Modes of parallel data transfer

In parallel I/O, all data bits are sent together. The method used to synchronise the processor with the peripheral gives the following modes.

1. Simple (unconditional) I/O: the processor sends or reads data whenever it wants, assuming the device is always ready (e.g. LEDs, switches). No control signals are used. This is 8255 mode 0.
2. Strobed (conditional) I/O: the processor first checks the device status (polling), or the device signals with a strobe that the data is valid, and only then is the data transferred. Used for keyboards.
3. Single handshake: one control line from the sender (strobe/DAV) says "data valid"; the receiver takes the data but sends no acknowledge. Suitable when the receiver is always fast enough.
4. Double handshake (interlocked): two lines are used: the sender puts out the data and asserts DAV; the receiver accepts the data and returns ACK; the sender then removes DAV and the receiver removes ACK. Reliable for devices of different speeds (8255 modes 1 and 2).

Other transfer methods at system level

• Interrupt-driven I/O: the device interrupts the processor when it is ready.
• DMA: a DMA controller transfers data directly between the device and memory.

 Handshake:  data  ====valid=======
             DAV   ___|‾‾‾‾‾‾‾‾|______
             ACK   _____|‾‾‾‾‾‾‾‾|____