Read/Write Operations

Data transfers require all components of the computer system to  synchronise correctly. Memory must be accessed, buses must be made available, peripherals must transfer data, etc. All these operations must happen in a specific sequence in order for the transfer to be successful.

In addition I/O devices are attached in a physically different way; requiring different approach for memory and peripherals.

For data to be read from an I/O device the sequence is as follows:

  1. Processor signals the device to be read by placing the memory address onto the address bus.
  2. Control unit of the processor turns on the I/O read line of the control bus.
  3. Data is placed onto the data bus from the peripheral.
  4. CPU reads data from data bus and places it into the MDR.

For data to be written to an I/O device the sequence is as follows:

  1. Processor signals the I/O device to be written by placing the memory address onto the address bus.
  2. Processor places the data to be written onto the data bus.
  3. Control unit of the processor turns on the I/O write line of the control bus.
  4. Data is written to the peripheral.

Next: Software Elements