I have some newbie assembly questions.....?

[Erimus]

I decided to try picking up some assembly yesterday.

If it matters, I'm using an Intel something-rather, and these are being saved with the DOS format (.com)

First... how would I make something like an if(1==1)? That's had me quite confused

And how would I make the program wait for a press from a particular key? There's xor ax,ax (I do not know what that does, as all I have is source code in which that is used - no tutorial or anything) but that waits for any key to be pressed... any way to make that wait for a particular key press? (I'm sure it's really obvious, but I have no idea O.O)

And I had another question, but I dont' remember what it is... Oh.. methinks i remember... How might I make something that would allow a person to type a message, and then echo the message, or something?

Any help is greatly appreciated!

 

[Kardinal]

The best way to learn assembly is to find a good tutorial book, because it is almost impossible to learn all the twists it has just by remembering. For example, I have a vague idea of how I once did the "type message, echo message" to screen once, some time ago, but I could not recollect the exact code to duplicate the effect: for assembly you always need a reference (i.e. a good book).

Here is a great online reference you might want to check:

http://www.arl.wustl.edu/~lockwood/class/cs306/books/artofasm/toc.html

With that said, let's see what can I help you with:

1) In assembly nothing is simple. Your 'if(1==1)' statement will probably be several lines of code in assembly. However, here is a simple code that will check if registers AX and BX are equal:

cmp AX, BX ; Compare AX with BX
jne label_else ; Jump if not equal to label_else

label_then:
.
.
.
jmp label_endif ; Here ends the 'then' part of the if, we must jump to label_endif

label_else:
.
.
.

label_endif:
; Continue with the code after the if statement from this point on.

2) We'll go to the key press problem in much less detail in just a moment. But first, lets see what xor AX, AX does: xor is short for exclusive-or (if you don't know what that means, google it now). xor AX, AX performs the xor bitwise operation between AX and AX, and saves the result in AX. In other words, it effectively zeroes register AX, no matter what value AX originally contained. So xor AX, AX means (in C): AX = 0;

So, as you see, xor AX, AX does not wait for a key to be pressed. It is probably just one line of many that gets that (difficult in assembly) thing to happen.

3) Wait for a particular key press... Make a person type a message... Echo a message to the screen... This (and in fact all things a computer can do) can be made in assembly. However, until you have a good understanding of basic register management in assembly this is far out of your reach.
Never the less, let me just give you a tip: what you are looking for is interrupt 21h (if you don't know what that is, google it later on). You will need this in order to make your first approach to a human being through assembly.

For the time being, I would suggest you find a good tutorial on assembly, check the reference I gave you and run your programs with a debugger to see how they work step by step. After you master that, the sky is the limit.

Patience, and good luck.