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Normally, when I film these videos,

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I’m at the Centre for Computing History
in Cambridge.

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Unfortunately, it is April 2020
as I record this,

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and times being as they are,

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I have to make do with a green-screen.

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But why simulate that same location?

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I could be anywhere with this, right?

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Well, first, this would have been a great
subject to film at the computer museum.

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But also: if I change things too much, it’ll
break the style,

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and people don’t like change.

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Actually, that’s not true:

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people like just the right combination of
novelty and familiarity.

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On that note: did you know that you can’t
name a file “Con” in Windows 10?

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...I promise that’s related.

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If you’re running a Windows machine,
try it right now.

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Go to File Explorer, create a
new folder, call it “Con”. C-O-N.

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The system is not going to let you do it.

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You also can’t name a text file “con.txt”
or an image “con.jpg”.

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Con is a first name,
not a common English one,

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but it’s a variant of Cornelius, Conrad,
Connor, Konstantinos.

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There are a good few people who
go by the name Con.

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So if you’re trying to save
a photo of one of them

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using their name on Windows,
you can’t.

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In fact, the system won't let you call a
file or folder

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CON, AUX, PRN, LST, COM0 through COM9,
LPT0 through LPT9, or NUL.

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And the reason for that goes back to MS-DOS,

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one of the old white-text-on-a-black-screen
command-lines

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that were just how computers worked
back in the 80s.

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Actually, it goes back
a bit further than that,

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because all the versions of DOS were copying
the idea from other places,

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but let’s not go too deep into history.

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In MS-DOS, Microsoft’s version of DOS,

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all those little words are the
names of “device files”.

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They were a quick and clever way of letting
programs on a computer interface with other stuff:

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printers, keyboards, basically anything
that you can plug in.

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Rather than every program having to understand
how to talk to every possible device,

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that job was given to a “device driver”.

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And that device driver…

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drove the device.

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So programmers didn’t have to worry about
how every single type of printer worked.

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Instead, they could reuse code that they
already had for reading and writing files.

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They’d just save whatever they wanted to
print to the right “device file”;

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it wouldn’t actually get written to disk,

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instead the device driver would pick it up
from there and do all the heavy lifting.

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But the programmer didn’t have to care about
how any of that worked:

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as far as they were concerned,

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they could just save a file and something
else would deal with it.

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And eventually, that meant that if
a new printer came out

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with a new, complicated,
shiny feature on it...

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the manufacturer would
bundle a new driver with it.

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All the old programs would still work,

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they could just save stuff to the device file,
same as they always did,

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and the driver would deal with
the messy real-world stuff.

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And if you wanted a really, really basic way
of using those device files,

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you could do it with the command line itself.

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If you typed in “dir”, you’d get a listing
on screen of

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whatever files were in the current
directory or folder.

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"D-i-r". They were called different
things back then.

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Add a redirect symbol, that angle bracket,

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and instead of the listing going to the screen,
it’d be saved to a file.

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But if you tried to save it to a file called LPT1,

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the system would spot that file name,
and instead of going to the hard disk,

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your listing would be sent to the device driver
for the parallel port,

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the one that was labelled LPT1
on the back of your computer.

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The place where you probably
have a printer plugged in.

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So even knowing nothing about how
the parallel port or printer worked,

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you could just... print.

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The side effect of that was:
those file names were reserved.

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You could not create a file called LPT1.

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Or CON,
which was short for “console”.

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If you tried, you’d end up sending stuff
to a printer or something instead.

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Oh, and the really early versions of MS-DOS
actually didn’t have the concept of directories,

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or folders, as we now call them,

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everything was just in one big heap of files.

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When directories were added in,

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those device file names still had to work
everywhere,

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which meant those file names
had to be blocked everywhere too.

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Same with con.txt, because file extensions
were stored differently back then.

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And there is a direct line from
thirty-year-old MS-DOS to Windows 10.

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Each version has built on the one before.

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If you know where to look,

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you can still find the Windows 3.1 file selection
box from '92 in Windows 10 today,

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and Windows 3.1 ran on top of MS-DOS.

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Now, there aren’t many Windows 10 PCs
out there with parallel ports:

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but I bet there’s a few somewhere.

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And Microsoft chooses to support them,

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because of “backwards compatibility”.

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Backwards compatibility means that you shouldn’t
have to buy a new printer

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just because you’ve got a new computer.

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New stuff should also support old stuff,
within reason.

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The trouble is that different companies have
very different definitions of “within reason”.

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Video gamers are used to the idea of
backwards compatibility:

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sometimes games for older consoles will still
work on modern ones.

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But usually, it doesn’t last too long:

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you can't play a Playstation 1 game
on a Playstation 4.

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At least, not by putting the disk in
and expecting it to work.

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The internal parts are so different that the
designers would have to emulate the old system:

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write a piece of software that translates
between the old game and the new console,

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changing the command calls on the fly,

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or even just create an entire virtual
Playstation 1 that sits in memory.

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It can be done.

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But the economic argument for backwards compatibility
in games isn’t great:

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if you support old games,

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does that mean people won’t buy as many
new ones?

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Couldn't you just sell them a remastered version
instead?

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And if you want to emulate an earlier system,

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you'll need to emulate everything.

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Even the bugs.

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To this day, Microsoft Excel thinks that the
29th of February, 1900 is a real day.

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It isn’t.
1900 was not a leap year,

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turn-of-the-century leap years are only
leap years if they’re divisible by 400.

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But when Excel first came out, in '87,

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it was competing with a program
called Lotus 1-2-3,

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and that program had a bug where
it got that leap year wrong.

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So Excel copied that bug,
for backwards compatibility.

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And more than three decades later,
that’s still the case,

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it’s in the formal specifications,

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it still gets that wrong because if Microsoft
changes that buggy behaviour now,

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they might silently break some really critical
spreadsheet somewhere in the world.

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Microsoft has always been the company
of Backwards Compatibility.

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It’s built into almost everything they do,

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it’s part of the deal of why a lot of companies
stick with Windows:

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they will not need to change their workflow.

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You can still open a Microsoft Word document
from thirty years ago in the current version.

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There are going to be folks out there, somewhere,

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who still need to use a very old bit
of software or hardware,

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the sort of thing where the company that made
it went bust decades ago, and for that to work,

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it needs to read from a magical device file
called CON.

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So Microsoft, to this day, doesn’t let you
save a file with that name.

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Because it might break something.

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It won’t last forever, of course.

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At some stage, old systems do become too
difficult or expensive to keep supporting.

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Or maybe there's a direction that the company
is trying to push their products in.

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Mac and Apple users will
know that all too well;

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new versions of Mac OS often just
don’t run old programs.

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New apps, even simple ones, aren’t allowed
on older phones.

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Headphone jacks get removed.

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People like a mixture of novelty and familiarity.

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If you change nothing, people get bored and
move on to something else.

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If you change everything, people hate it,
it breaks their workflow,

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and they move on to something else.

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Backwards compatibility, with reason,
helps to find that balance.

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So yes, I couldn’t film this video in the
actual Centre for Computing History.

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But, at least for a while,

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I don’t have to go to an actual museum to
run old applications and games.

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Catalogues of software and the documents created
with them can still be read,

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even if the exact combination of hardware
they were designed for isn’t available.

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No-one’s expecting your iPhone to read
punch cards, but sometimes,

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a little backwards compatibility can be a
good thing.