All sun2 machines are equipped a diagnostic display of eight Light Emitting Diodes (LEDs), located on either the front (deskside chassis) or the back (desktop machine) of the system unit.
The kernel changes the display during periods of idle processor activity according to a stored sequential pattern list. The
/dev/leds interface provides a way of manipulating the pattern list via simple file I/O.
The structure of the file is as follows:
struct led_patterns {
u_char divisor;
u_char patlen;
u_char pat[256];
};
divisor
The number of idle periods to wait before switching to the next pattern in the array.
patlen
The number of patterns stored in the array.
pat
The array of patterns to display.
When a clock interrupt occurs while the processor is idle, a pattern countdown timer is decremented. When the countdown timer reaches zero it is reset with the
divisor value and the next pattern in the array is selected and displayed.
Each 8-bit pattern describes the state of the diagnostic LEDs. A set bit in a pattern indicates that its corresponding LED should be extinguished, while a reset bit indicates an LED to be illuminated.