7. I/O

Three techniques are possible for I/O operations:

Programmed I/O

• Data are exchanged between the processor and the I/O module

• Processor executes a program that gives it direct control of the I/O operation

• When the processor issues a command it must wait until the I/O operation is complete

• If the processor is faster than the I/O module this is wasteful of processor time

Interrupt-driven I/O

• Processor issues an I/O command, continues to execute other instructions, and is interrupted by the I/O module when the latter has completed its work

Direct memory access (DMA)

• The I/O module and main memory exchange data directly without processor involvement

I/O Mapping

Memory mapped I/O

There is a single address space for memory locations and I/O devices

A single read line and a single write line are needed on the bus

• Devices and memory share an address space I/O looks just like memory read/write

• No special commands for I/O

  • Large selection of memory access commands available

Isolated I/O

• Separate address spaces

• Need I/O or memory select lines

• Special commands for I/O

  • Limited set

Q

1. In what circumstances that a modem can become an I/O device?

   NIC

2. List all the factors that may affect the performance of an I/O of a system.

   I/O Technique

   Bus System

3. List and explain briefly the principle I/O techniques.

   Programmed I/O - data exchanged between the processor and the I/O module. The processor executes a program that gives it direct control of the I/O operation, including sensing device status, sending a read or write command, and transferring the data

   Interrupt-driven I/O - processor issues I/O command, then goes off to do other things until the I/O module interrupts the processor to request service when it is ready to exchange data with the processor

   Direct Memory Access - involves an additional module on the system bus. The DMA module is capable of mimicking the processor, and, indeed, of taking over control of the system from the processor

4. What is the difference between memory-mapped I/O and isolated I/O?

   Sharing memory location between processor and I/O or not

   Memory mapped I/O

   • Devices and memory share an address space I/O looks just like memory read/write

   • No special commands for I/O

     • Large selection of memory access commands available

   Isolated I/O

   • Separate address spaces

   • Need I/O or memory select lines

     • Special commands for I/O Limited set

5. When a device interrupt occurs, how does the processor determine which device issued the interrupt?

   1. multiple interrupt lines

   2. software poll

   3. daisy chain(hardware poll,vectored)

   4. bus arbitration(vectored)

6. When a DMA module takes control of a bus, and while it retains control of the bus what does the processor do?

   1. The processor pauses for each bus cycle stolen by DMA module

   The processor either doesn't need to use the bus at the time, or it is forced to suspend operation temporarily, The processor deals with other things, while the bus operation is left to DMA