I/O Modules and Structure

• secondary memory devices
• communications equipment
• terminals

CPU much faster than I/O devices

• waiting for I/O operation to finish is inefficient
• not feasible for mouse, keyboard
• I/O module sends an interrupt to CPU to signal completion
• Interrupts normal sequence of execution
• I/O requests can be handled synchronously or asynchronously.
  • In a synchronous system, a program makes the appropriate operating system call and, as the CPU is now executing operating system code, the original program's execution is halted i.e. it waits.
  • In an asynchronous system, a program makes its request via the operating system call and then its execution resumes. It will most likely not have had its request serviced yet!
  • The advantage of having an asynchronous mechanism available is that the programmer is free to organize other CPU activity while the I/O request is handled.
• Software that communicates with controller is called device driver
• Most drivers run in kernel mode
• To put new driver into kernel, system may have to
  • be relinked
  • be rebooted
  • dynamically load new driver
• we must have an event driven I/O system handling all of the pending I/O requests (maybe these are triggered when data arrives, or a peripheral device such as a CD drive indicates it is ready etc.).
• Requests that the operating system has not yet been able to service might mean that the program is currently ``sleeping'' or ``waiting''.