Computer Buses SJSU - Fall 2008 CS 147 Vu Luu

Contents 1. Concepts 2. Measurement 3. Operation

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 ISA EISA VLB PCI bit PCI 2.1 (66 MHz) AGP 8x322,133 USB 21Slow-Speed: 1.5 Mbit/s Full-Speed: 12 Mbit/s Hi-Speed: 480 Mbit/s Firewire 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

4. CPU takes data from data lines and then de-asserts MREQ and RD to release the bus

1. CPU puts address on the bus. 1

2. CPU asserts MREQ and RD lines. 1 2

3. CPU asserts MSYN line. Memory controller locates and loads data from memory to data lines

4. Memory controller locates, loads data from memory to data lines, and asserts SSYN line

5. CPU takes data from data lines and then de-asserts MREQ, RD, and MSYN

6. Finally, memory controller de-assert SSYN

Bridge-based bus architectures System includes a lot of buses which are segregated by bridges. Advantage: buses can simultaneously operate. Intel architecture:

Bridging with AMD processors

Bridging with VIA C7 processors

Internal Communication Methodologies Programmed I/O (polling) Interrupt-drive I/O Direct Memory Access (DMA)

Programmed I/O (polling) CPU polls each device to see if it needs servicing. Drawback: The CPU wastes time for polling devices (busy- wait.)

ModeMaximum transfer rate (MB/s) mode 03.3 mode 15.2 mode 28.3 mode mode mode 520 mode 625 Programmed I/O modes in the ATA interface

Interrupt-Drive I/O (PIO) Device requests service through a special interrupt request line that goes directly to the CPU. No busy-wait. More efficient than PIO.

Direct Memory Access (DMA) Devices transfer data directly to and from memory bypasses the CPU. Very efficient mode. CPU is free to do other operations.

ModesMaximum transfer rate (MB/s) Multi-word DMA Multi-word DMA Ultra DMA Ultra DMA Ultra DMA Ultra DMA Ultra DMA 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., p.303 – p.316. Kozierok, Charles. The PC Guide.