What is a bus? • Buses are defined not only by their placement but by their width. • The larger the bus width, the more information that can be transferred in a specific time period. • Example: A 32 bit data bus can transmit 32 bits of data across the bus at any one time, whereas an 8 bit data bus can only transmit ¼ of the data. Bus Width Bus width refers to the number of wires in the bus over which data can travel. Buses • There are two buses on every motherboard: - internal bus and - external bus. Bus Speed • A computer or device's bus speed or throughput is always measured in bits per second or megabytes per second. Internal Bus • The Internal bus (also called the system bus) is the wiring that connects the CPU, memory, and all the other components on the motherboard. • The internal bus is represented by the wiring on the bottom of the motherboard Parts of the Internal Bus Part Function Power The power bus is the wires that provide electrical power to every part of the motherboard. Control The control bus sends out timing signals so that other components on the motherboard can stay in time with the CPU. Address The address bus is used for sending information on memory addressing. This information tells the components on the motherboard where to find instructions and data in memory. Data The data bus does exactly what it sounds like. It is responsible for transmitting the actual data between the system components. External Bus • The External bus (or expansion bus) connects all the peripherals and attachments to the motherboard. • The most common form of external bus connector is the expansion slot. • Types of expansion slots include: - 8-Bit, ISA, MSA, EISA, IDE, VLB, PCI, AGP 4 PCI Express bus card slots • Every external bus slot carries a certain number of signals through it's copper pathways to the external bus. • Some of these signals are used for power, some for IRQ requests, and some connect directly to the data and address bus. Some important connections to the external bus include: • Clock Signal • Interrupt Request (IRQ) • Direct Memory Access (DMA) • Input/Output Address (I/O) • Bus Mastering Clock Signal • The clock signal is used to provide timing information in order for the bus clock to synchronize it's transmissions with the system buses. Interrupt Request (IRQ) • Sometimes the external bus must carry a request to the CPU that needs urgent or special attention. This is called an Interrupt Request, or IRQ. • Each external bus device is given an IRQ value so that the CPU knows which IRQ's are coming from which devices. • IRQ's can be shared between devices, but only if the two devices NEVER attempt to use the Interrupt at the same time. Direct Memory Access (DMA) • DMA allows certain types of instructions to totally bypass the processor and access main memory themselves. • Unlike IRQ's, two devices can not share a DMA channel, although current system buses provide more DMA channels than were previously available. • Hard drives and video cards use DMA extensively, mainly because they have on- board processors that don't necessarily require CPU access. Input/Output Address (I/O) • As opposed to an IRQ sending an interrupt to the CPU, I/O addresses allow the processor to directly access the device. • Each expansion port is identified by a unique I/O address, which lets the CPU send instructions directly to the device. • The device then responds through it's DMA channel or the data bus. • No two devices can share the same I/O address. Bus Mastering • Sometimes a device requires the ability to talk directly to another device. • This is called bus mastering, and is controlled by a bus mastering chip installed directly on the expansion device.