Ed, many of the errors are just plain ERRORS that couldnt be the result of editing.
Below are some of them, some noted by others before me:
>The Scroll Lock key on IBM PC-compatible computer keyboards first appeared on the keyboard of the IBM 3270, which is sometimes thought of as a combination mainframe terminal and personal computer.
Just plain WRONG. The 3270 was dumber than dumb. It had exactly ZERO computing power. All it could do, and poorly at that, was to display a page of 25 lines of 80 characters that was blasted to it from the mainframe. A whole page at a time.
No scrolling of the text either. There was a SCROLL LOCK key, but all it did was send a command to the mainframe, which it could obey or ignore, as it wished.
The writer may have been confused by a few hybrid products, the PC3270, which was basically an IBM PC with special 3270-like emulation software built-in, or the PC/370 which was a PC with a miniaturized mainframe and 3270 emulator in one box. These were not 3270’s any more than Ru Paul is Diana Ross.
>some 3270 keys such as Transmit, Dup, and Field Mark were left behind.
Those keys had to do with the 3270’s “block mode” of operation, which thankfully the PC didnt inherit any of.
>The original intent of the Scroll Lock key was to stop text from scrolling as it was transmitted from a mainframe or minicomputer–
While there could have been some communications program that implemented the “pause output” function on the SCROLL LOCK key, most programs used the standard ASCII Control-S, Control-Q key combo for this. I used the PC for many years as a dumb terminal, and I don’t recall any of the comm programs mentioning this feature existing on the SCROLL LOCK key.
>that way you could read it while continuing to download the rest of the text.
Hmmm, that would be a nice feature, I just don’t recall it working that way.
>On a Macintosh, the Scroll Lock key sometimes served as a “power/reset” button.
I stopped using Macs around 1991, but I doubt if Apple would ever do something so un-user-friendly. On the older Macs, before the Apple Desktop Bus, the power switch was on the back, by the power cable. On ADB keyboards, the power switch is a big oddly-shaped bar across the top of the keyboard, with a right-pointing triangle on it. I’ve never heard of “Scroll Lock” ever being a power or reset key.
>In layman’s terms that means, “You can make a program monitor a specific location in your computer’s hardware so it can do something cool (or lame, depending on your point of view) when the SysReq key is pressed.”
That’s an incorrect interpretation. Here’s the real scoop:
Since the early 1970’s IBM had this “Virtual Machine” concept going. The idea is a really cool one (at least for us Computer Scientists). It goes something like this: You’d have one Uber-machine monitor-- often called the VMM, virtual machine monitor. It’s a very clever program, one that can simulate several virtual computers running on one CPU. The virtual machines have absolutely no knowledge or ability to influence each other. This comes in very handy when you want to, say, run three completely different operating systems simultaneously on the same computer. Each operating system thinks it has the whole computer to itself. You can even have one or more systems “crash” while the other virtual machines keep on merrily running.
This presented a problem to the terminal. If each OS thought it had complete control of the keyoard, how do you switch between typing to different virtual machines? It couldnt be with a regular key, as the ground rules stated that each OS had to run with NO changes. So IBM had to add a totally NEW key, one that could be intercepted by the VMM, and wouldnt conflict with the virtual machines view of the keyboard.
The VMM idea never caught on in the PC world, so the key is kind of an orphan.
>Some DOS communication programs used the Break key as a shortcut to quickly terminate a modem connection.
The Break key goes a bit farther back, to the days of telegraphy. In those simple days, a telegraph loop consisted of a key, a clicker, a battery, then a wire to the opposite guy, who also had a key, a clicker, and a battery. To receive a message, you’d flip a switch on your key to short it out, then the other guy would tap out a message on his key. If you lost track of the message you were receiving, you’d flip open your switch, which would break the series circuit, so the guy on the sending end would notice his clicker wasnt clicking in step with his sending. If he was smart, he’d remember that this either menat the indians had broken the wire, or you wanted to say something (a primitive way of saying “OVER”). Then you’d tap out your response, something like "I lost you after “Greetings,” ".
This “BREAK the line current” protocol continued on into teletype protocol, then into PC communications protocol. When you press the BREAK key, the PC doesnt send a character, it actually sends a burst of the “space” tone, a long stream of 0000 bits, which isnt a valid character as it doesnt have the required prefix start bit or the suffixed stop bit.
On an old mechanical teletype, this WAS a signal to the telephone line switching equipment that you wanted to hang up the connection.
On a PC it usually was a signal to the computer on the other end to STOP whatever it was doing. One sometimes side-effect of sending an overly-long break signal was to cause the modem on the other end to hang-up the phone.
This whole BREAK signal was cheap and effective for telegraph and teletype operations, but soon became a big pain-in-the-butt for computer communications. Being a special, non-character-based signal, it required special handling, with special wires, circuits, IC’s, and computer handshakes to properly pass this “BREAK” signal along. This required an inordinate amount of extra hardware, and many systems just ignored BREAK. Even those that did handle BREAK properly denigrated this usage and encouraged people to use a designated control character for this, usually Control-C for most systems, sometimes Control-Y for VAX systems.
