29.14 Caveats

29.14.1 Unix Is Protected

For one thing, it may simply not be possible. Unix runs in protected mode. Only the kernel and device drivers are allowed to access hardware directly. Perhaps a particular Unix clone will let you read the keyboard scan codes, but chances are a real Unix operating system will not. And even if one version may let you do it, the next one may not, so your carefully crafted software may become a dinosaur overnight.

29.14.2 Unix Is an Abstraction

But there is a much more important reason not to try accessing the hardware directly (unless, of course, you are writing a device driver), even on the Unix-like systems that let you do it:

Unix is an abstraction!

There is a major difference in the philosophy of design between MS DOS and Unix. MS DOS was designed as a single-user system. It is run on a computer with a keyboard and a video screen attached directly to that computer. User input is almost guaranteed to come from that keyboard. Your program's output virtually always ends up on that screen.

This is NEVER guaranteed under Unix. It is quite common for a Unix user to pipe and redirect program input and output:

    % program1 | program2 | program3 > file1

If you have written program2, your input does not come from the keyboard but from the output of program1. Similarly, your output does not go to the screen but becomes the input for program3 whose output, in turn, goes to file1.

But there is more! Even if you made sure that your input comes from, and your output goes to, the terminal, there is no guarantee the terminal is a PC: It may not have its video memory where you expect it, nor may its keyboard be producing PC-style scan codes. It may be a Macintosh, or any other computer.

Now you may be shaking your head: My software is in PC assembly language, how can it run on a Macintosh? But I did not say your software would be running on a Macintosh, only that its terminal may be a Macintosh.

Under Unix, the terminal does not have to be directly attached to the computer that runs your software, it can even be on another continent, or, for that matter, on another planet. It is perfectly possible that a Macintosh user in Australia connects to a Unix system in North America (or anywhere else) via telnet. The software then runs on one computer, while the terminal is on a different computer: If you try to read the scan codes, you will get the wrong input!

Same holds true about any other hardware: A file you are reading may be on a disk you have no direct access to. A camera you are reading images from may be on a space shuttle, connected to you via satellites.

That is why under Unix you must never make any assumptions about where your data is coming from and going to. Always let the system handle the physical access to the hardware.