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 know 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

Register operand is used where the data which the processor wants to reach is stored or is intended to be stored in the register. If we assume that a and b are both registers named R0 and R1 the instruction for copying data from R0 to R1 would look like this:

 copy R1, R0

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 can be only one in the instruction and 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. Instruction which initialises register R1 with the constant (immediate) value of 5 looks like this:

 copy R1, 5

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. If we assume that the var represents the address of the variable the instruction which copies data from the variable to R1 can look like this:

 copy R1, var

Indirect memory operand is accessed by specifying the name of the register which value represents the address of the memory location to reach. We can compare the indirect addressing to the pointer in high-level languages where the variable does not store the value but points to the memory location where the value is stored. Indirect addressing can also be used to access elements of the table in a loop, where we use the index value which changes every loop iteration rather than a single address. Different assemblers have different notations of indirect addressing, some use brackets, some square brackets, and others @ symbol. Even different assembler programs for the same processor can differ. In the following example, we assume the use of square brackets. The instruction which copies the data from the memory location addressed by the content of the R0 register to R1 register would look like this:

 copy R1, [R0]