How can a computer use the drive's momentum for emergency power?

Question

I remember reading an answer about a computer that, in the event of a power failure, would write the important parts of the memory to disk - using the momentum of the still-spinning drive to do so. This was an early form of hibernation and, if this worked how I am imagining, a far superior version to today's.

What I can't work out is how this could possibly work. Unless the drive contains a generator I see no way that the computer could have enough power to read the memory (unless it contained a backup battery, which I don't think was the case), although writing is still plausible if the drive's still running.

How can a computer hibernate "using the momentum of the disk drive"?

Answer 1

Any real attempt to use the momentum of the spinning disk to generate power would cause the disk to come to a halt very quickly. In essence, this is the principle behing regenerative braking of electric vehicles. Otherwise, you would have a perpetual motion machine.

However, there certainly were computers that used the momentum of a disk that was spinning to be able to write to it.

There were various generations and configuations of the DEC PDP-11, both in terms of the CPU and the disk drives. They had one common feature, that disk writes (and for that matter tape deck writes) were cached. The disk controller held an amount of RAM, writes to the drive by the CPU went into this RAM buffer. The buffer was written to the physical drive when it reached a defined threshold.

The configuration of these drives and the threshold at which writing occured could be changed by the System Manager, or other privileged account. In the process control uses, for which I used to use these computers, increasing the write frequency reduced the risk of data loss due to power failure at the expense of disk speed.

The drives also contained their own power supplies, with large capacitors. If the power failed, an interrupt would cause the disk controller to write the contents of the RAM buffer to the drive before the drive span down and the memory was lost.

There is good information about the configuration of various generations of PDP-11 disks here.

This was not a guaranteed fail-safe mechanism. Sometimes data was lost. However, the system did successfully prevent data loss most of the time.
The later MicroVAX computers also had this system and would very rarely lose data on power loss - in my experience.
(Of course, the VAX-FT could survive all sorts of failures - but that was a different animal.)

The system wasn't intended to hibernate the computer, in the way that a modern laptop can hibernate. It just prevented the loss or corruption of open files, including the page file and swap file.

Answer 2

The PDP-11 had a 'power fail' trap that would be invoked when a problem with AC power was detected by the power supply. The system then had 2ms of runtime (powered by capacitors in the power supply, not disk momentum) to save volatile data. Typically, this was limited to the contents of the CPU registers (including the program counter and stack pointer) and possibly certain device registers as well. 'Main' memory was not originally considered volatile because it used magnetic cores that would retain their content even without power, and in any event 2ms was not enough time to write more than a tiny fraction of main memory anywhere.

I suppose it would also have been theoretically possible to write the CPU registers to a dedicated track on a drum memory device. It could not have been a moving-head disk because head seeks take much longer than 2ms. Also, an entire drum track would have been needed because 2ms is not even enough time to wait for the drum to rotate to a specific position; you would have required the ability to start writing at whatever position the drum happened to be in when the power fail trap occurred. I do not know whether DEC produced any drum devices that had such a write-anywhere ability (or the ability to quickly determine where the drum was currently positioned, which is effectively the same thing).

In any event, saving registers in core memory seems like a much better idea. 2ms is not a lot of time (I would guess fewer than 1000 instruction executions), and writing to core is much faster than writing to a drum. Plus, you needed to have core memory anyway to make a power failure survivable, so there was little point in designing a solution that required you to have both core and drum memory. Far easier (and cheaper) to just allocate a few words of core to save your registers.

When power was restored, that same 'power fail' trap would be invoked again, and the power fail handler would then be able to re-initialize I/O devices, restore registers from wherever they had been saved, and resume normal operation.

Answer 3

Any electrical motor can, with the right driver circuit, act as a generator.

The easiest case which does not need any outer circuit at all (it's all built in) is a shunted DC motor, it acts as a voltage source as long it is rotating. That one is called Counter-Electromotive Force. How many volts depends on the motor characteristic and the speed. When the voltage applied from outside is higher than the internal voltage, it's working as a motor and it's torque in rotation direction is driving the load. When the outside voltage is smaller than the internal, it's working as a generator and the torque works in the opposite direction of the rotation direction – it's braking the load.

At constant speed, there is an equilibirum between inside voltage and outside voltage and no current flows. (Of course, that only applies to a drive without losses.)

For a harddisk drive, things are more complicated as those usually are synchronous AC motors, but the principle stays the same.

Answer 4

From that I remember, the old type hard drives were very sensitive to the unexpected power loss - the machine I used (SM-4) required to power them down manually before switching off the main power supply. The main problem was the need to move the heads into parking position before the drive comes to halt. Otherwise loss of the results of the currently running execution is just the smallest of the problems you have afterwards.

The drive momentum was used to provide the energy to move the heads into parking position. This did not help to save any data but in most of cases prevented drive and head damage during the unexpected power loss.