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Buses in a Graphics Context:. PCI vs. AGP. Context: Why use a specialized bus?. Given: I/O takes a significant amount of time, esp. Graphics I/O. If Graphics I/O moves into separate bus, system buses freed up for other devices.

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context why use a specialized bus
Context: Why use a specialized bus?
  • Given: I/O takes a significant amount of time, esp. Graphics I/O.
  • If Graphics I/O moves into separate bus, system buses freed up for other devices.
  • Why use a specialized graphics bus (e.g. AGP) over a universal bus (e.g. PCI)?
features what is a bus
Features: What is a Bus?
  • A way of connecting multiple devices together so that data can be transferred between them
  • Shared by multiple devices --> Only 1 device can send at any particular time
  • Has multiple lines each transferring a single bit. # of lines known as bus width
  • Different types of lines: control, address, and data
  • Multiplexed
  • Buses can be synchronous or asynchronous
  • Intro
    • Created in 1992 - replaced ISA.
    • Solved many technical issues of ISA bus
      • IRQ
      • Address conflicts (jumpers)
    • Features Plug-and-Play technology
    • Because of high speed data transfer only 3-4 devices can be plugged into one PCI bus, as opposed to 6-7 on ISA board.
    • Two PCI buses can be connected with PCI-to-PCI bridges
    • These bridges divide into primary and secondary bridges and are physically isolated.
pci cont d1
PCI (cont’d)
  • Technical Attributes
    • PCI supports up to 64 bit data transfer and 133MHz clock which allows for transfer of 1066 megabytes per second.
    • Features direct memory access (DMA)
    • Bus arbitration: Fair, Round Robin, Master Capable
    • Several pins on the bus are selected to implement bus mastering
      • any device can take over the control of the bus at any time
      • if possible, one device may occupy the whole bandwidth
pci cont d2
PCI (cont’d)
  • Interrupts:
    • Interrupts are not synchronized with other signals on the bus, since they can be activated and used at any time.
    • PCI design does not specify how interrupts are to be shared – this is resolved by motherboard manufacturers.
agp cont d
AGP (cont’d)
  • Higher throughput, both peak and sustained
    • Transferring data on rising and falling edges of clock ticks
    • Uses demultiplexed data and address lines
    • Sideband addressing allows pipelining (i.e. Sending requests while receiving data
    • High peak throughput of 533 MB/s on 32 data lines
agp cont d1
AGP (cont’d)
  • Memory access improvements
    • Direct Memory Execution (DIME)
    • Uses system memory as own, using GART
    • Allows concurrent system memory usage
    • Saves money on expensive graphics memory
    • Useful for on-board graphics

Address/data multiplexed


Peak at 133MB in 32 bits

Multi-target, single master

Connects to multiple peripherals besides memory I/O


Address/data de-multiplexed

Pipelined requests

Peak at 533MB/s in 32 bits

Single target, single master - makes DIME possible

Memory read/write only, no other I/O operations

comparisons cont d
Comparisons (cont’d)
  • Data Transfers
    • AGP makes multiple requests for data during a bus or memory access.
    • PCI makes one request, and does not make another until the data it request has been transferred.
  • AGP does not replace the PCI bus, but it is a dedicated connection that can used only by the graphics subsystem
  • AGP is faster
  • PCI is more flexible
  • AGP Information:
    • [email protected] PCI Information. http://home.hyperlink.net.au/~chart/program.htm#agp (Feb. 2003).
    • Intel Corporation. Accelerated Graphics Port Technology Home. http://www.intel.com/technology/agp/ (Feb. 2003).
  • General Bus Information:
    • Kozierok, Charles M. The PC Guide. http://www.pcguide.com/ (Feb 2003).
    • Stallings, William. Computer Organization and Architecture: Designing for Performance. 5th ed. New Jersey: Prentice Hall, 2000.
  • PCI Information:
    • Adaptec, Inc. Adaptec: A Fundamental Element of the PCI Architecture is the Concept of Bridges. http://www.adaptec.com/worldwide/support/supporteditorial.html?sess=no&language=English+US&prodkey=PCI_bridge_whitepaper (Feb. 2003).
    • Apple Computer, Inc. PCI Bus Overview1. http://developer.apple.com/techpubs/hardware/DeviceManagers/pci_srvcs/pci_cards_drivers/PCI_BOOK.5.html (Feb. 2003).
    • Quatech, Inc. PCI Bus Overview. http://www.quatech.com/Application_Objects/FAQs/comm-over-pci.htm (Feb 2003).
    • Sundance Multiprocessor Technology Ltd. & Sundance Italia S.r.l & Sundance DSP Inc., E&OE. A brief description of the PCI Bus. http://www.sundance.com/edge/files/pci.htm (Feb. 2003).