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  The ROM BIOS establishes the keyboard-hold (pause) state when it detects a
  Ctrl-Num Lock or Pause keystroke. During keyboard hold, the ROM BIOS
  executes a do-nothing loop until a printable key is pressed; it doesn't
  return control of the computer to whatever program is running until this
  happens. This feature is used to suspend the operation of the computer.

  During keyboard hold, all hardware interrupts are handled normally. For
  example, if a disk drive generates an interrupt (signaling the completion
  of a disk operation), the disk interrupt handler receives the interrupt
  and processes it normally. But when the interrupt handler finishes
  working, it passes control back to whatever was happening when the
  interrupt took place--which is that endless do-nothing loop inside the ROM
  BIOS. So, during the keyboard hold, the computer can respond to external
  interrupts but programs are normally completely suspended. The keyboard
  BIOS continues to handle interrupts that signal key actions, and when it
  detects a normal keystroke (for example, the Spacebar or a function key,
  but not just a shift key), it ends the keyboard hold, finally returning
  control to whatever program was running.

  The keyboard-hold state is of no practical use in programming, except that
  it provides a standard way for users of our programs to suspend a
  program's operation.

  Be aware that the keyboard-hold state is not "bullet-proof." A program can
  continue working through the keyboard hold by acting on an external
  interrupt, such as the clock-tick interrupt. If a program really wanted to
  avoid being put on hold, it could set up an interrupt handler that would
  work through the hold state, or it could simply turn the hold state off
  whenever the hold state was turned on.