How did the SysRq key get onto modern keyboards if it's rarely used?

Question

Was the SysRq key ever used on any common operating system?

Google tells me it wasn't. But then how did it end up on so many keyboards?

Answer 1

Before IBM PC even existed, terminal keyboards for IBM Mainframes make extensive use of the SysRq or (System Request) key. I recall using SysReq key on IBM 5251 and 5291 terminals for IBM S/36. Probably the key (and function) was already present in the 1970s IBM 3270 terminal.

Its function was to suspend the current job and display the System Request Menu. The System Request Menu allows the user to send and display messages, transfer to a second job, or end the current job.

It was included in the first IBM PC 5150 just because it was used also as a mainframe terminal (with the appropiate card).

Answer 2

SysRq is popular because the PC AT was popular, and its clones were too. PC clone manufacturers copied all the features of the computers they were cloning, with the exception of BASIC in ROM, and that included SysRq — both the physical key, and its associated handling in ROM (interrupt 0x15 function 0x85).

Any OS or software running on the AT could end up relying on SysRq, so it became entrenched, even though in practice it wasn’t used much.

Answer 3

I think you're coming at this kind of backwards. The commonly used keys are commonly used because they're on keyboards and are widely available. Software follows hardware in this instance. The system request key was put there because the hardware designers thought it would be useful, and even if they were wrong, removing keys from the standard keyboard as dictated by whatever IBM was doing would mean introducing potential incompatibilities.

Answer 4

As other answers and comments have already said, the SysRq key was first introduced with the keyboard of the PC/AT of 1984, and had parallels to similar keys on IBM mainframe keyboards.

Even if it was rarely used, keyboard manufacturers kept including it, as removing it would make their keyboard incompatible with the few pieces of software that did use it. Only in recent years have manufacturers started to drop it and other little-used keys, chiefly on laptops where space is at a premium.

So why was this key so rarely used on the home PC, rare enough that you ask whether any OS ever used it?

In the mid 1980s, only the brand-new AT (and compatibles) would have the SysRq key. If a software developer wanted to support the older XT and PC machines, they'd have to stick to using keys that existed on both keyboard variants. New versions of MS-DOS, for example, couldn't use it without losing backward compatibility with earlier machines.

As described on Wikipedia's page about the SysRq* key, it behaves differently to all other keys on the keyboard, and bypasses the keyboard buffer and the OS' usual input processing. Pressing SysRq can only be detected with a dedicated BIOS function (int. 0x15, function 0x85). Under DOS, any user application could access the BIOS functions directly, but that was no longer possible under Windows. For good or ill, the developers of Windows saw no need to make special use of this special key, other than some specialist debugging purposes.

Linux, however, does use it. The 'magic SysRq key' can be used to issue simple commands to an unresponsive computer: remounting filesystems, terminating processes, or restarting the machine. This uses SysRq's ability to bypass the normal keyboard-handling interface, and issue signals directly to the underlying operating system.

*Note: At the time of writing, the SysRq article on Wikipedia includes a weakly sourced claim that the key was meant for switching between operating systems on the PC/AT

Answer 5

As for how it got on keyboards, I'd assume that's because the basic 104+ key IBM style keyboard had it.

Though you might not use it often, it's still got a purpose. On linux, hitting alt+sysreq allows you to send messages to the kernel even if everything else is locked up. https://en.wikipedia.org/wiki/Magic_SysRq_key I've had to use it to reboot a number of times.

Answer 6

History. It was on the original IBM keyboard for the IBM PC. Later, IBM rearranged the keys on the keyboard, but they kept the original keys. Most keyboard manufacturers copied the IBM layout because "IBM compatibility" was a big deal back then.

There are lots of useless keys on standard keyboards. There's the entire "numeric keypad" with the number-key arrangement upside down. (OK, accountants probably use it.) There's the Caps Lock key, whose existence is an annoying nuisance for most people.

For a really annoying keyboard, take a look at the Swiss keyboard. There's a key devoted to § and ° , and in order to make room for ü and some accented letters, it sacrifices | and ~ while also requiring awkward alt-key combinations for {}.