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Define Variables

To define a variable, we use instructions like the code below:

var_name_here db 12h
ano_var_name dw 5678h

As you can see, db and dw could decide the length of the variable.

• db Define Byte(s)
• dw Define Word(s)

Variable could be considered a convenient alias for a specified address.

Where variable stores?

It stores in your code segments, and it's in the same position as where you write it in the asm code. You can use your 8086 emulator to try it out yourself.

For more info, check this Github ASM Learning Book

Some interesting attempt

We already known that variables store in the code segment, means it's possible that the ip register point to the position of a defined variable, so can we use variable definition to make the CPU run some specified code?

Here we can see the corresponding HEX code of instructions:

mov dx, 0ffffh
ret

is: C3FF FFBA

Then what will happen if we use variable declaration to put these specified numbers into our code segment?

Let compile and run the ASM below:

mov ax, 1

var1 dw 0ffbah
var2 dw 0c3ffh

mov ax, 2

And here is what we got!

DX value has been updated to ffffh, and program halted, and this is what command mov dx, 0ffffh and ret actually do!

Based on the result we know, the definition of a var before ret make affect the program, so currently we suggest that all variables be declared after a ret command, or at a place where IP register would not go through.

Variables Invalidation

I found out this when testing about DS data segment register. First we should know that a variable actually is just an alias to a address. For example you now can use a name var_a to represent a special address like 0b00h.

However we have already know that the final addresss be accessed not only related to the offset address, but also related to the base address (usually the value in DS). So if we changed the DS and then try to directly use the variable, we may got an unexpected result. See screenshot below:

Here we try to read the value from a variable val after we changed the value of ds to 0200h (It's initial value is 0100h). And we can find out that bx will not turn to 1234h. Since currently, ds is 0200h, so the CPU actually trying to moving the value from address 2000d to ax.

You can try to set value back to 0100h of ds and see if the program would work again.

Define An Array

byte_arr db 'Hello World', 0
word_arr dw 5 DUP(1234h)
word_arr dw 5 DUP(1, 2, 3)
word_arr dw 1234, 5678, 1234, 0000

There is some points we need to take attentions to:

• When declaring string array, we need to set a stop 00h at the last of the array. However we won't know where the string array ends.
• We could use dup_times DUP(dup_value) to create an array with identical value in every element.
• DUP() could receive more than one parameter. For example you can use DUP(1, 2) to general array like 1, 2, 1, 2, ...
• We could add several contigunous values to an array by using a comma to seperate them.

Access Data In Array

Similar to the one in C/C++, we could use subscript, or you can say, index, to access the value at the certain postion in the array. See example above.

mov ax, arr[3]
mov arr[5], bx