In addition to a power button, some Macintosh models have one or two other buttons: a reset button and often an interrupt, or programmer's, button. The former is typically used to recover from crashes that leave the system unresponsive, while the latter is for debugging and applying firmware updates. Where they appear on your computer, or even if they do, depends on which model you have. For more information, see the following articles in Apple's knowledge base:

  http://docs.info.apple.com/article.html?artnum=14117
  Desktop Macintosh: Power, Reset, and Interrupt Buttons (archive.org)
  Desktop Macintosh: Power, Reset, and Interrupt Buttons. Page 1 (MacintoshRepository.org)


  http://docs.info.apple.com/article.html?artnum=8622
  Desktop Macintosh: Power, Reset, and Interrupt Buttons, Part 2 (archive.org)
  Desktop Macintosh: Power, Reset, and Interrupt Buttons. Page 2 (MacintoshRepository.org)

However, even if your computer doesn't have a reset or interrupt button, it is usually possible to duplicate their function. See the instructions below:

The Reset button

If your keyboard has a power button, you may be able to reproduce the effects of the reset button by pressing the following key combination:

  Ctrl-Cmd-Power

If you have one of the newer keyboards that doesn't have a power button, then try the following:

  Ctrl-Cmd-Eject

Especially if you have a USB keyboard, this sequence sometimes won't work. In that case, if your computer doesn't have a separate reset button, try holding down the power button on your computer for six seconds.

The Interrupt button

If your keyboard has a power button, you may generate an interrupt by pressing the following key combination:

  Cmd-Power

If you have one of the newer keyboards that doesn't have a power button, then use the following:

  Cmd-Eject

If you have a debugger installed, this will bring it up, though only in Mac OS 9.x or earlier, and the Classic environment. In Mac OS X, it will halt the processor for as long as you have the keys held down, but otherwise has no effect.

If you are applying a firmware update, you can't generate an interrupt from the keyboard. If you don't have an interrupt button, consult your documentation for information about how to replicate its functionality. For newer models, hold down the power button on the computer until it sounds a tone and/or begins to flash.

 

 

Article source : (iu.edu) , edited with working links to old docs.info.apple articles.