Fundamentals of Addressing Modes

Addressing Modes is the way in which the argument of an instruction is specified. The addressing mode defines a rule for interpreting the address field of the instruction before the operand is reached. Addressing mode is used in instructions which operate on the data or in instructions which change the program flow.

Instructions which reach the data have the possibility of specifying the data placement. The data is an argument of the instruction, sometimes called an operand. Operands can be of one of the following: register, immediate, direct memory, and indirect memory. As in this part of the book the reader doesn't have the knowledge about any assembler instructions we will use the hypothetic instruction copy that copies the data from the source operand to the destination operand. The order of the operands will be similar to high-level languages where the left operand is the destination and the right operand is the source. Copying data from a to b will be done with such an instruction.

 copy b, a

An immediate operand is a constant or the result of a constant expression. The assembler encodes immediate values into the instruction at assembly time. The operand of this type is always at the source place in the operands list. Immediate operands are used to initialise the register or variable, as numbers for comparison. An immediate operand as it's encoded in the instruction, is placed in code memory, not in data memory and can't be modified during software execution.

A direct memory operand specifies the data at a given address. An address can be given in numerical form or as the name of the previously defined variable. It is equivalent to static variable definition in high-level languages.

Data in the memory can be accessed using different addressing modes.