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This article is from the 3b1 computers FAQ, by John B. Bunch with numerous contributions by others.

8.1. What are the LED's left side of the machine for?

This is from John Milton's HwNotes series #1.

0 RED:	  This is the "user LED". It can be turned on and off
	  with the syslocal(2) call. It is not used by any
	  existing applications.
1 GREEN:  This is the one most people get wrong. This LED
	  toggles every time there is a process context
	  change, and is cleared on the whole second.
2 YELLOW: This is the idle LED. When it is on, there are no
	  processes in the ready to run state.
3 RED:    Heart beat LED. This is toggled on the whole second.

This and other interesting information is available in the HwNote series 1-15 that are archived on OSU.

 

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