Expansion BusChapter 5

System crystals

Every device soldered into the motherboard is designed to run at the speed of the crystalWhat 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

Expansion bus crystal

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

ISA expansion bus

On first PC’s, called “PC bus”8-bitran at 8.33 MHz maximum, per IBMIBM shared the technology with everyone, allowed others to produce cards that fitIBM patented the technology, but not the cards that use it

ISA expansion bus

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

MCA

IBM developed for use with the PS/232 bit for use with 386’s and 486’s All devices using it had an installation disk that you had to haveOptions disk automatically configured the device properly

MCA

Had some major drawbacks:were incompatible with ISA cardsMCA was licensed by IBM and they did not release it to the public which made it very expensiveit was not backwardly compatible

Only showed up in IBM computers and is basically a dead technology

Enhanced ISA (EISA)

Also 32-bit An industry group of clone makers created it as a competitor to MCAIt beat MCA for 2 reasons:

It did everything MCA did, but much cheaperbackwardly compatible with ISA

It also died due to Microsoft Windows and its graphical demands

VESA “VL” BUS

Called the VESA Local Bus because it tapped into the local bus to run at a faster speedWas 32-bit, so it died due to the release of the Pentium which ran at 64-bit

PCI

Peripheral Component Interconnectdesigned by Intelreleased to the public domain, so was quite successfulnot tied to the CPU, so Apple machines can use itCan send at 64-bit speed

PCI

Can transfer data between PCI devices while the CPU is doing other stuffDoesn’t use IRQ’s; simply a plug and play technologyuses a powerful “burst mode” that makes data transfers very efficientPCI is fairly standard now

Advanced Graphics Port (AGP)

A single connector that looks like a PCI slot bit is slightly shorter and usually brownOnly video cards use AGP

PC cards

Once known as (PCMCIA) Personal Computer Memory Card International AssociationPrincipally used in laptops3 types

Type I - 3.3 mm - memoryType II - 5 mm - NICs and modemType III - 10.5 mm - hard drives

Plug and Play

Software technology, not bus technologyNeed three things for it to occur:

PnP support in BIOSPnP operating system (Windows 95)PnP device (adapter card)

No one makes non-PnP devices anymore!

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 RAMIf this wire is charged, devices will get data

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 CPUOnce a device has an IO address, another can not share itIO addresses defined by IBM on page 275

I/O Addresses

Know these I/O Addresses for the exam:

COM1 - 3F8COM2 - 2F8COM3 - 3E8COM4 - 2E8LPT1 - 378LPT2 - 278

IRQ’s (interrupt requests)

IO addresses are good, but must be initiated by CPU, otherwise CPU doesn’t know which device is calling itIRQ’s allow devices to get CPU’s attentionEvery CPU has an INT wire; if charged it will stop what it is doing and listenIt will then run the BIOS routine to find out what to do with that device

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 sharingMap on pages 282-283

IRQ’s

Map of IRQ’s:0 - system timer 8 - real-time clock1 - keyboard 9 - video card

2 - (redirected to IRQ9) 10 - Open3 - Com 2, Com 4 11 - Open4 - Com 1, Com 3 12 - PS/2 Mouse5 - Sound or LPT2 13 - math coprocessor

6 - Floppy 14 - hard disk7 - LPT1 (printer) 15 - Open

DMA (direct memory access)

Allows system to run background applications without interrupting the CPUDiagrams on page 286-288another chip, the 8237, is the traffic cop by controlling all the DMA functionsthe DMA chip sends data along the external data bus when the CPU is busy and not using the external data bus

DMA

Was designed to be used with ISA (8 bit), then EISA (16-bit) so it was rather slowBus mastering is now used

directly bypasses the 8237 chipdevices 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