[CUBE] Why Sleep (Was PL Upgrade)

[Tom Davidson]

Sleep in a laptop is much easier than a desktop because in a laptop,
you have control of what hardware is there. The only variables are
the PCMCIA cards. Even the, sometimes they dont properly power up.
I've got one laptop that always sleeps and wakes OK, but the 802.11
card almost never works when it wakes.

Deep sleep is tough to do properly if you dont account for the state
of every piece of silicon in the computer. There can be interactions between
videocards, hard drives, USB devices, CD-ROM drives, Airport Cards
that cause a lockup or improper restart. Even different sub-revisions of 
these
may have restart problems. The software has to completely store the state
of every device in the system on disk before powering down, miss one bit
and you dont get to power up again properly. Something like a video card
has dozens of internal registers and up to 64MB of RAM alone. Your hard
disk has a 2MB or more buffer that has to be flushed to disk or saved, every
manufacturer has a slightly different command set. Even the processor has
internal registers that can not be directly read by software, processor 
revision
specific instruction sequences have to be executed to completely set the CPU
to a known state before shutdown and after restore. There are also several
revisions of the Cube Motherboard, each probably has it's own unique set of
things that must be done to properly shut down and restore.

The CPU card is one of the least complicated things in the cube, there 
are only
3 or 6 chips on it that store any state, the CPU and 2 cache chips. But 
it's not the
original CPU revision that the Cube was built with, it's a 7455 not a 
7400/7410
But, if  the system software makes some 7400 assumption, it's going to make
a change in how the system is vs how it should be shut down, all it 
takes is 1 bit
in some register somewhere to get the wrong value and you don't reboot.

I do embedded systems design and have an appreication for how utterly 
complex
a current computer like the Cube is. There must be at least 10 different 
microprocessors
in a Cube system, each with it's own local software running and having 
volitile storage
and internal registers.