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<title>05 TAJGA FASM Tutorial</title>
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<center><b>CHAPTER 5 - Jumps and branching</b></center><br><br>
You should know a little about how instructions are processed by processor. It
fetches instruction in machine code, executes it, and then moves to next
instruction. This is repeaten until instruction <code>int 20h</code> is reached.
In this chapter we will learn something about instructions which changes this
behaviour.<br><br>
<br><b>5.1. instruction pointer</b><br>
Processor loads first instructions (it determines number of bytes the
instrution consists from), executes it and then moves to another instruction.
But how this mechanism works? Processor has a special word register "ip" which
holds address of currently executed instruction. After instruction is executed
processor adds it's size to "ip" and executes instruction on address in "ip".
Mechanism works like this:
<ul>
<li>Loop
<ul>
<li>Execute instruction on <code>ip</code>
<li>size = size of instruction on <code>ip</code>
<li>ip = ip + size
</ul>
<li>Until instruction <code>int 20h</code> is found
</ul>
<b>NOTE:</b> as with others pointers, <code>ip</code> doesn't hold full address
of instruction, only offset part. Be we don't care about this now.<br><br>
<b>NOTE:</b> "ip" stands for "instruction pointer".<br><br>
<br><b>5.2. Jumps</b><br>
Register <code>ip</code> is not like other registers (like <code>ax</code>,
<code>ah</code>,<code>bp</code>...). It's contents can't be changed using
<code>mov</code> instruction. <code>mov ip,5</code> doesn't work.
But there is a special instruction which can change value in <code>ip</code>
register. It is <code>jmp</code> ("jmp" = "jump") instruction. This instruction
has one operand, new address for <code>ip</code> register. So
</code>jmp 5</code> has effect like <code>mov ip,5</code> would if it were an
instruction. Example:
<blockquote class="code"><pre>
org <font color=#339933>256</font>
jmp Start
text db <font color=#bb0000>'Text to output'</font>
Start<font color=#333399>:</font>
mov ah<font color=#333399>,</font><font color=#339933>9</font>
mov dx<font color=#333399>,</font>text
int <font color=#339933>21h</font>
int <font color=#339933>20h</font>
</pre></blockquote>
First instruction sets value of <code>ip</code> to address of
<code>mov ah,9</code> instruction (address is held in label <code>Start</code>).
Thus processor won't try to execute bytes
defined by "Text to output" string and this program will work.<br><br>
<b>NOTE:</b> of course, when <code>ip</code> is changed by <code>jmp</code>
instruction, then size of this instruction is NOT added to it.<br><br>
<a name=3></a>
<b>5.3. Comparing and conditonal jumps</b><br>
If you can code any language already you should know branching, eg.
conditional execution of some parts of code. For example: say you want value
not greater than 10 in <code>al</code>. So if value in <code>al</code> register
is > 10, you will set <code>al</code> to 10. This is branching - if some
condition is true then something is executed, otherwise it is not executed.
Assembly impletation of this mechanism is that when condition is false you will
jump over conditional code, when condition is true you will just cotinue
execution. It is as if C code:
<blockquote><pre>
if (condition)
ConditionalCode(); //this can be any C code, not just function call
</pre></blockquote>
would be writen this way:
<blockquote><pre>
if (!condition) goto LabelAfterConditionalCode;
ConditionalCode(); //this can be any C code, not just function call
LabelAfterConditionalCode:
</pre></blockquote>
First problem is how to decide whether condition is true. In assembly, there
is instruction which can compare two operands. It is <code>cmp</code>. It's
operands follow same rules as <code>mov</code>'s operands (almost every
instruction follows these or very similar rules). So examples of comparing:
<blockquote class="code"><pre>
cmp ax<font color=#333399>,</font>bx <font color=#777777>; compare value of "ax" to value of "bx"</font>
cmp al<font color=#333399>,</font>byte <font color=#333399>[</font>SomeLabel<font color=#333399>]</font> <font color=#777777>; comapre value of "al" to byte at SomeLabel</font>
cmp ax<font color=#333399>,</font><font color=#339933>5</font> <font color=#777777>; compare value of "ax" to 5</font>
cmp ax<font color=#333399>,</font>al <font color=#777777>; wrong, operands have different size</font>
</pre></blockquote>
This instructions checks whether first operand is same, greater or lesser than
second.
<blockquote class="term">
instruction: <b>cmp</b>
</blockquote>
OK, we can compare, but how are results of comparison stored? CPU (processor)
has special register called "flags register" in which it stores results of
comparison (and some other things too). This register can't be accessed with
<code>mov</code> or similar instructions (same like <code>ip</code>), it's value
is set by <code>cmp</code> instruction. Now you don't have to care HOW is result
of comparison stored in this register, you would need understand bit arithmetics.
<blockquote class="term">
register: <b>flags</b>
</blockquote>
OK, we can compare, we know result is stored in <code>flags</code>. Only thing
we need now is conditional jump itslef. Conditional jump is jump which is taken only
when condition specified by you is true (in flags register). It will be best
explained on example. We compare <code>ax</code> to <code>bx</code>
(<code>cmp ax,bx</code>). Now conditional jump can jump if <code>ax</code> <
<code>bx</code>, or when <code>ax</code> = <code>bx</code>, or when
<code>ax</code> >= <code>bx</code> etc.
These jumps are (op1 is first operand of <code>cmp</code>, op2 is second):
<ul>
<li><code>je</code> - jump if op1 = op2 (op1 is "equal to" op2)
<li><code>ja</code> - jump if op1 > op2 (op1 is "above" op2)
<li><code>jb</code> - jump if op1 < op2 (op1 is "below" op2)
<li><code>jae</code> - jump if op1 >= op2 (op1 is "above or equal to" op2)
<li><code>jbe</code> - jump if op1 <= op2 (op1 is "below or equal to" op2)
</ul>
Example code: (but don't try to compile it, it is not .COM executable, it is
just piece of code)
<blockquote class="code"><pre>
cmp ax<font color=#333399>,</font><font color=#339933>10</font>
jbe AX_lesser_than_10
mov ax<font color=#333399>,</font><font color=#339933>10</font>
AX_lesser_than_10<font color=#333399>:</font>
</pre></blockquote>
this piece of code will check whether value in <code>ax</code> is below or equal
to 10, and if not (if the value in <code>ax</code> is above 10) it will set
<code>ax</code> to 10. Corresponding C code is:
<blockquote><pre>
if (ax>10) ax=10;
</pre></blockquote>
or more similar to our assembly version:
<blockquote><pre>
if (ax <= 10) goto AX_lesser_than_10
ax=10;
AX_lesser_than_10:
</pre></blockquote>
Another example: get maximum of {ax,bx} and store it in ax:
<blockquote class="code"><pre>
cmp ax<font color=#333399>,</font>bx
jae AX_already_contains_bigger_value
mov ax<font color=#333399>,</font>bx
AX_already_contains_bigger_value<font color=#333399>:</font>
</pre></blockquote>
So compare <code>ax</code> to <code>bx</code>, if it is bigger or equal then it
already contains the bigger value, we dont need to change anything. If
<code>ax</code> is lesser than <code>bx</code> then we must move value in
<code>bx</code> (=bigger value) to <code>ax</code>.<br><br>
More complicated version: store maximum of {ax,bx} in cx:
<blockquote class="code"><pre>
cmp ax<font color=#333399>,</font>bx
ja AX_bigger
mov cx<font color=#333399>,</font>bx
jmp done
AX_bigger:
mov cx<font color=#333399>,</font>ax
done<font color=#333399>:</font>
</pre></blockquote>
so we compare <code>ax</code> to <code>bx</code>, then if <code>ax</code> is
lesser than <code>bx</code> jump won't be taken and we continue by
<code>mov cx,bx</code> which stores bigger value in <code>cx</code> as
wanted, and then <code>jmp done</code> skips instructions used in case
<code>ax</code> is bigger.
If <code>ax</code> is bigger than <code>bx</code>, then <code>jmp AX_bigger</code>
will be taked, so next instruction is <code>mov cx,ax</code> which moves greater
value (in <code>ax</code>) to <code>cx</code>. You see, code was divided into two
"branches" one for ax>bx, other for ax<=bx.
Finally both branches goes to instruction behind <code>done:</code>, and at this
place <code>cx</code> always holds bigger value. By the way, there could be
<code>jae</code> instead <code>ja</code>, because for case when ax=bx both
branches have same effect.
<blockquote class="term">
instructions: <b>je, ja, jb, jae, jbe</b>
</blockquote>
But what can we do if we want to jump when operands are NOT equal? We could do
something like this:
<blockquote class="code"><pre>
cmp ax<font color=#333399>,</font>bx
je Same
jmp NotSame
Same<font color=#333399>:</font>
...
NotSame<font color=#333399>:</font>
</pre></blockquote>
but this is not needed because there are instructions which jumps when
condition is not true. These are <code>jne</code>,<code>jna</code>,
<code>jnb</code>,<code>jnae</code> and <code>jnbe</code>. Instruction
<code>jne</code> jumps when operands are not equal, <code>jna</code> when first
operand is not above second operand etc., so:
<blockquote class="code"><pre>
cmp ax<font color=#333399>,</font>bx
jne NotSame<font color=#333399>:</font>
...
NotSame<font color=#333399>:</font>
</pre></blockquote>
and ... part is executed only if value in ax is equal to value in bx.<br><br>
<b>NOTE:</b> <code>jna</code> is same as <code>jbe</code>, <code>jnb</code> is
same as <code>jae</code>, <code>jb</code> is same as <code>jnae</code>,
<code>ja</code> is same as <code>jnbe</code>.
<blockquote class="term">
instructions: <b>jne, jna, jnb, jnae, jnbe</b>
</blockquote>
<b>NOTE:</b> (important) many instruction change <code>flags</code> register,
not only <code>cmp</code>, so conditional jumps should be right behind
<code>cmp</code>, no instructions between them.
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