Expansion Bus Chapter 5

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

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

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

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

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

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

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

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

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

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

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

PC cards Once known as (PCMCIA) Personal Computer Memory Card International Association Principally used in laptops 3 types Type I - 3.3 mm - memory Type II - 5 mm - NICs and modem Type III - 10.5 mm - hard drives

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!

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

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

I/O Addresses Know these I/O Addresses for the exam: COM1 - 3F8 COM3 - 3E8 COM4 - 2E8 LPT1 - 378 LPT2 - 278

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

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 282-283

IRQ’s Map of IRQ’s: 0 - system timer 8 - real-time clock 1 - keyboard 9 - video card 2 - (redirected to IRQ9) 10 - Open 3 - Com 2, Com 4 11 - Open 4 - Com 1, Com 3 12 - PS/2 Mouse 5 - Sound or LPT2 13 - math coprocessor 6 - Floppy 14 - hard disk 7 - LPT1 (printer) 15 - Open

DMA (direct memory access) Allows system to run background applications without interrupting the CPU Diagrams on page 286-288 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

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