1. Computer Buses Ref: Burd, Chp 6. 206 – 220 Englander, Chp 7 p 177-183

2. Outline Definition of bus Types of bus lines and their role Interrupts
Bus standards
Ports

3. 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.

4. Buses
ALU
Control
unit
CPU
video
controller
memory
disk
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.

5. 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 >100 lines, including all 4 types
Data lines provide path for moving data (including instructions) between system components. It’s 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

6. Address lines used by the CPU to carry a digital code which uniquely identifies each memory location and I/O port. It’s size dictates maximum addressable memory. eg 8 bit address bus allows only 28 memory locations 00 01 10 11
1 or 0
Possible combinations = 4 (or 22 )
00, 01, 10, 11
2 line address
BUS
MEMORY LOCATIONS

7. Evolution of Intel Microprocessor
Name: Pentium PII
Year:
Data Bus
Address
bus width
Addressable MB MB GB GB G
Memory

8. 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 => bus protocols designed to minimize these.

9. For module to send data to another, must obtain use of bus by sending bus request signal on control line
bus request
write 1
CONTROL
LINES
When request granted, puts module address on address lines, and transfers data via data bus lines. 1
ADDRESS
LINES 1
DATA
LINES

10. 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.

11. 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

12. 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.

13. Fetch/ Execute/ Interrupt Cycle Fetch the next instruction START
Interrupts
disabled
Execute the
Instruction
Interrupts enabled
Check for interrupt,
process if req’d.

14. 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.

15. 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

16. 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

17. 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

18. 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.

19. 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.

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