Editing Basic Assembly Programming (section)

===== If-Then and If-Then-Else =====

Sometimes we want to execute a little piece of code only if some condition holds. And maybe we want to execute a different piece of code only if it fails. In “structured” programming languages, these constructs are called <code>if</code> and <code>if ... else</code>. Just like for loops, our lives get a little bit simpler if we ''invert'' the condition. This is the standard in assembly language programming, so really make sure you get used to that. Demorgan’s Laws can also be quite helpful.

Suppose we want to read an input and output 1 if the number is 10, and zero otherwise. In a structure language, that might look like this:

<syntaxhighlight lang="c">if (input == 10) {
  output 1
}
else {
  output 0
}</syntaxhighlight>

Let’s translate that into assembly. Firstly, we can’t do <code>== 10</code>. What we ''can'' do is <code>input - 10</code>. If the result is 0, the input was 10.

<pre>move|inp|d1 # read from input to register 1
loadi|10
move|s0|d2  # get 10 into register 2
sub         # compare them
loadi|else
jnz         # jump to the else case if input was not 10

label then  # fall through to here if the input was 10
loadi|1
move|s0|out
loadi|done
j

label else
loadi|0
move|s0|out

label done</pre>

A deeper if-elseif-else chain is implemented by simply repeating this inside the <code>else</code> block.