8086 Assembler Tutorial for Beginners (Part 3)
Variable is a memory location. For a programmer it is much easier to have some value be kept in a variable named "var1" then at the address 5A73:235B, especially when you have 10 or more variables.
Our compiler supports two types of variables: BYTE and WORD.
As you probably know from part 2 of this tutorial, MOV instruction is used to copy values from source to destination.
Let's see another example with MOV instruction:
Copy the above code to MicroAsm source editor, and press F5 key to compile it. Then open the executable in any disassembler (emu8086 or any other).
Compiler is not case sensitive, so "VAR1" and "var1" refer to the same variable.
The offset of VAR1 is 0108h.
The offset of var2 is 0109h, this variable is a WORD so it occupies 2 BYTES. It is assumed that low byte is stored at lower address, so 34h is located before 12h.
You can see that there are some other instructions after the RET instruction, this happens because disassembler has no idea about where the data starts, it just processes the values in memory and it understands them as valid 8086 instructions (we will learn them later).
You can even write the same program using DB directive only:
Copy the above code to MicroAsm text editor, and press F5 key to compile and load it in the emulator. You should get the same disassembled code, and the same functionality!
As you may guess, the compiler just converts the program source to the set of bytes, this set is called machine code, processor understands the machine code and executes it.
ORG 100h is a compiler directive (it says to compiler how to handle the source code). This directive is very important when you work with variables. It says to compiler that the executable file will be loaded at the offset of 100h (256 bytes), so compiler should calculate the correct address for all variables when it replaces the variable names with their offsets. Directives are never converted to any real machine code.
Why executable file is loaded at offset of 100h? Operating system keeps some data about the program in the first 256 bytes of the CS (code segment), such as command line parameters and etc.
Though this is true for COM files only, EXE files are loaded at offset of 0000, and generally use special segment for variables. Maybe we'll talk more about EXE files later.
Arrays can be seen as chains of variables. A text string is an example of a byte array, each character is presented as an ASCII code value (0..255).
Here are some array definition examples:
a DB 48h, 65h, 6Ch, 6Ch, 6Fh, 00h
b DB 'Hello', 0
b is an exact copy of the a array, when compiler sees a string inside quotes it automatically converts it to set of bytes. This chart shows a part of the memory where these arrays are declared:
You can access the value of any element in array using square brackets, for example:
MOV AL, a
You can also use any of the memory index registers BX, SI, DI, BP, for example:
MOV SI, 3
MOV AL, a[SI]
If you need to declare a large array you can use DUP operator.
The syntax for DUP:
number DUP ( value(s) )
number - number of duplicate to make (any constant value).
value - expression that DUP will duplicate.
c DB 5 DUP(9)
is an alternative way of declaring:
c DB 9, 9, 9, 9, 9
one more example:
d DB 5 DUP(1, 2)
is an alternative way of declaring:
d DB 1, 2, 1, 2, 1, 2, 1, 2, 1, 2
Of course, you can use DW instead of DB if it's required to keep values larger then 255, or smaller then -128. DW cannot be used to declare strings!
The expansion of DUP operand should not be over 1020 characters! (the expansion of last example is 13 chars), if you need to declare huge array divide declaration it in two lines (you will get a single huge array in the memory).
Getting the Address of a Variable
There is LEA (Load Effective Address) instruction and alternative OFFSET operator. Both OFFSET and LEA can be used to get the offset address of the variable.
LEA is more powerful because it also allows you to get the address of an indexed variables. Getting the address of the variable can be very useful in some situations, for example when you need to pass parameters to a procedure.
In order to say the compiler about data type,
these prefixes should be used:
BYTE PTR - for byte.
WORD PTR - for word (two bytes).
BYTE PTR [BX] ; byte access. or WORD PTR [BX] ; word access.MicroAsm supports shorter prefixes as well:
b. - for BYTE PTR
w. - for WORD PTR
sometimes compiler can calculate the data type automatically, but you may not and should not rely on that when one of the operands is an immediate value.
Here is first example:
Here is another example, that uses OFFSET instead of LEA:
Both examples have the same functionality.
LEA BX, VAR1
MOV BX, OFFSET VAR1
are even compiled into the same machine code: MOV BX, num
num is a 16 bit value of the variable offset.
Please note that only these registers can be used inside square brackets (as memory pointers): BX, SI, DI, BP!
(see previous part of the tutorial).
Constants are just like variables, but they exist only until your program is compiled (assembled). After definition of a constant its value cannot be changed. To define constants EQU directive is used:
name EQU < any expression >
The above example is functionally identical to code:
Read the following section only if you are using emu8086 - 8086 microprocessor emulator:
You can view variables while your program executes by selecting "Variables" from the "View" menu of emulator.
To view arrays you should click on a variable and set Elements property to array size. In assembly language there are not strict data types, so any variable can be presented as an array.
Variable can be viewed in any numbering system:
You can edit a variable's value when your program is running, simply double click it, or select it and click Edit button.
It is possible to enter numbers in any system, hexadecimal numbers should have "h" suffix, binary "b" suffix, octal "o" suffix, decimal numbers require no suffix. String can be entered this way:
'hello world', 0
(this string is zero terminated).
Arrays may be entered this way:
1, 2, 3, 4, 5
(the array can be array of bytes or words, it depends whether BYTE or WORD is selected for edited variable).
Expressions are automatically converted, for example:
when this expression is entered:
5 + 2
it will be converted to 7 etc...