8086 assembler tutorial for beginners (part 3)

Variables

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.

Syntax for a variable declaration:

name DB value

name DW value

DB - stays for Define Byte.
DW - stays for Define Word.

name - can be any letter or digit combination, though it should start with a letter. It's possible to declare unnamed variables by not specifying the name (this variable will have an address but no name).

value - can be any numeric value in any supported numbering system (hexadecimal, binary, or decimal), or "?" symbol for variables that are not initialized.


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: 


ORG 100h

MOV AL, var1
MOV BX, var2

RET    ; stops the program.

VAR1 DB 7
var2 DW 1234h

Copy the above code to the source editor, and press F5 key to compile it and load in the emulator. You should get something like:



As you see this looks a lot like our example, except that variables are replaced with actual memory locations. When compiler makes machine code, it automatically replaces all variable names with their offsets. By default segment is loaded in DS register (when COM files is loaded the value of DS register is set to the same value as CS register - code segment).

In memory list first row is an offset, second row is a hexadecimal value, third row is decimal value, and last row is an ASCII character value.

Compiler is not case sensitive, so "VAR1" and "var1" refer to the same variable.

The offset of VAR1 is 0108h, and full address is 0B56:0108.

The offset of var2 is 0109h, and full address is 0B56:0109, 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: 


ORG 100h

DB 0A0h
DB 08h
DB 01h

DB 8Bh
DB 1Eh
DB 09h
DB 01h

DB 0C3h

DB 7

DB 34h
DB 12h

Copy the above code to the source 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 tells compiler how to handle the source code). This directive is very important when you work with variables. It tells 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

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[3]

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.

for example:
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.


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.


Reminder:
In order to tell the compiler about data type,
these prefixes should be used:

BYTE PTR - for byte.
WORD PTR - for word (two bytes).

For example:
BYTE PTR [BX]     ; byte access.
    or
WORD PTR [BX]     ; word access.
assembler supports shorter prefixes as well:

b. - for BYTE PTR
w. - for WORD PTR

in certain cases the assembler can calculate the data type automatically.



Here is first example: 


ORG 100h

MOV    AL, VAR1              ; check value of VAR1 by moving it to AL.

LEA    BX, VAR1              ; get address of VAR1 in BX.

MOV    BYTE PTR [BX], 44h    ; modify the contents of VAR1.

MOV    AL, VAR1              ; check value of VAR1 by moving it to AL.

RET

VAR1   DB  22h

END


Here is another example, that uses OFFSET instead of LEA: 


ORG 100h

MOV    AL, VAR1              ; check value of VAR1 by moving it to AL.

MOV    BX, OFFSET VAR1       ; get address of VAR1 in BX.

MOV    BYTE PTR [BX], 44h    ; modify the contents of VAR1.

MOV    AL, VAR1              ; check value of VAR1 by moving it to AL.

RET

VAR1   DB  22h

END

Both examples have the same functionality.

These lines:
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

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 >

For example:

k EQU 5

MOV AX, k


The above example is functionally identical to code:

MOV AX, 5



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...




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