What The Heck Does That Key Do?

6 minute read

You know what keys I'm talking about. The ones that say things like Scroll Lock, Break, and SysRq. When was the last time you pressed one of those keys? Here's how they got to be there, what they originally did, and if there's still anything you can use them for now.

Print Screen
The Print Screen key barely fits in this category--quite a few people know what the Print Screen key does, and it's the only one of these keys that I use regularly. Originally, as its name suggests, the Print Screen key dumped the contents of the screen to the printer. So why doesn't it perform this handy function today? Well, quite simply, graphical user environments. This key was envisioned when text-based operating systems like MS-DOS were the norm, and printing text was quite sensible. But when Windows came along, the Print Screen key had to be deactivated in Windows--dot-matrix printers and graphics don't mix, and even if your printer printed graphics, that would have been a lot of ink. (Dot-matrix printers are the really noisy kind that operate by punching pins in the shape of letters. If you go to a few stores with old cash register equipment today, you might still encounter a dot-matrix receipt printer.)

But not being able to get a record of the current contents of the screen wasn't a good solution either, so the function was modified to copy an image of the screen to the computer's clipboard instead. This remains its function today. If you've been reading along with this newsletter every week, you've probably encountered a few of my screenshots--an common way to create a screenshot is to press the Print Screen key, paste the image into an image editing program, and save it.

(Note: Macs don't have a print screen key, but you can take a screenshot by pressing Command-Shift-3.)

Scroll Lock
If you know anything about the scroll lock key, it's probably that it makes one of those little lights in the upper-right-hand corner of the keyboard come on. But chances are good that you have never known what it's for. If you're exceedingly (un)lucky, you might have experienced your cursor in Excel refusing to move when you press the arrow keys. And that, it turns out, is the solution to the mystery of Scroll Lock.

The scroll lock key was envisioned back when most computers didn't have mice. By pressing the scroll lock key and then the arrow keys, you could scroll around your document without moving the cursor within the document. Nowadays, with the advent of mice and scroll bars, this key is almost entirely useless (except for making the little light light up). A few programs still preserve the old behavior (like Excel), but by and large almost no programs use the key, and you have no real reason to utilize it.

Those unlucky souls who have noticed their computer suddenly start typing over text rather than inserting it probably know something about the Insert key--namely that it's annoyingly easy to press by mistake. The purpose of the key is to toggle insert and overtype mode--that is, when you type more, whether the computer should insert it before text that comes after, or whether the text after it should be replaced as you type. (If you're confused, try it: fire up Word, type a few words, press Insert, and try inserting text between the words.)

You probably can't think of any uses for overtyping text, and by and large there aren't any, but I can think of a few. For instance, if you have some text in Word that will wrap to the next line and totally mix up the formatting, or you're filling out a form where adding extra spaces could mess things up very quickly, turning on Insert will let you change or fill in the space without adding extra characters.

Fortunately, pressing this key does not break your computer. In fact, chances are very, very good that pressing the key will do nothing at all. Believe it or not, the origin of the key dates back to the days of the telegraph. Telegraph keys had a switch that would short the contacts of the key, generating a continuous signal. This switch would be set anytime the telegraph was not in use--this way, when the operator was ready to send again, he could turn off the switch and thereby alert the receiving party that he was about to send. (Additionally, if the signal was broken and no message arrived, they would know there was a problem with the line.)

When teletypewriters came into common use (they were used for a time to control early mainframes), the Break key was created to emulate this functionality. Pressing the key would cause the system at the other end to terminate a program, prepare for a login, or something similar. (When controlling a teleprinter at the other end instead of a computer, the printer would generate a continuous series of punches or DELETE characters, which was sometimes used to make a loud noise and alert the operator.)

So is there anything you can do with the Break key? Well, maybe. If you ever run MS-DOS programs, pressing Ctrl-Break may terminate a program. When your computer is beginning to boot and doing the POST (Power-On Self Check, where you see the computer's hardware or manufacturer's logo displayed onscreen), you may be able to freeze the output with Pause. But the only thing that most users are likely to be able to do with the Break key is to open the System Properties screen on a Windows computer, which you can do by holding the Windows key and pressing Break.)

I'm tempted to pronounce this "sys wreck" and say that it makes your system explode. But it actually stands for "system request," and goes down in history as something that was completely superseded and outdated about twenty years ago, yet still remains a standard feature of PC keyboards. It's usually combined with the Print Screen key and accessed by holding down Alt before pressing the key. The key was originally introduced by IBM as a way to switch between operating systems. Since then it has had...well, basically no use at all. A tiny number of systems use it as a sort of "panic" signal, to trigger a hard reboot or terminate a program.

The key does have one useful function on Linux computers. There's a function called the "magic SysRq key" that can be used to terminate the entire desktop environment (essentially logging you out immediately) or perform various other debugging functions. The key has absolutely no use whatsoever to Windows users.

Soren "scorchgeek" Bjornstad

If you have found an error or notable omission in this tip, please leave a comment or email me: webmaster@thetechnicalgeekery.com.

Copyright 2011 Soren Bjornstad.
Verbatim copying and redistribution of part or all of this article
is permitted, provided this notice is preserved.

Primary sources for this article were my memory (information thus collected over many years from various unknown sources) and the Wikipedia articles on each, which are all quite extensive and helpful.