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<title>02 TAJGA FASM Tutorial</title>
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<center><b>CHAPTER 2 - First program</b></center><br><br>
You may have asked why am i fooling with creating some text files when you
want to learn assembly. But text files are just some "arrays" of bytes. You
didn't just learn how to create text file, you learnt how to define file
containing any data you want. And this is what runnable program is - special
"data" file, array of numeric values, called "machine code". You only have to
know meaning of these values :). Of course it is very hard to remember all
values and their's meanings, and this is what assembler is for. It translates
programs from human acceptable language to machine code. So you only have to
learn this human acceptable language :)<br><br>
<blockquote class="term">
<b>machine code</b><br>
array of numeric values, that represents instructions to processor (CPU)
</blockquote>
Now we will care about DOS .COM programs (rarely called "memory image", you
will learn why later, when you get into thing). These are most-simple
executable (runable) files under DOS and Windows.<br><br>
So let's create first .COM file, which won't do anything.
<blockquote class="code"><pre>
org <font color=#339933>256</font>
int <font color=#339933>20h</font>
</pre></blockquote>
Compile this to .COM file and run it. Nothing should happen. Now let's look
what that two lines means. (This will be funny...)
<blockquote class="code"><pre>
org <font color=#339933>256</font>
</pre></blockquote>
I won't explain what this directive does now. Just put this line in the
begginning of every .COM file! It doesn't define any data, even does nothing
you can notice now. We will get to this later.
<blockquote class="code"><pre>
int <font color=#339933>20h</font>
</pre></blockquote>
This is "instruction". Instruction is command for processor, which is stored
in created file as one or more bytes. When you run .COM executable file,
processor walks thru it and decodes instructions from machine code and does
what these instructions instruct it to. Instruction <code>int 20h</code> says
that this is end of execution of file. So first instruction in this says
processor to stop execution, so executable file does nothing, as you saw.<br><br>
<blockquote class="term">
<b>instruction</b><br>
single command to processor
</blockquote>
(by the way - int 20h is NOT instruction to processor which ends execution of
.COM program. It is instructs processor to call some system procedure.
System procedure is chosen by number following <code>int</code>, in this case number 20h
(it IS sort of number) which means procedure to end .COM file . <code>int</code> can be
followed by another number and another system procedure will be called. But
for now we can abstract from this, forget about it and take <code>int 20h</code> as
instruction to stop program.)<br><br>
So "machine code" is set of "instructions". Differ between directives and
instructions. Directive is command for compiler, how it should define data
and what data should it define. Instructions are defined data which encodes
what processor will do when you execute program. For example <code>db 0,0</code> is
directive which defines two zero bytes, but it is instruction in case it is
executed, because two zero bytes have special meaning for processor (don't
care what is their meaning). <code>org 256</code> is directive, not instruction,
because it doesn't define any data. You will get into this by practice.<br><br>
Instruction <code>int 20h</code> is simple, it don't need any arguments (=parameters, or
values which changes it's effect). But what if some instruction DOES need any
arguments? For this reason processor has it's own "variables" (variable is
general term for space which stores some value). These variables are called
"registers". First registers we'll learn are <code>al</code>, <code>ah</code>,
<code>bl</code>, <code>bh</code>, <code>cl</code>, <code>ch</code>,
<code>dl</code>, <code>dh</code> which are byte sized (they contain value in
range 0 to 255).<br><br>
<blockquote class="term">
<b>register</b><br>
"internal" processor's variable
</blockquote>
(by the way <code>int 20h</code> takes argument in AL register, but, again, we can
abstract from this. And, in fact, value 20h is instruction argument too, but
we abstracted from this before. This is what i was talking about when i wrote
that "it will be funny").<br><br>
Now how to set value of register? There is a instruction which does this, for
example:
<blockquote class="code"><pre>
mov al<font color=#333399>,</font><font color=#339933>10</font>
</pre></blockquote>
this instruction sets value of <code>al</code> register to 10. <code>mov</code>
stands for "move". Destination of "moving" follows
<code>mov</code> (separated with spaces), in this case it is <code>al</code>
register. Then comes source of moving separated with comma (<code>,</code>), in
this case it is number 10. So this instruction "moves" value 10 to register
<code>al</code>.
Another example of moving:
<blockquote class="code"><pre>
mov al<font color=#333399>,</font>bl
</pre></blockquote>
This copies value in <code>bl</code> register to <code>al</code> register. It
won't change value in <code>bl</code> register. Source of <code>mov</code> always stays
unchanged.
<b>NOTE:</b> You will often (always?) see some people talking about
<code>mov</code> instruction. But <code>mov</code> is not instruction, and
<code>int</code> is not an instruction too. <code>mov al,bl</code> or
<code>int 20h</code> is instruction for example. <code>mov</code> or
<code>int</code> is called "instruction mnemonics". But accept this, everyone calls it
"instruction" and you probably will too after some time (and i probably
will too, sorry :). Arguments of instruction (part of instruction without
instruction mnemonics, like <code>al</code> and <code>10</code> in
<code>mov al,10</code> are called "instruction operands" (or "instruction
arguments" :)
<blockquote class="term">
<b>instruction mnemonics</b> (this term is not so improtant now)<br>
<b>instruction operand</b>
</blockquote>
Now let's go to usage of registers. We will use <code>int 21h</code> instruction
which can do MANY things depending on value in <code>ah</code> register. We
won't learn meaning of all values, now we will talk only about value 2. If
value 2 is in <code>ah</code> register when <code>int 21h</code> instruction is
executed then character in <code>dl</code> (more extactly: character whose ASCII
code is in <code>dl</code>) is writen to screen (console).<br><br>
<b>NOTE:</b> if you are using some Windows and file manager (like Total Commander)
You will see window appear for very short time and then disappear. But our
character is displayed in this window and you probably can't notice that.
You must run shell (<code>cmd</code> on XP, <code>command</code> on older
windozes) and run your program from it. Anyway, if you can't handle this, forget
about assembly for a while and learn using your operating system. And then, dont
forget to return to assembly!<br><br>
Okay so let's look on program which writes character "a":
<blockquote class="code"><pre>
org <font color=#339933>256</font>
mov ah<font color=#333399>,</font><font color=#339933>2</font>
mov dl<font color=#333399>,<font color=#bb0000>'a'</font></font>
int <font color=#339933>21h</font>
int <font color=#339933>20h</font>
</pre></blockquote>
so anylisis
<blockquote class="code"><pre>
mov ah<font color=#333399>,</font><font color=#339933>2</font>
</pre></blockquote>
sets value of "ah" register to 2, this should be clear
<blockquote class="code"><pre>
mov dl<font color=#333399>,<font color=#bb0000>'a'</font></font>
</pre></blockquote>
this moves "a" character into <code>dl</code> register. (In fact, there is nothing like
"character a" in assembly. You could have noticed i wrote registers can
contain numeric values. Nothing about characters. Way this works is that compiler
translates character enclosed in apostrophes into it's numeric (ASCII) code,
which is recognized by <code>int 21h</code> as code for this character. In assembly
character "a" means ASCII code for character "a")
<blockquote class="code"><pre>
int <font color=#339933>21h</font>
</pre></blockquote>
In this case, when "ah" contains value 2, this writes character in "dl"
<blockquote class="code"><pre>
int <font color=#339933>20h</font>
</pre></blockquote>
And we can't forget to stop execution. Otherwise program will most-probably
crash.<br><br>
<b>NOTE:</b> in assembly character enclosed in apostrophes is same as ASCII
code for this character<br><br>
So writing multiple characters ("ab") is:
<blockquote class="code"><pre>
org <font color=#339933>256</font>
mov ah<font color=#333399>,</font><font color=#339933>2</font>
mov dl<font color=#333399>,<font color=#bb0000>'a'</font></font>
int <font color=#339933>21h</font>
mov dl<font color=#333399>,<font color=#bb0000>'b'</font></font>
int <font color=#339933>21h</font>
int <font color=#339933>20h</font>
</pre></blockquote>
we don't have to set <code>ah</code> to 2 again for second <code>int 21h</code>
because value 2 will reamin in <code>ah</code> from previous settings. Value in
<code>dl</code> will remain too, so code
<blockquote class="code"><pre>
org <font color=#339933>256</font>
mov ah<font color=#333399>,</font><font color=#339933>2</font>
mov dl<font color=#333399>,<font color=#bb0000>'a'</font></font>
int <font color=#339933>21h</font>
int <font color=#339933>21h</font>
int <font color=#339933>21h</font>
int <font color=#339933>20h</font>
</pre></blockquote>
will write "aaa".
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