Concepts A bus is a collection of wires and connectors through which the data is transmitted. Bus = address bus + data bus –Data bus: transfers actual data. –Address bus: transfers information about data and where it should go.
Concepts (cont.) Bus protocol: rules determining the format and transmission of data through bus. Parallel bus: data is transmitted in parallel. –Advantage: fast –Disadvantage: high cost for long distance transmission, interference between lines at high frequency. Serial bus: data is transmitted in serial. –Advantage: low cost for long distance transmission, no interference. –Disadvantage: slow Bus master: The device controls bus. Other devices are slaves.
Concepts (cont.) Local (system) bus: CPU main memory. Front side bus: –Original concept: CPU components –Modern Intel architecture: CPU NorthBridge chipset Back side bus: CPU L2 cache Memory bus: Northbridge chipset main memory AGP bus: Northbridge chipset GPU ISA, EISA, VLB, PCI, Firewire, USB, PCI-Express bus: motherboard peripheral devices.
Measurement Bus width: indicates the number of wires in the bus for transferring data. Bus bandwidth: refers to the total amount of data that can theoretically be transferred on the bus in a given unit of time.
BusWidth (bit)Bandwidth (MB/s) 16-bit ISA1615.9 EISA3231.8 VLB32127.2 PCI32127.2 64-bit PCI 2.1 (66 MHz)64508.6 AGP 8x322,133 USB 21Slow-Speed: 1.5 Mbit/s Full-Speed: 12 Mbit/s Hi-Speed: 480 Mbit/s Firewire 4001400 Mbit/s PCI-Express 16x version 2168,000 Width and Bandwidth of Some Typical Buses
Synchronous Bus vs. Asynchronouse Bus A bus can be classified as one of two type: synchronous and asynchronous. Synchronous bus: there is a common clock that synchronizes bus operations. Asynchronous bus: there is no common clock. Bus master and slaves have to handshake during transmission process.
1. CPU places address of the location it wants to read on the address lines. 1
2. After the voltages on the address lines have become stable, CPU asserts MREQ and RD lines. 1 2
3. Memory controller locates memory location and loads it into data lines. 1 2 3
4. CPU takes data from data lines and then de-asserts MREQ and RD to release the bus. 1 2 3 4
ModesMaximum transfer rate (MB/s) Multi-word DMA 113.3 Multi-word DMA 216.6 Ultra DMA 016.7 Ultra DMA 125.0 Ultra DMA 233.3 Ultra DMA 344.4 Ultra DMA 466.7 Ultra DMA 5100 Ultra DMA 6133 DMA modes in the ATA interface
References Murdocca, Miles and Heuring, Vincent. Computer Architecture and Organization: An Integrated Approach. John Wiley & Sons, Inc., 2007. p.303 – p.316. Kozierok, Charles. The PC Guide. http://www.pcguide.com/. http://www.pcguide.com/