Computer Buses

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Computer Buses

• Ref Burd, Chp 6. 206 220
• Englander, Chp 7 p 177-183
• Chp 8, p 211- 218, 228-236

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Outline

• Definition of bus
• Types of bus lines and their role
• Interrupts
• Bus standards
• Ports

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Buses

• Data travels between components (CPU, memory
  I/O devices) of the computer along communication
  paths called buses. (Note can also have buses
  external to system)
• Bus is made up of multiple lines, which may be
  wires or conductors on a printed board.
• Bus may carry data between two components only
  (point-to-point) or it may be shared between
  many (multipoint). System bus connects CPU with
  main memory other system components.

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Buses
Point-to point bus
Multipoint / system bus
Buses may be serial (one bit at time) or parallel
(multiple bits at time. Most internal buses are
parallel, most external are serial.
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Bus Lines

• Bus lines classified as data, address, control
  and power lines. Depending on the function of
  the bus, it may have all or some of these types.
• Current system bus may have gt100 lines, including
  all 4 types
• Data lines provide path for moving data
  (including instructions) between system
  components. Its width usually same or multiple
  of CPU word size (eg 64 bit CPU usually has 64 or
  128 bit data bus) to enhance system performance

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Address lines used by the CPU to carry a digital
code which uniquely identifies each memory
location and I/O port. Its size dictates maximum
addressable memory. eg 8 bit address bus allows
only 28 memory locations

MEMORY LOCATIONS
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Evolution of Intel Microprocessor

• Name 8080 80286 80386 Pentium
  PII
• Year 1974 1980 1985
  1993 1997
• Data Bus 8 16 32
  64 64
• Address 20 24 32
  32 36
• bus width
• Addressable 1MB 16MB 4GB
  4GB 64G
• Memory

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Bus lines

• Control lines include
• memory read/write I/O read/write
• transfer acknowledge
• bus request/grant
• interrupt request/acknowledge
• clock used to synchronize operations
• Control signals necessary to effectively share
  bus, but may consume many bus cycles gt bus
  protocols designed to minimize these.

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For module to send data to another, must obtain
use of bus by sending bus request signal on
control line
CONTROL LINES
When request granted, puts module address on
address lines, and transfers data via data bus
lines.
ADDRESS LINES
DATA LINES
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Bus Speed

• Bus clock speed and data bus width together
  determine theoretical max data transfer rate of
  bus eg 64 bits 100 mhz.
• However, actual speeds are much less than this
  due to reliability problems caused by with
  electrical interference, slow peripheral devices
  etc.
• Power lines carry electrical power to the module
  they are communicating with.

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Interrupts

• An interrupt is a signal that interrupts normal
  processing of CPU to indicate some event has
  occurred. It may be come from several sources
• I/O device ( e.g keyboard, printer, disk drive)
• timer (running from system clock)
• program (software interrupt)
• hardware failure (eg memory error)
• Interrupts carried on special bus control lines
  to cpu, eg in pcs 15 lines, IRQ1,IRQ8 etc

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Interrupts

• Interrupt Handlers are operating system service
  routines used to process each interrupt. They are
  loaded into memory at start up.
• Interrupts are assigned priorities to allow some
  interrupts (eg hardware failure) to take
  precedence over less important ones.
• I/O device controllers use interrupts to
  communicate with cpu as these devices are so slow
  compared to cpu.

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START
Fetch/ Execute/ Interrupt Cycle
Fetch the next instruction
Interrupts disabled
Execute the Instruction
Interrupts enabled
Check for interrupt, process if reqd.
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Bus Standards

• Many different standards (protocols) for BUS
  architecture
• Standard specifies format meaning of messages
  sent across bus
• A computer system will contain multiple diff bus
  types, with some interconnected to form backplane
  or system bus.
• Bus controller (bus interface unit) used to
  interconnect different bus types by converting
  signals to correct format.

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BUS standards

• ISA . Was standard system bus- initially 8 bit,
  then 16 bit when AT (286) introduced. Latest
  version EISA is 32 bit. Still used to connect
  slow I/O devices
• As processor speed and need for high volume high
  speed data transfer increase, bus can cause
  serious bottlenecks in I/O processing. One
  solution is Local BUS dedicated to input/output
  devices.
• PCI is recent 32 (or 64) bit Local BUS standard
  used to connect many fast I/O devices. It
  multiplexes data addresses on same lines
• AGP - graphics only internal bus

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External bus standards

• USB - recent standard for external serial bus
  with high throughput, allows daisychaining (up
  to 127 devices). Eliminates need for separate
  serial and parallel ports for common devices.
• SCSI bus family - use a specialized parrallel
  port, that can accept and provide addressing for
  multiple different devices such as disk drives,
  CD-ROMs, scanners at very high data rates.
• IEEE FireWire - extremely high speed external
  bus, developed for peripherals such as video
  cameras

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Ports

• Many I/O devices connect into bus via standard
  interface called a port. An I/O port is a
  connector at end of bus into which device can be
  plugged.
• Older I/O ports were low speed, still in limited
  use
• Serial (e.g RS-232, 25 pin tech. spec, used for
  mouse, modem,network interface) one bit at a
  time, over one line.
• Parrallel (e.g. Centronics, used for printers)
  multiple bits at a time, over multiple lines.
• Newer ports, eg USB port handle multiple devices
  and much higher data throughput

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Storage Device Controllers

• Storage I/O devices connected to system bus
  through a device controller.
• Device Controller
• implements bus interface standard.
• translates commands from cpu into device-specific
  format
• Allows multiple devices to share bus connection
• EIDE and SCSI controllers are complex devices
  with on-board processor, able to control large
  range of devices disk, tape, CD, DVD drives.

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Technology Focus
20
Bus Interface units

• Systems frequently have many different types of
  buses within the one system.
• The bus interface unit provides the translation
  mechanism for moving data from one type of bus to
  another.

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CPU
CPU BUS
MEMORY
PCI BUS
AGP BUS
USB Port Interface
SCSI Disk controller
Video Adapter
ISA BUS
Parallel Port Interface
Serial Port Interface
printer
modem