untergeek's New Writeups

raking it in (idea)

2000-08-20T21:36:41Z

A euphamism for "making lots of money"

machine code (thing)

2000-06-13T22:05:28Z

Ones and zeros. It doesn't get any lower level than this, this is the code that the computer actually reads. I'm always amazed that people used to program in this. I have enough trouble with assembly.

low level (idea)

2000-06-13T21:57:13Z

The nuts and bolts of something, where generalists fear to tread.

In terms of programming it tends to refer to assembly language. Of course to someone who programs in assembly it's probably going to refer to machine code and to someone who deals exclusively in algorithms it could very well mean C, it's all relative.

jump table (thing)

2000-06-13T21:45:07Z

A low level programming construct used in situations where a numerical value x determines what block of code gets executed next. It generally takes the form of an array of addresses. The indices of the array correspond to possible values of x. Thus x is used as an offset into the array, the address is read from the array and control jumps to that address. Ideally this can all be done without any expensive comparison operations. In the real world there will probably be two comparisons to make sure that x is not above the maximum index in the array or below the minimum.

A high level representation of this concept is the switch() which is typically compiled into a jump table. If you've understood what I've said so far you'll also understand that it's a Good Thing to try and keep the conditions in your switch contiguous so as to avoid situations where most of the space the produced jump table is taking up is used to jump to the default code.

I'd supply a nice…

break (thing)

2000-06-13T21:06:26Z

In C and programming languages with C style syntax break is used to exit a block of code. Primarily it is used to exit loops when some condition is met.


while (1)
{
  //do something...
  if (done)
  {
    break;
  }
  //do something else...
}

Of course break is also used quite heavily in switches to keep all hell from breaking loose.

switch() (thing)

2000-06-13T20:48:30Z

The variable given to a switch must be an int or a defined type that resolves to an int (long int might work too, it's been awhile). The reason for this is that a switch compiles into a jump table. This is important because finding what you're looking for in a jump table would take O(1) time instead of the O(n) time (where n is the number of conditionals) it could take to check every conditional if you were to implement this as a series of if()/elseif() statements.

It's also worth pointing out that you'll probably want to put a "break;" at the end of every block of code after a condition. If you don't your code will go to the correct condition and then execute all the code after that, including code intended for other conditions. Of course this may be what you want. See Duff's Device.

Update: wharfinger has brought it to my attention that a switch doesn't always compile to a jump table. This caused me to drag out my copy of the Dragon Book. On…