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PROCESSING & COMMUNICATION INSIDE A COMPUTER Compiled by : S. Agarwal, Lecturer & Systems Incharge St. Xavier’s Computer Centre, St. Xavier’s College Kolkata. March-2003

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Basic CPU Components The CPU consists of The control unit which controls all operations of the computer system. The arithmetic and logic unit which performs arithmetic and logical operations. Arithmetic operations involve the general mathematical calculations like addition, subtraction, multiplication and division. Logical operations involve comparisions like > < = etc.  Registers which hold temporary data for a specific purpose or function. Interface which communicate with external memory, I/O units, and possibly other CPUs.

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CPU CLOCK SPEED The rate of speed that the processor executes commands is measured by clock speed, or often called MHz (megahertz). The processor requires a fixed number of clock cycles to perform each instruction. The faster the clock speed, the more instructions it can execute. Thus resulting in a faster running computer system.

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Listed below are some of the more common processors and clock speed on the market today.

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Obsolete CPUs 1. Intel Celeron 266 - 533 MHz.2. Intel Pentium II 233 - 450 MHz3. AMD K6-2 266 - 550 MHz4. VIA Cyrix III 553 - 667 MHz5. AMD K6-3 400 and 450 MHz 6. Cyrix MII 7. Pentium8. AMD K69. Cyrix 686

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ARITHMETIC-LOGIC  UNIT The  arithmetic-logic  unit  (ALU)  performs   all arithmetic operations   (addition,    subtraction, multiplication,   and   division)   and   logic   operations. Logic  operations  test  various  conditions  encountered during processing and allow for different actions to be taken based on the results. The data required to perform the arithmetic and logical functions are inputs from the designated CPU registers and operands. The  ALU  relies  on  basic  items  to  perform  its operations.  These  include  number  systems,  data  routing circuits   (adders/subtracters),   timing,   instructions, operands,   and   registers.

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Control Unit The   control   unit   maintains   order   within   the computer   system   and   directs   the   flow   of   traffic (operations) and data.

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How the CPU works The CPU is centrally located on the motherboard. Since the CPU carries out a large share of the work in the computer, data pass continually through it. The data come from the RAM and the units (keyboard, drives, etc.). After processing, the data is sent back to the RAM and the units. The CPU continually receives instructions to be executed. Each instruction is a data processing order. The work itself consists mostly of calculations and data transport.

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The Instruction-Execution Cycle Many types of personal computers can execute instructions in less than one-millionth of a second; supercomputers can execute instructions in less than one-billionth of a second. The CPU performs four steps in executing an instruction: The control unit gets the instruction from memory. The control unit decides what the instruction means and directs the necessary data to be moved from the memory to the arithmetic logic unit. The arithmetic logic unit performs the actual operation on the data. The result of the operation is stored in memory or a register. The first two instructions make up what is called the instruction time. The last two instructions make up what is called the execution time. The combination of these two is called a machine cycle.

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Each central processing unit has an internal clock (or system clock), which produces pulses at a fixed rate to synchronise all computer operations. A single machine cycle instruction is made up of a number of subinstructions, each of which must take at least one clock cycle. Each type of CPU is designed to understand a specific group of instruction called the instruction set. How the CPU finds Instructions and Data The location in memory for each instruction and each piece of data is identified by an address, or a number that stands for a location in the computer memory. An address may be compared to a mailbox in everyday life, except that the address can hold only one item - a fixed amount of data, a number or a word - at any one time.

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The following is an example of a simple case of adding two numbers together and placing the result in a location X. The command executed is - Let X = N1 + N2.

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Interrupts (IRQs) An interrupt is basically what it sounds like, a message from one part of the computer to another (normally to the system processor) that tells it that it needs to stop what it is doing, and do something else instead. An IRQ is an interrupt request, and is the name for the actual signal that is used when a peripheral requests an interrupt of the processor. Interrupts play a key role in how the processor performs input/output processing, and interfaces with every peripheral in the computer, from the keyboard and mouse to the hard disk and modem.

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The PC Bus The bus is actually a set of circuits that run throughout the board and connect all the expansion slots, memory, and CPU, etc. together. The various components and devices must be linked together to perform a function. The 'bus' (or should we say 'BUSES'?) provides a highway for passing information between the devices on the system.

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The bus ties these devices together, so that A signal from the keyboard is displayed on the screen (CRT) A record from a file on a hard disk drive is read into memory and processed at the direction of the CPU A file is sent to a printer for printing, etc. SO we can say, electrical signals representating information flows along the bus from one device to another.

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The computer may contain several types of buses (all located on the motherboard). Some of the more common buses found are: CPU bus or 'system' bus. An address bus Memory bus I/O or Expansion Bus ISA Bus PCI Bus Micro Channel Bus EISA Bus External Buses (Can have external cables connecting devices) SCSI Bus PC Card Bus USB Bus

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VARIOUS BUSES : The Processor Bus: This is the highest-level bus that the chipset uses to send information to and from the processor. The Cache Bus: Higher-level architectures, such as those used by the Pentium Pro and Pentium II, employ a dedicated bus for accessing the system cache. This is sometimes called a backside bus. Conventional processors using fifth-generation motherboards and chipsets have the cache connected to the standard memory bus. The Memory Bus: This is a second-level system bus that connects the memory subsystem to the chipset and the processor. In some systems the processor and memory buses are basically the same thing.

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The Local I/O Bus: This is a high-speed input/output bus used for connecting performance-critical peripherals to the memory, chipset, and processor. For example, video cards, disk storage devices, high-speed networks interfaces generally use a bus of this sort. The two most common local I/O buses are the VESA Local Bus (VLB) and the Peripheral Component Interconnect Bus (PCI). The Standard I/O Bus: Connecting to the above three buses is the "good old" standard I/O bus, used for slower peripherals (mice, modems, regular sound cards, low-speed networking) and also for compatibility with older devices. On almost all modern PCs this is the Industry Standard Architecture (ISA) bus.

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All buses are located on the motherboard with the exception of the External buses. External buses connect to standard ISA or PCI expansion slots via a controller/adapter card. This card acts as an interface between the ISA/PCI bus architecture. The cables connected to the adapter/controller card are actually considered as the bus. Thus the 50 pin cable connecting SCSI devices to the controller is the actual bus. There is a new bus called the "Universal Serial Bus" that fits into this category. The difference is that this bus connects directly to the system bus on the motherboard.

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A motherboard is the physical arrangement in a computer that contains the computer's basic circuitry and components. On the typical motherboard, the circuitry is imprinted or affixed to the surface of a firm planar surface and usually manufactured in a single step. The computer components included in the motherboard are: The microprocessor (Optionally) coprocessors Memory BIOS Expansion slots Interconnecting circuitry Additional components can be added to a motherboard through its expansion slots. The electronic interface between the motherboard and the smaller boards or cards in the expansion slots is called the bus.

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Microprocessor A microprocessor is a computer processor on a microchip. It's sometimes called a logic chip. It is the "engine" that goes into motion when you turn the computer on. A microprocessor is designed to perform arithmetic and logic operations that make use of small number-holding areas called registers. Typical microprocessor operations include adding, subtracting, comparing two numbers, and fetching numbers from one area to another. These operations are the result of a set of instructions that are part of the microprocessor design. When the computer is turned on, the microprocessor is designed to get the first instruction from the basic input/output system (BIOS) that comes with the computer as part of its memory. After that, either the BIOS, or the operating system that BIOS loads into computer memory, or an application program is "driving" the microprocessor, giving it instructions to perform.

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Microchip A microchip (sometimes just called a "chip") is a unit of packaged computer circuitry (usually called an integrated circuit) that is manufactured from a material such as silicon at a very small scale. Microchips are made for program logic (logic or microprocessor chips) and for computer memory (memory or RAM chips).

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MEMORYA computer requires a memory to store and retrieve instructions and data. There are a variety of storage devices including semiconductor memories and magnetic memories. Generally, the term memory refers to only the small integrated circuits called chips, which are used as a computer's internal memory.

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BIOS BIOS is a layer between the hardware and the software. If the software wants to access the hard drive, it has to go through the BIOS to make sure that the hard drive is working. The BIOS translates between the two because hardware speaks a machine language and software speaks a programming language. The BIOS is considered neither a hardware nor a software but a firmware. BIOS is like software because it contains instructions, but it is a hardware as it is an intricate part of the motherboard’s circuitry.

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BIOS is an integral part of the computer and comes with it when you bring it home. (In contrast, the operating system can either be preinstalled by the manufacturer or vendor or installed by the user.) BIOS is a program that is made accessible to the microprocessor on an eraseable programmable read-only memory (EPROM) chip. When you turn on the computer, the microprocessor passes control to the BIOS program, which is always located at the same place on EPROM. When BIOS boots up (starts up) the computer, it first determines whether all of the attachments are in place and operational and then it loads the operating system (or key parts of it) into the computer's random access memory RAM from the hard disk or diskette drive. With BIOS, the operating system and its applications are freed from having to understand exact details (such as hardware addresses) about the attached input/output devices. When device details change, only the BIOS program needs to be changed. Sometimes this change can be made during the system setup. In any case, neither the operating system or any applications you use need to be changed.

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If you open the PC and examine the system motherboard you will notice that it has a number of different expansion slots with expansion cards inserted into them. By using a collection of wires and protocols, each slot allows you to add functionality to the PC by inserting printed circuit boards (expansion boards), such as soundcards, graphics cards, TV cards etc. These boards communicate with the other hardware devices in the system by means of the data bus which facilitates communication with the microprocessor.

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There are three types of expansion slots: ISA (Industry Standard Architecture) This bus architecture was developed by IBM for their PC/XT and PC/AT machines and became a de facto industry standard. Generally these are the long black slots you see on the motherboard. PCI (Peripheral Component Interconnect) Starting in the early 90s, ISA began to be replaced by the PCI local bus architecture. These are the smaller, white slots on the motherboard. PCI is a 64-bit bus, though it is usually implemented as a 32-bit bus.It can run at clock speeds of 33 or 66 MHz. At 32 bits and 33 MHz, it yields a throughput rate of 133 MBps. Although it was developed by Intel, PCI is not tied to any particular family of microprocessors. AGP ( Accelerated Graphics Port) This is a interface specification developed by Intel Corporation. These are the smallest, brown slots on the motherboard.. AGP is based on PCI, but is designed especially for the throughput demands of 3-D graphics. Rather than using the PCI bus for graphics data, AGP introduces a dedicated point-to-point channel, so that the graphics controller can directly access main memory. The AGP channel is 32 bits wide and runs at 66 MHz. This gives a total bandwidth of 266 MBps, as opposed to the PCI bandwidth of 133 MBps. AGP allows 3-D textures to be stored in main memory rather than video memory.

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