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Archive for August, 2006

DOS "debug" fun

with 2 comments

Sometimes you get the urge to do some programming, but you don’t have access to a compiler. Luckily, virtually every Windows PC in the world has a utility called “debug” installed. This little program lets you input 16-bit x86 assembly language, and allows you to write it out to create a .com file. This file can then be executed inside of a DOS shell.

The following is an example of something simple but cool you can do with this program. I will be using some basic DOS interrupts in order to do console input and output. (Lookup ‘DOS interrupt 21h’ on google to find out more).

I’ll highlight everything i’m typing in green so you can try this at home. You won’t need to type the optional comments (anything after the semicolon is ignored), and they get lost when you write the file anyway.


-a 100
1476:0100 mov cl, d7 ; default cl to character 'd7'
1476:0102 mov ah, 6  ; ah:6 is console input/output
1476:0104 mov dl, ff ; dl:ff specifies input
1476:0106 int 21     ; interrupt 21 call
1476:0108 jz 10c     ; skip next instruction if failure
1476:010A mov cl, al ; save result of console input
1476:010C mov dl, cl ; load current character as output
1476:010E int 21     ; interrupt 21 call
1476:0110 jmp 102    ; loop forever!
-r cx
CX 0012
Writing 00012 bytes

Now, when you execute “”, you will see the screen swamped with the funky ‘d7’ ASCII character.

Whenever you type a character, the screen with update to display that character. Try alternating between visible characters and not visible characters (like space). For more fun, try to as quickly as possible type “\|/-\|/-” (animates like a little progress bar).

Note that you cant really exit the program. Pressing ctrl+c will actually just display a little heart character. It would be easy to modify the code to accept a character (like escape or control+c) to exit, but i’ll leave that up to you :].

This is just one basic thing you can make a .com file do. For a cooler example, check out neetro. Neetro is a little bit more complex than the program above, so it would have been a pain to write using “debug”. The source is instead compiled using nasm.

Written by caustik

August 20th, 2006 at 10:00 pm

Ridiculous C-code

with 2 comments

For no particular reason I decided to write the following confusing line of C code today…

const _=0;do;while(0?0,1:!_,_);

Might make an interesting interview question. Can you parse it in your head?

Here are some reasons why this seems very confusing:

  1. const _=0;This is not much different than “const int a = 0;”… It is just using the implied “int” type, and rudely using the single underscore character, which is acceptable (but taboo) as a variable name.
  2. do;whileThis looks odd, but is actually valid. Typically, people implement a do/while as “do{}while(…);” – however, it is equally valid to replace the {}’s with a single statement, even an empty do-nothing statement (“;”).
  3. a?b:cI am tempted to fail somebody in an interview immediately if they have no clue what this operator does. It isn’t that it’s particularly critical or anything -but if you’re a C coder and you’ve been around the block – it would be very odd for you to not be familiar with the conditional operator. Still, it is rare enough to be difficult to brain-parse.All this does is evaluate “a”, and if the result is true, evaluates “b”. If the result if false, it evaluates “c”. Simple.
  4. !_,_ Okay so this one is nasty. I might not expect everybody to be familiar with this guy. What happens here is the variable “_” is being evaluated as negated, then the variable “_” itself is being evaluated. The trick is, only the right side of the comma is actually used as the value of the expression. Huh?

    This is primarily useful in things like a “for” loop. Since the general syntax of a for loop is “for(a;b;c)”, you at first may feel a little limited. How can I increment two variables on each step? Easy. “for(a;b;d++,e++)” – note the comma operator allows you to squeeze two subexpressions into “c”. In this case, nobody knows or cares that the expression value is, in the end, defined only by e++. The result is not being used at all.

    I’d be interested to hear some real world valueable uses for the comma operator, aside from a “for” loop.

Written by caustik

August 17th, 2006 at 8:25 am

Posted in Funny,Programming

Tagged with ,

Here goes

with one comment

This is my first blog posting. Should get interesting. I won’t bother to explain the title of this blog, because that is sort of the point.

Written by caustik

August 17th, 2006 at 7:35 am

Posted in Hacking