Assembler directives

The following are some more widely used directives of the 8051.

ORG (origin)

The ORG directive is used to indicate the beginning of the address. The number that comes after ORG can be either in hex or in decimal. If the number is not followed by H, it is decimal and the assembler will convert it to hex. Some assemblers use “. ORG” (notice the dot) instead of “ORG” for the origin directive. Check your assembler.

EQU (equate)

This is used to define a constant without occupying a memory location. The EQU directive does not set aside storage for a data item but associates a constant value with a data label so that when the label appears in the program, itp constant value will be substituted for the label. The following uses EQU for the counter constant and then the constant is used to load the R3 register.

When executing the instruction “MOV R3, ttCOUNT”, the register R3 will be loaded with the value 25 (notice the # sign). What is the advantage of using EQU? Assume that there is a constant (a fixed value) used in many different places in the program, and the programmer wants to change its value throughout. By the use of EQU, the programmer can change it once and the assembler will change* all of its occurrences, rather than search the entire program trying to find every occurrence.

END directive

Another important pseudocode is the END directive. This indicates to the assembler the end of the source (asm) file. The END directive is the last line of an 8051 program, meaning that in the source code anything after the END directive is ignored by the assembler. Some assemblers use “. END” (notice the dot) instead of “END”.

Rules for labels in Assembly language

By choosing label names that are meaningful, a programmer can make a program much easier to read and maintain. There are several rules that names must follow. First, each label name must be unique.

The names used for labels in Assembly language programming consist of alphabetic letters in both uppercase and lowercase, the digits 0 through 9, and the special characters question mark (?), period (.), at (@), underline (_), and dollar sign ($).

The first character of the label must be an alphabetic character.

In other words it cannot be a number. Every assembler has some reserved words that must not be used as labels in the program. Foremost among the reserved words are the mnemonics for the instructions. For example, “MOV” and “ADD” are reserved since they are instruction mnemonics. In addition to the mnemonics there are some other reserved words. Check your assembler for the list of reserved words.