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 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