Addressing Modes

Each instruction requires certain operands on which it has to operate. There are various technique to specify the operand of an instruction. These techniques are called addressing modes. The most common addressing techniques, or modes are:

1.   Immediate Addressing:

The simplest form of addressing is immediate addressing, in which the operand value is present in the instruction itself. This mode can be used to define and use constants or set initial values of variables.

The advantage of immediate addressing is that no memory reference other than the instruction fetch is required to obtain the operand, thus saving one memory or cache cycle in the instruction cycle. The disadvantage is that the size of the number is restricted to the size of the address field, which, in most instruction sets, is small compared with the word length.

2.   Direct Addressing:

In this modes of addressing the address of the operand is specified in the instruction. The operand will be in memory. Thus, the address field contains the effective address of the operand.

It requires only one memory reference and no special calculation. The obvious limitation is that it provides only a limited address space.

3.   Indirect Addressing:

In this mode the address field of the instruction gives the address where the effective address is stored in the memory.

With direct addressing, the length of the address field is usually less than the word length, thus limiting the address range. One solution is to have the address field refer to the address of a word in memory, which in turn contains a full-length address of the operand. This is known as indirect addressing.

There does not appear to be any particular advantage to this approach, and its disadvantage is that three or more memory references could be required to fetch an operand.

4.   Register Addressing:

In this modes of addressing, the instruction specifies the name of the register in which the operand is available.

The advantages of register addressing are that (1) only a small address field is needed in the instruction, and (2) no time-consuming memory references are required. The disadvantage of register addressing is that the address space is very limited.

5.   Register Indirect Addressing:

In this mode of addressing, the instruction specifies the register in which address of data is available. Here, operand will be in the memory and address in register pair.

The advantages and limitations of register indirect addressing are basically the same as for indirect addressing. In both cases, the address space limitation (limited range of addresses) of the address field is overcome by having that field refer to a word length location containing an address. In addition, register indirect addressing uses one less memory reference than indirect addressing.

6.   Displacement Addressing:

Displacement addressing is a very powerful mode of addressing that combines the capabilities of direct addressing and register indirect addressing.

Displacement addressing requires that the instruction have two address fields, at least one of which is explicit. The value contained in one address field (value = A) is used directly. The other address field, or an implicit reference based on opcode, refers to a register whose contents are added to A to produce the effective address.

7.   Stack Addressing:

A stack is a linear array of locations. It is sometimes referred to as a pushdown list or last-in-first-out queue. Items are appended to the top of the stack. Associated with the stack is a pointer whose value is the address of the top of the stack. Alternatively, the top two elements of the stack may be in processor registers, in which case the stack pointer references the third element of the stack. The stack pointer is maintained in a register. Thus, references to stack locations in memory are in fact register indirect addresses.

The stack mode of addressing is a form of implied addressing. The machine instructions need not include a memory reference but implicitly operate on the top of the stack.