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Chapter 2

Central Processing Unit

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CPU

• The "brain" of the computer system is called the central processing unit.

• Everything that a computer does is overseen by the CPU.

• CPUs have for years increased their performance.

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CPU History

• closely tied to the IBM and Intel. The CPUs have their roots to Intel's chip 4004 from 1971.

• seven or eight CPU generations up till today.

• The first microprocessor to make into a home computer was the Intel 8080.

• PC market moved from the 8088 to 80286 to 80386 to 80486 to Pentium, Pentium II, Pentium III and Pentium 4.

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Functions of CPU

• executes a collection of machine instructions that tell the processor what to do.

• Based on the instructions, a microprocessor does three basic things.

– perform mathematical operations. – process and transfer data – make decisions

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The CPU – speed measurement

• clock speed and data width.

• Norton System Information (SI).

– a relative number – 8086=1– Computing Index (CI) – CPU Speed– Disk Index (DI) – Disk Speed– Performance Index (PI) – Combined CI & DI

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CPU Frequencies

• The internal clock frequency is the speed inside the CPU.

• The external clock frequency (Front Side Bus or System Bus) is the speed between the CPU and RAM.

Internal clock frequency is higher than External clock frequency

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Bus & Bus Speed• connects one part of the motherboard to another.

• speed measured in megahertz (MHz)

– front side bus (FSB) – CPU to Northbridge.

– back side bus - CPU to level 2 (L2) cache.

– memory bus - Northbridge to the memory.

– AGP bus - the video card to the memory and the CPU.

– IDE or ATA bus - Southbridge to the disk drives.

– PCI bus - PCI slots to the Southbridge.

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Bus Connections

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How does a CPU work?

• centrally located on the motherboard.

• data come from the RAM and other units.

• receives and sends its data from and to buses. They can be divided into:

– system bus, which connects the CPU with RAM – I/O buses, which connect the CPU with other components.

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System Bus

• Any computer will have three major system buses

– Address Bus– Data Bus and – Control Bus.

CPUSystem Bus RAML2

CACHE

I/O BusesI/O UNITS

(Drives, keyboard,

port, adapter, etc)

Bridges(Chipset)

• System bus is the central bus of the PC that connect CPU with RAM.

• A bridge connects the I/O buses with the system bus and on to RAM. The bridge is part of the PC chipset.

• Chip sets are a bunch of intelligent controller chips, which are on a motherboard.

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Address Bus

• Unidirectional – one direction only

• carries addresses by CPU to memory and I/O elements.

• The size of the address is determined by the number of lines in the bus.

• The lines determine the number of memory locations and I/O, the CPU can address to.

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Address Bus - Example

If the address bus is composed of 16 lines, the CPU will be able to generate 216, or 65,536 distinct address codes. (1k = 1024). If the address bus size is increased to a 20-bit word size, what will be the possible addresses?

Possible addresses = 220 = 1,048,576 addresses

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Data Bus

• Bi-directional – both directions.

• Write operation – data from the CPU to memory.

• Read operation – data moves from memory to the CPU.

• size of the data bus usually corresponds to the word size of the computer.

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Control Bus

• Bi-directional.

• carries the timing and control signals.

• Some are output signals or input signals to/from the CPU to/from I/O elements.

• Each microprocessor has its own unique set of control signals.

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Types of data

• Instructions:

– called program code.

– include the commands send to your PC using your keyboard and mouse.

– e.g. Commands to print, save, open, etc.

• Data:– user data. – e.g. actual contents (the text or the letters).

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I/O Bus

• move data.

• connect all I/O devices with the CPU and RAM.

• buses speed is lower than the system bus.

• On modern PCs, you will usually find four buses:

– The ISA bus, an old low speed bus.

– The PCI bus, which is a new high speed bus.

– The USB bus (Universal Serial Bus), which is a new bus.

– The AGP bus (Accelerated Graphics Port) which is used for the graphics card.

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Physical aspects of the I/O buses

• consists of tracks on the printed circuit board.

• These tracks are used as:

– Data– Address – Control

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ISA bus (Industry Standard Architecture)

• Since about 1984, it is the standard bus for PC I/O functions.

• ISA was an improvement over the original IBM XT bus, which was only 8 bit wide. IBM's trademark name is AT bus.

• can run as 16 bit wide but it requires 2-3 clock ticks to move 16 bits of data.

• ISA bus works synchronous with the CPU.

• ISA bus has a theoretical transmission capacity of about 8 MB/s. (Actual speed does not exceed 1-2 MB/s.)

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Disadvantages of ISA Bus

• narrow or limited bandwidth – only 8 bits wide

• It is slow. It requires 2-3 clock ticks to move a16 bits of data.

• not “intelligent”, since the CPU has to control the data transfer across the bus.

• When PC communicates with the floppy drive, the rest of the PC is waiting for it to complete.

• It is not in used in the new Pentium 4 computer.

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PCI bus (Peripheral Component Interconnect)

• high speed bus of the 1990s made by Intel – 32 bit wide

• is processor independent. Therefore, it can be used with all 32 or 64 bit processors.

• is backward compatible with the ISA bus.

• is buffered in relation to the CPU and the peripheral components.

• handles the transmission in its own tempo.

• operate asynchronous.

• is intelligent – All PCI adapter cards configure themselves. Plug and Play. (PnP).

• is the central I/O bus, in all PCs.

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Plug and Play

• is part of the PCI specification. It means that all PCI adapter cards are self-configuring.

• During startup, communication takes place between the PC’s startup programs, the PCI controller and each PCI device (adapter).

• The adapter must be able to inform the I/O bus which I/O addresses and IRQ’s it can operate with and be able to configure itself to use the resources allocated to it by the I/O bus.

• When the exercise is successful, the adapter is configured automatically, and is ready to be used by the operating system.

• All the components involved (adapter, motherboard and Windows) have to be Plug and Play compatible for the system to work.

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Schematic overview of Plug and Play

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Extended System Configuration Data (ESCD)

• is a small data area which is stored in the motherboard’s CMOS storage.

• ESCD store is used to save adapter configuration information; e.g. PC’s configuration.

• ESCD also allows the user to manually allocate an IRQ etc. This can be done using the motherboard’s setup program.

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What is a chip set?

• The chip sets are a bunch of intelligent controller chips; it controls the system and its capabilities.

• All components communicate with the processor through the chipset transfer and organize the steady flow of data.

• makes a bridge between the CPU and other components. It permitted concurrent activity in all three locations, (multitasking). All data transfer to and from I/O units cross this intersection..

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Typical Chipset

PCI BusSystem Bus North Bridge

South Bridge

CPU

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MCH and ICH• a new architecture was introduced by Intel in 1999.

• Memory Controller Hub (MCH) replacing the north bridge.

• I/O Controller Hub (ICH) replacing the south bridge.

• The MCH is a controller located between the CPU, RAM and AGP. It regulates the flow of data to and from RAM. This new architecture has two important consequences:

– The connection between the two hubs is managed by a special bus (link channel), which can have a very high bandwidth.

– The PCI bus comes off the ICH, and doesn’t have to share its bandwidth with other devices.

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Two Bridges

• a north and a south bridge.

• Both bridges are essentially routers.

• north bridge takes all the heavy traffic

• south bridge routes in to a lot of different narrow routes

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Typical Chipset