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System Buses. Val Grinblat Dan Hummell Ryan McKenica. Introduction. 1980s More power Better performance Enhance operation. PC/XT. Released in 1981 Extension of the motherboard Processor and complete control 62 pins. AT Bus. Limited memory handling Therefore AT was developed

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System buses

System Buses

Val Grinblat

Dan Hummell

Ryan McKenica


  • 1980s

  • More power

  • Better performance

  • Enhance operation

Pc xt

  • Released in 1981

  • Extension of the motherboard

  • Processor and complete control

  • 62 pins

At bus
AT Bus

  • Limited memory handling

  • Therefore AT was developed

  • Two separate oscillators

Physical differences
Physical Differences

  • A second connector

  • Backwards compatible

  • Smooth bridge to the 16-bit data bus

Isa bus

  • Industry Standard Architecture

  • 16-bit addressing limit

  • Twice as many interrupts and DMA channels

Problems with isa
Problems with ISA

  • Complete access to system resources

  • No central registry

  • Limited number of interrupts

  • Limited number of available ports

  • Quatech


  • IBM developed MCA bus in 1987

  • Design began around 1983

    • AT bus shows hint of MCA bus design

  • First 32-bit bus for the personal computer

  • Revolutionized design and standard of the bus

  • Replaced the PC/AT standard for PCs

  • Inspired by the best ideas from mainframe computers

  • IBM patents design to corner the market


  • Programmable Option Select

    • No longer need dip switches, jumpers, and headers

  • Small Architecture

    • Many grounds close to the high frequency digital signals

    • Smaller PCs

  • Multitasking

  • 32-bit bus width

  • First plug & play board


  • Microprocessor Speed vs. Bus Speed

    • 8 Mhz Bus Clock Signal lock

    • 10 Mhz Capability

  • Higher speed than ISA

  • Extra data lines:16 more data lines for 32 bits total

  • More address lines:4 gigabytes address memory

  • Channels for improved audio and video

  • Smaller mount components and expansion boards

  • FCC Certification easy to attain

    • Every fourth pin had electrical ground


  • Obsolete

  • Costly for manufactures and users to upgrade from PC/AT-based PCs

  • Not backward compatible with ISA-based boards

  • IBM had total control over architecture

  • Bus speed not fast enough for modern processors and PCs


  • Introduced in 1998

  • “Gang of Nine”

    • Nine companies united to develop a design to match IBM’s MCA bus

  • Improvements based on MCA

    • Compatible with ISA-based systems

    • Not patent to one company

  • Molded from the AT bus design


  • Plug & Play feature

  • ISA cards work on EISA slots

  • Addressing Enhancement

    • 32-bit address bus

  • Bus width signaling

  • New transfer modes

    • Burst mode


  • Backward compatible with ISA

  • Supports bus mastering for greater efficiency

  • 32-bit bus width

  • Interrupt Sharing

  • Design was open to all manufactures


  • More expensive than other systems

  • Few EISA-based cards available

  • Performance is lower than more modern, popular buses such as the VESA local bus and PCI

  • Not widely used

Vesa or vl bus

VESA or VL Bus

Video Electronics Standards Association Bus


Video Local Bus

Brief history
Brief History

  • Developed by the Video Electronics Standards Association

  • To replace slow ISA bus for video components and high-speed devices

  • Can be used a local video bus or an expansion bus

Technical details
Technical Details

  • Data width 32 bits

  • Bus speed 33MHz

  • Throughput 250 MB/sec

  • Transfer rate 8 times faster then the ISA bus

  • Bus connector original ISA slot plus additional VESA slot (see figure)

Technical details cont
Technical Details Cont

  • VESA bus connector

  • Two devices on one expansion card

  • Limited number of VESA devices connected to system board at one time

What happen to the vesa bus
What happen to the VESA bus?

  • It had a good design and good features

  • Plans from VESA for a 64-bit version of the bus for the Pentium processor

  • However, the PCI bus become more popular when the Pentium processor became the standard processor

Pci bus


Peripheral Component Interconnect Bus

Brief history1
Brief History

  • Developed in 1993 by the Intel Corporation

  • Originally design for better graphics capabilities on Intel computers

  • Has become the standard general-purpose bus

  • Used in PCs and Macintosh computers

Technical details1
Technical Details

  • Data width 32 bits

  • Bus speed 33 MHz

  • Throughput 265 MB/sec

  • Can run at speeds different then the CPU

  • Short slot length then the both the ISA and VESA bus (see figure)

Technical details cont1
Technical Details Cont

  • PCI bus connector

  • Plug and Play compatible

  • IRQ sharing

  • PCI Bridge

Where is the pci bus today
Where is the PCI bus today?

  • Standard in most all PCs sold today

  • Still the standard general-purpose bus

  • Not often used for video adapters since development of the AGP bus