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Bus structures
Unit objectives Describe the primary types of buses,
and define interrupt, IRQ, I/O address, DMA, and base memory address
Describe the features and functions of the PCI bus
Describe the features and functions of the various video buses
Buses
Communication pathway Defined by
– How many bits it transmits at one time– Signaling technique– Data transfer speed
Four types– Address– Data– Expansion– Video
continued
Buses, continued
Address and data buses enable: – Basic CPU operation – Interactions with memory
Expansion bus– Communication pathway for non-core
components to interact with core components
– Adapter cards add functionality– PCI bus predominant– Older buses: ISA, EISA, Micro Channel,
and PC bus
Video bus
Generates signals sent to monitor Can be built into motherboard or
adapter card Communicates over expansion or
video bus Enormous amount of data strains
expansion bus Graphic buses
– VESA or AGP– Transmit video data at high speeds
The riser bus
Brings the basic wiring and control of a function to a motherboard
Decreases cost Three riser standards:
– Advanced Communication Riser (ACR) – Audio/Modem Riser (AMR) – Communication and Networking Riser
(CNR)
System interaction
Gain the attention of the CPU Access shared memory locations Extend the system BIOS Transfer data across the bus
Interrupts
Signal CPU that attention is needed– CPU stops what it was doing– Services the device request– Returns to its previous task
Polling– Inefficient alternative to interrupts
IRQs
Numerical addresses Help CPU identify interrupt source Enforce priority of interrupts Common IRQs
– IRQ 1: Keyboard– IRQ2: Cascade IRQs 9-15– IRQ4: COM1– IRQ6: Floppy– IRQ12: PS/2 mouse– IRQ14: Primary IDE hard drive
DMA channels
DMA controller relieves CPU Dedicated channels Largely replaced by other techniques,
such as bus mastering
Base memory addresses
Devices extend system BIOS with new routines– Display adapters– SCSI controllers– IDE controllers
System BIOS locates and loads BIOS extensions using mapped memory location
PC/XT bus
8-bit bus– IBM PC and IBM XT
4.77 MHz clock speed 1.6 Mbps maximum data transfer rate
(0.4 MBps) Supported IRQs 0-8 Configured with DIP switches or
jumpers
PC/AT and ISA bus
16-bit bus– IBM AT, clones, 80386/486, current PCs
8 MHz clock speed 8 MBps maximum data transfer rate
PCI bus
32- or 64-bit bus– Pentium PCs
33 or 66 MHz clock speed 133-533 MBps maximum data transfer rate Up to 8 functions on a single card Up to 5 cards/slots per system Requires PnP
PCIe
Newer standard Uses serial communication Link Lanes x1 (by one) x1, x2, x4, x8, x12, x16, and x32 bus
widths Can up-plug Can’t down-plug
Multifunction cards
PCI spec supports multifunction cards Up to 8 functions per card Five slots/cards per system Total of 40 expansion devices
Need for video buses
Older PC designs just used expansion bus
Graphical interfaces involve massive amounts of graphics data
Specialized buses were developed to be fast enough
VESA local bus
32- or 64-bit bus– Pentium PCs
33 or 66 MHz clock speed Popular on 80486-based PCs Generally software configurable No longer used
PCI-based video
Low-end systems: video adapter built into motherboard
Three type of video slots– PCI– PCIe– AGP
PCI is slowest of three types Share bus with all other PCI-based
devices Work well for two-monitor system
AGP
Technically a port, not a bus Provides direct connection between video
adapter and CPU Referred to as #X Original performance benefit was accessing
and using main system memory– Direct Memory Execute (DIME)
Modern AGP cards use onboard memory, except in laptops
Multiple-monitor support Being phased out for PCIe
AGP characteristics
32-bit bus Multiple of 33 MHz clock speed Speed “pumped” to as much as 533
MHz 266-2133 MBps maximum data
transfer rate PnP configurable
AGP slots
Typically brown; sometimes maroon or other dark color
Separated from other bus slots
High-end systems include multiple, independent AGP slots
PCIe
16x PCIe card has 4 Gbps bandwidth Dual line technology allows up to 8
GBps Simultaneous data movement
upstream and downstream Ideal for gaming and photography,
videography