Why is the maximum boot.ini delay 11 million seconds?
I mentioned in passing that
the maximum delay you can specify
in boot.ini is about 11 million seconds.
I'm disappointed but sadly not surprised that everybody focused on that
number and completely missed the point of the article.
First of all, the value of 11 million was not a conscious limitation.
It's just an artifact of other limitations.
The delay is specified in seconds in boot.ini,
but internally it is converted to BIOS clock ticks.
(Remember, this is a boot loader;
there's not much infrastructure available yet.)
The conversion is done in 32-bit arithmetic,
and 4 billion BIOS clock ticks at 18.2 ticks per second
is about 110 million seconds.
Wait, but you said 11 million seconds, not 110 million seconds.
Well, yes, but the conversion is done in 32-bit integer arithmetic:
BiosTicks = BootIniSeconds * 182 / 10.
A value for BootIniSeconds larger than about 11 million
will result in a signed integer overflow in the intermediate product.
That's why the limit is 11 million seconds.
It's a storage limitation, not an arbitrary cutoff.
And it is indeed
the limit of a natural data type,
just hidden behind some other computations.
(SB
figured this out,
though I fear that this shout-out will only serve to encourage
people to speculate even more wildly than they already do
in the hopes of earning a future shout-out.)
Now you'd think this would be plenty enough of a boot delay.
I mean, it's already much longer than the maximum amount you can delay
with a single call to Sleep
or WaitForSingleObject.
And who would ever need a computer to pause four months before booting?
Yet there's one commenter who
thought 11 million seconds wasn't enough:
"What if I want a timeout of 129 days?"
If you need a timeout of 129 days, then
go buy a custom hardware timer
device that waits 129 days after the onset of power before allowing
current to flow.
This same commenter has difficulty comprehending that just because
you're capable of delaying something for a specific length of time
and you're capable of never doing it at all,
that doesn't mean that you are also capable of delaying something for
an arbitrary length of time.
One commenter suggested that
the problem could be solved by using a bignum library.
Before you solve a problem, you first have to be sure that what you
have really is a problem in the first place.
I have yet to hear of any customers who are running into the
11 million second limit.
Using bignums here would be a solution in search of a problem.
"Hey, let's consume valuable developer and tester resources in order to
add a bunch of code to a tightly-constrained environment
to cover a case that nobody cares about."
(And I failed to live up to
my promise to Ulric
merely to incorporate this lecture by reference in the future.)
