CS 2734 Computer Organization II

Read pages 55-60 (Section 2.3) of Tanenbaum and pages 249-253 (Sections 10.1-10.4) of Paul

• By popular request, I'll go over the pipeline question on the exam.

• Last time:
  • We discussed the disparity between I/O device times and CPU cycle times (scaled in human terms 1 second for a fast processor versus 50 days for a disk drive).

  • A disk drive is a block device. Access is performed a block at a time.

  • Character devices such as keyboards produce a sincgle character for each I/O operation.

• There are four approaches to performing I/O:
  • Programmed I/O with busy waiting
  • Interrupt driven I/O
  • DMA (Direct Memory Access) I/O
  • Data channel I/O

• How devices interface with a CPU:
  • Devices are now usually connected to the bus through an appropriate controller (see Figure 2-21 of Tanenbaum).

  • I/O devices are often memory mapped - the device is associated with a particular memory address. On the Sparc, addresses 0xfff00000 to 0xffffe000 designate device registers.

  • For example a simple keyboard, might have two registers
    • Status register: ready error intr .....
    • Data register
    The following code loads the keycode into %o0

            set   0xffff0008, %o1
            ldub  [%o1], %o0
    

• How do you know when something is available?
  • In programmed I/O you test the ready bit of the status register
  • In interrupt driven I/O you set the interrupt bit and process the I/O in an interrupt service routine.

• Monitors are interesting I/O devices:
  • Screen retraced 30 to 60 times per second
  • Three guns sweep out an almost horizontal trace in 50 microseconds.
  • Video RAM:
    • Character-mapped
    • Bit-mapped - each pixel has a color:
      • B/W
      • Color-mapped
      • RGB

Revision Date: 4/11/97