Buses

1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. Plug and Play continued • All PC devices are Plug and Play • Non-Plug and play devices are considered to be legacy equipment