A simple bus

¢ Wires:

“ Uni-directional or bi-directional

“ One line may represent multiple wires

¢ Bus

“ Set of wires with a single function

¢ Address bus, data bus

“ Or, entire collection of wires

¢ Address, data and control

¢ Associated protocol: rules for communication

¢ Conducting device on periphery

¢ Connects bus to processor or memory

¢ Often referred to as a pin

“ Actual pins on periphery of IC package that plug into socket on printed-circuit board

“ Sometimes metallic balls instead of pins

“ Today, metal pads connecting processors and memories within single IC

¢ Single wire or set of wires with single function

“ E.g., 12-wire address port

¢ Most common method for describing a communication protocol

¢ Time proceeds to the right on x-axis

¢ Control signal: low or high

“ May be active low (e.g., go™, /go, or go_L)

“ Use terms assert (active) and deassert

“ Asserting go™ means go=0

¢ Data signal: not valid or valid

¢ Protocol may have subprotocols

“ Called bus cycle, e.g., read and write

“ Each may be several clock cycles

¢ Read example

“ rd™/wr set low,address placed on addr for at least tsetup time before enable asserted, enable triggers memory to place data on data wires by time tread

¢ Suppose a peripheral intermittently receives data, which must be serviced by the processor

“ The processor can poll the peripheral regularly to see if data has arrived “ wasteful

“ The peripheral can interrupt the processor when it has data

¢ Requires an extra pin or pins: Int

“ If Int is 1, processor suspends current program, jumps to an Interrupt Service Routine, or ISR

“ Known as interrupt-driven I/O

“ Essentially, polling of the interrupt pin is built-into the hardware, so no extra time!