Why can't computers boot instantly? (2013)

// What happens is that S1 is different for different computer configurations, and processing needs to happen.

The computer and OS config doesn't change often.

And it's doesn't seem slow to test that.

So caching at boot should work.

And caching is a low effort optimization. So it should have been implemented early.

But we don't have caching at boot, why?

Can't tell if you're joking but actually like this idea.

Yes, but it's just one of the factors, and actually all major user-facing OSes do care about boot times. Other factors are the design by committee and platform fragmentation. Hardware manufacturers don't care about initialization times, and standard makers are issuing insanely complex specifications with lots of compatibility cruft and edge cases.

All of the answers here are missing something. In the old days, the OS was often on a ROM that was simply mapped into some unified memory location. Thus the system went from a state of "off" to "ready to go" pretty much immediately. There wasn't a need to copy the data from the ROM to RAM and then execute some stuff to get into an initialized mode.

If you hunt around a bit for things like "atari 800 memory map" you can find diagrams showing how this all worked. In fact on some systems, you could selectively map certain things from ROM into the memory map to get more RAM if you wanted.

Today, the move to put the OS onto disk, the memory hierarchy, and virtual memory eliminate this approach. You basically have to let the virtual memory system map things from disk into some page space, and often things need to be copied into RAM, work their way up the cache hierarchy and be executed somewhere in order to set the system into an initialized state. I've honestly forgotten nearly all of the details so maybe somebody else can provide a better explanation.

So the reason computer can't boot instantly:

1) We have a complex memory system these days that prevent mapping the OS into the memory address space.

2) OS's aren't in ROM anymore. They sit on disk in a state different than when under operation.

3) Simple engineering willpower. Nobody wants to bother to figure it out because once the system is booted it doesn't really matter (with modern sleep modes and whatnot).

The two hard things in computer science are naming things, and cache invalidation.

Remember; the computer could quite literally have been completely reconfigured out from under the BIOS. Testing that everything is the same from the computer's point of view is anything but straightforward.

Also, most recognizable boot time isn't spent waiting on BIOS and POST; it's spent waiting for either the boot loader, disk encryption, or the OS to spin up all the background stuff (graphics stacks, session managers, HAL's, message busses, desktop environments, network services... etc) it needs to offer for fulfilling the particular purpose the system is intended for.

Computation is amazingly fast, but by no means either free or magic.

Everything takes time. However, I make no excuses for Windows: They had 3-5 second boot nailed for a while. Then, they done bloated it to the point of unrecognizability.