1. Expansion Bus
Chapter 5

2. System crystals
Every device soldered into the motherboard is designed to run at the speed of the crystal
What happens when you try to add a device that did not come with your motherboard?
An extension to the external data needed to be made that ran at its own speed
Expansion bus crystal - a different crystal added that controlled the part of the external bus that was connected to the expansion slots

3. Expansion bus crystal
The frontside bus runs at the speed of the motherboard
The expansion slots run at another, much slower speed
The chipset acts as the divider between the two buses, compensating for the speed difference with wait states and special buffering areas

4. ISA expansion bus On first PC’s, called “PC bus” 8-bit
ran at 8.33 MHz maximum, per IBM
IBM shared the technology with everyone, allowed others to produce cards that fit
IBM patented the technology, but not the cards that use it

5. ISA expansion bus
A new 16-bit version appeared on the 286 because of its 16-bit external data bus
was downwardly compatible
called “AT bus”
still ran at 8.33 MHz, but was 16-bit

6. MCA IBM developed for use with the PS/2
32 bit for use with 386’s and 486’s
All devices using it had an installation disk that you had to have
Options disk automatically configured the device properly

7. MCA Had some major drawbacks:
were incompatible with ISA cards
MCA was licensed by IBM and they did not release it to the public which made it very expensive
it was not backwardly compatible
Only showed up in IBM computers and is basically a dead technology

8. Enhanced ISA (EISA) Also 32-bit
An industry group of clone makers created it as a competitor to MCA
It beat MCA for 2 reasons:
It did everything MCA did, but much cheaper
backwardly compatible with ISA
It also died due to Microsoft Windows and its graphical demands

9. VESA “VL” BUS
Called the VESA Local Bus because it tapped into the local bus to run at a faster speed
Was 32-bit, so it died due to the release of the Pentium which ran at 64-bit

10. PCI Peripheral Component Interconnect designed by Intel
released to the public domain, so was quite successful
not tied to the CPU, so Apple machines can use it
Can send at 64-bit speed

11. PCI
Can transfer data between PCI devices while the CPU is doing other stuff
Doesn’t use IRQ’s; simply a plug and play technology
uses a powerful “burst mode” that makes data transfers very efficient
PCI is fairly standard now

12. Advanced Graphics Port (AGP)
A single connector that looks like a PCI slot bit is slightly shorter and usually brown
Only video cards use AGP

13. PC cards
Once known as (PCMCIA) Personal Computer Memory Card International Association
Principally used in laptops
3 types
Type I mm - memory
Type II - 5 mm - NICs and modem
Type III mm - hard drives

14. Plug and Play Software technology, not bus technology
Need three things for it to occur:
PnP support in BIOS
PnP operating system (Windows 95)
PnP device (adapter card)
No one makes non-PnP devices anymore!

15. I/O Addresses
If everything in the computer connects to both the external data and address bus, how does the CPU know to talk to a particular device?
Extra wire on address bus called IO/MEM used to tell address bus that CPU is sending data to a device, and not to RAM
If this wire is charged, devices will get data

16. I/O Addresses Which device is the CPU talking to?
Defined as the IO address in bus
All devices must have an IO address in order to “talk” to the CPU
Once a device has an IO address, another can not share it
IO addresses defined by IBM on page 275

17. I/O Addresses Know these I/O Addresses for the exam: COM1 - 3F8
COM3 - 3E8
COM4 - 2E8
LPT
LPT

18. IRQ’s (interrupt requests)
IO addresses are good, but must be initiated by CPU, otherwise CPU doesn’t know which device is calling it
IRQ’s allow devices to get CPU’s attention
Every CPU has an INT wire; if charged it will stop what it is doing and listen
It will then run the BIOS routine to find out what to do with that device

19. IRQ’s
Virtually every device in a system requires its own unique and individual IRQ
one exception is the joystick
IBM came up with a map of IRQ’s and associated devices to prevent from sharing
Map on pages

20. IRQ’s Map of IRQ’s: 0 - system timer 8 - real-time clock
1 - keyboard video card
2 - (redirected to IRQ9) 10 - Open
3 - Com 2, Com Open
4 - Com 1, Com PS/2 Mouse
5 - Sound or LPT math coprocessor
6 - Floppy hard disk
7 - LPT1 (printer) Open

21. DMA (direct memory access)
Allows system to run background applications without interrupting the CPU
Diagrams on page
another chip, the 8237, is the traffic cop by controlling all the DMA functions
the DMA chip sends data along the external data bus when the CPU is busy and not using the external data bus

22. DMA
Was designed to be used with ISA (8 bit), then EISA (16-bit) so it was rather slow
Bus mastering is now used
directly bypasses the 8237 chip
devices have circuitry that enables them to watch for other devices using the external data bus and can “get out of the way” on their own
DMA assignments on page 291