Click above to get retro games delivered to your door ever month!

  The 8086, 80286, and 80386 interact with the circuitry around them in
  three ways: through direct and indirect memory access, through
  input/output (I/O) ports, and with signals called interrupts.

  The microprocessor uses memory by reading or writing values at memory
  locations that are identified with numeric addresses. The memory locations
  can be accessed in two ways: through the direct memory access (DMA)
  controller or through the microprocessor's internal registers. The disk
  drives and the serial communications ports can directly access memory
  through the DMA controller. All other devices transfer data to and from
  memory by way of the microprocessor's registers.

  Input/Output ports are the microprocessor's general means of communicating
  with any computer circuitry other than memory. Like memory locations, I/O
  ports are identified by number, and data can be read from or written to
  any port. I/O port assignment is unique to the design of any particular
  computer. Generally, all members of the IBM PC family use the same port
  specifications, with just a few variations among the different models.
  (See page 37.)

  Interrupts are the means by which the circuitry outside the microprocessor
  reports that something (such as a keystroke) has happened and requests
  that some action be taken. Although interrupts are essential to the
  microprocessor's interaction with the hardware around it, the concept of
  an interrupt is useful for other purposes as well. For example, a program
  can use the INT instruction to generate a software interrupt that requests
  a service from DOS or from the system ROM BIOS. Interrupts are quite
  important when programming the PC family, so we'll devote a special
  section to them at the end of this chapter.