Buses

• All devices in the computer are connected to the External Data Bus

• Extension to External Data Bus called Expansion Bus

– Used for devices that might not run at speed of CPU

– Regulated by the expansion bus crystal

External Data Bus:

• Two buses make up external data bus:

– expansion bus, which conects expansion slots and runs at speed of expansion bus crystal

– system bus, which supports RAM and the CPU and runs at speed of system crystal

History of Buses

• The 8088 had 8 bit external data bus and ran at 4.77 MHz

• The 286 had a 16 bit external data bus and ran at 8.33 MHz

• For many years the only choice in expansion slots were 8 and 16 bit ISA’s

IO address• All different patterns used by CPU to talk to

devices• All devices must have an I/O address• No two devices can share an I/O address• When used, the first 16 bits of the address

bus are occupied • I/O addresses are hexadecimal numbers• The 0 is dropped at the end

Interrupts• A way for a device to “interrupt” the CPU

to get its attention• No 2 devices can share an IRQ• Original IBM’s only had 8 IRQ’s• When more serial ports were added they

had to share an IRQ

– This was possible as long as the sharing devices didn’t use the Interrupt at the same time

LPT Ports

• Used for Parallel connections• Printers use Parallel connections which use

LPT ports• COM and LPT’s are just the IO address and

IRQ, they are not physical things• The Parallel connectors, 25 pin female and

the Serial connectors 9 pin and 25 pin males, are physical things

Interrupts expanded• More IRQ’s were needed for more devices• Another 8259 chip had to be added

– In the early PC’s an 8259 chip controlled the first 8 interupts

– Now another 8259 would be added to add 8 more interupts

– The new 8259 chip had to use IRQ 2

– IRQ 9 was cascaded to IRQ 2 for devices that had been using IRQ 2

DMA

• Direct Memory Access

– Allows devices to access memory directly

– The 8237 chip controls DMA the way the two 8259 chips control IRQ’s

– Only floppy drives and sound card use DMA’s

New Expansion Buses

• MCA- Micro Channel Architecture

– Came with 386

– First 32 bit bus (matched 386 32 bit external data bus)

– Had problems, is now dead, not used• The EISA bus and the VL-bus didn’t last

long either

PCI

• Predominant expansion bus of the PC world• Capabilities independent of CPU• Uses own PCI bus to connect with

expansion bus• Masters or takes over the external data bus

In Order to use Plug and Play

• The system BIOS must support PnP• The motherboard and its chipset must

support PnP• The operating system running on the PC

must support PnP• The bus of the expansion slot that the device

occupies must be compatible with PnP

Plug and Play continued

• All PC devices are Plug and Play• Non-Plug and play devices are considered

to be legacy equipment