There are essentially three uses of the preprocessor--directives, constants, and macros.

A cache line can be in one of three states: invalid, clean or dirty. Invalid means that the cache line has no useful data, and no memory operation will hit it. Clean means that the cache line contains an up-to-date copy of a piece of main memory. Dirty means that the cache line has been written to, and main memory is out of date.

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In the Intel world, the CPU performs something called "cache snooping". This means that a dedicated piece of hardware looks at all memory operations to main memory, and checks to see if the CPU's cache has a more up-to-date version of the memory. It also looks at memory writes, and makes sure that the CPU's cache has the most up to date version of the data.
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If a jack is plugged in and the THING is turned on, things become interesting:

• As soon as the THING is turned on, voltage on pin 5 drops from +2.5V to 0V for about 0.5 seconds => this provides any external device plugged onto the remote port with a cold reset, as was previously identified
• After this reset phase, +2.5V is turned back on but it is only maintained if the remote device replies to a specific query within 5 secs.
• If no proper reply came from the external device within 5 secs, external voltage is turned off, until the THING itself is powered off in

Pin	Wire color	Function
1	Brown	? Shield ? (GND) - (unused by standard Remote/Headphones)
2	Blue	Digital ground (GND)
3	Orange	TXD
4	Green	Sense? (+2.5V, seems to be controlled by THING) - (unused by standard Remote/Headphones)
5	Yellow	+2.5V (0V when Plug isnt inserted) *1
6	Grey	RXD
Tip	Pink	Left audio (plus 600mv DC BIAS)
Ring	Red	Right audio
Sleeve	Black	Audio ground (GND)