMAl33 also support PIO modes 1, 3, and 4, and multiword DMA mode 2 at 16.6 mbps.

DMA Transfer Types

Memory To Memory Transfer   

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In this mode block of data from one memory address is moved to another memory address. In this mode current address register of channel 0 is used to point the source address and the current address register of channel is used to point the destination address in the first transfer cycle, data byte from the source address is loaded in the temporary register of the DMA controller and in the next transfer cycle the data from the temporary register is stored in the memory pointed by destination address. 

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After each data transfer current address registers are decremented or incremented according to current settings. The channel 1 current word count register is also decremented by 1 after each data transfer. When the word count of channel 1 goes to FFFFH, a TC is generated which activates EOP output terminating the DMA service.

Auto initialize 

In this mode, during the initialization the base address and word count registers are loaded simultaneously with the current address and word count registers by the microprocessor. The address and the count in the base registers remain unchanged throughout the DMA service.

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After the first block transfer i.e. after the activation of the EOP signal, the original values of the current address and current word count registers are automatically restored from the base address and base word count register of that channel. After auto initialization the channel is ready to perform another DMA service, without CPU intervention.

DMA Controller 

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The controller is integrated into the processor board and manages all DMA data transfers. Transferring data between system memory and an 110 device requires two steps. Data goes from the sending device to the DMA controller and then to the receiving device. The microprocessor gives the DMA controller the location, destination, and amount of data that is to be transferred. Then the DMA controller transfers the data, allowing the microprocessor to continue with other processing tasks. When a device needs to use the Micro Channel bus to send or receive data, it competes with all the other devices that are trying to gain control of the bus. This process is known as arbitration. The DMA controller does not arbitrate for control of the BUS instead; the I/O device that is sending or receiving data (the DMA slave) participates in arbitration. It is the DMA controller, however, that takes control of the bus when the central arbitration control point grants the DMA slave's request. 

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What is BIOS (basic input/output system)?

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  A BIOS (Basic Input/Output System) Short for ROM is boot firmware program that a computer uses to successfully start operating. The BIOS is located on a chip inside of the computer and is designed in a way that protects it from disk failure.

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When you turn on a PC, the BIOS first conduct a basic hardware check, called a Power-On Self Test (POST), to determine whether all of the attachments are present and working. Then it loads the operating system into your computer's random access memory, or RAM. The BIOS also manages data flow between the computer's operating system and attached devices such as the hard disk, video card, keyboard, mouse, and printer. The BIOS stores the date, the time, and your system configuration information in a battery-powered, non-volatile memory chip, called a CMOS (Complementary Metal Oxide Semiconductor) after its manufacturing process. The main functions of the BIOS are:

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Functions of BIOS 

(i) BIOS Power on Self Test (POST): It is a built-in diagnostic program. This self test ensures that the computer has all of the necessary parts and functionality needed to successfully start itself, such as use of memory, a keyboard and other parts. Then additional tests are done during booting. If errors are detected during the test, the BIOS instruct the computer to give a code that reveals the problem. Error codes are typically a series of beeps heard shortly after startup.

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The BIOS also works to give the computer basic information about how to interact with some critical components, such as drives and memory that it will need to load the operating system. Once the basic instructions have been loaded and the self-test has been passed, the computer can proceed with loading the operating system from one of the attached drives. Computer users can often make certain adjustments to the BIOS through a configuration screen on the computer. The setup screen is typically accessed with a special key sequence during the first moments of startup. This setup screen often allows users to change the order in which drives are accessed during startup and control the functionality of a number of critical devices. Features vary among individual BIOS versions.

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We can also use flash-memory cards to hold BIOS information. This allows users to update the BIOS version on computers after a vendor releases an update. This system was designed to solve problems with the original BIOS or to add new functionality. Users can periodically check for updated BIOS versions, as some vendors release a dozen or more updates over the course of a product's lifetime. 

Mother board (System) BIOS, Video adapter firmware (BIOS), Drive controller firmware (BIOS), Modem Card firmware (BIOS), Network adapter board BIOS, SCSI adapter BIOS. The mother board BIOS provides routines to support motherboard features. BIOS ROM chips for major sub systems of computer such as video and drive control must also be included.

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Actually BIOS can be placed in between the computer and external devices as its name tells it is used for reading the keystroke, displaying values on screen, Reading and writing to and from floppy and hard disks etc.

The keyboard is assigned the port number 60, which is known to BIOS. BIOS read this port and data from keyboard goes to computer.

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(ii) Bootstrap Loader: To boot the operating system. The BIOS contains a program known as bootstrap loader whose responsibility is to search and start the operating system boot program. Then the boot program of operating system controls the computer system and boots the operating system. 

(iii) BIOS Setup Utility Program: A non volatile memory (NVRAM) is used to store information about the computer system. During installation of a system, the user run BIOS setup program and enter the correct parameters. The settings of memory, disk types and other settings are stored in NVRAM and not in BIOS chip itself. To construct NVRAM, the material required is CMOS (Complementary metal oxide semiconductor). These CMOS chips are very efficient storage devices as they store and maintain data on very low values of current. The system's configurations therefore are also termed as CMOS settings, which we can set using BIOS set up program. The BIOS reads the parameters from CMOS RAM as and when required.

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CMOS settings can be maintained by battery backup either by using capacitor or by a battery built into NVRAM chip. This chip also has system clock. If there is no battery, the setting remains for short period of time and we need to reset the system. With it there is loss of BIOS password which protects BIOS set up program.

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To clear the CMOS RAM contents, two methods used are

(i) By using clear CMOS jumper.

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(ii) By holding down enter key during booting of the system.

For Pentium III motherboards, different set ups are there in AMI BIOS. These are:

• Standard CMOS Setup:It is used to set time date, hard disk type, type of floppy drive, type of monitor and keyboard.

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Advanced CMOS Setup:It is used to set typematic rate and delay, above 1 MB memory test, memory test tick sound, Hil < Del> message display, system boot up sequence etc.

• Advanced Chipset Setup:It is used to set features of chipset.

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• Power Management Setup:It is used to control power conservation options.

• PCI/Plug and Play Setup:It is used to set options of PCI bus and that of plug and play devices.

• Peripherals Setup:It is used to control options related to I/O controllers.

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• CPU Configuration Setup:This setup is used to select the types of CPU installed in the motherboard. In AMI BIOS, the settings are auto as it automatically finds out the type of CPU in the computer system. 

(iv) System Service Routines: The BIOS provides various software routines (subprograms) that can be called by higher-level software such, as DOS, Windows, or their applications, to perform different tasks. Virtually every task that involves accessing the system hardware has traditionally been controlled using one or more of the BIOS programs (although many newer operating systems now bypass the BIOS for improved performance). This includes actions like reading and writing from the hard disk, processing information received from devices, etc.

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BIOS services are accessed using software interrupts, which are similar to the hardware interrupts except that they are generated inside the processor by programs instead of being generated outside the processor by hardware devices. One thing that this use of interrupts does is to allow access to the BIOS without knowing where in memory each routine is located.

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Normally, to call a software routine you need to know its address. With interrupts, a table called an interrupt vector table is used that bypasses this problem. When the system is started up, the BIOS puts addresses into this table that represent where its routines are located for each interrupt it responds to. Then, when DOS or an application wants to use a BIOS routine, it generates a software interrupt. The system processes the interrupt, looks up the value in the table, and jumps to the BIOS routine automatically. DOS itself and application programs can also use this interrupt vector table.

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