DATA DECLARATION

The first instruction of our first assembly language program was “mov ax, 5.” Here MOV was the opcode, AX was the destination operand, while 5 was the source operand. The value of 5 in this case was stored as part of the instruction encoding. In the opcode B80500, B8 was the opcode and 0500 was the operand stored immediately afterwards. Such an operand is called an immediate operand. It is one of the many types of operands available.

Writing programs using just the immediate operand type is difficult. Every reasonable program needs some data in memory apart from constants. Constants cannot be changed, i.e. they cannot appear as the destination operand. In fact placing them as destination is meaningless and illegal according to assembly language syntax. Only registers or data placed in memory can be changed. So real data is the one stored in memory, with a very few constants. So there must be a mechanism in assembly language to store and retrieve data from memory.

To declare a part of our program as holding data instead of instructions we need a couple of very basic but special assembler directives. The first directive is “define byte” written as “db.”

db somevalue

As a result a cell in memory will be reserved containing the desired value in it and it can be used in a variety of ways. Now we can add variables instead of constants. The other directive is “define word” or “dw” with the same syntax as “db” but reserving a whole word of 16 bits instead of a byte. There are directives to declare a double or a quad word as well but we will restrict ourselves to byte and word declarations for now. For single byte we use db and for two bytes we use dw.

To refer to this variable later in the program, we need the address occupied by this variable. The assembler is there to help us. We can associate a symbol with any address that we want to remember and use that symbol in the rest of the code. The symbol is there for our own comprehension of code. The assembler will calculate the address of that symbol using our origin directive and calculating the instruction lengths or data declarations in-between and replace all references to the symbol with the corresponding address. This is just like variables in a higher level language, where the compiler translates them into addresses; just the process is hidden from the programmer one level further. Such a symbol associated to a point in the program is called a label and is written as the label name followed by a colon.