The three basic modes of addressing are −

1. Register addressing
2. Immediate addressing
3. Memory addressing

Register Addressing

In this addressing mode, a register contains the operand. Depending UPON the instruction, the register may be the first operand, the second operand or both.

For example,

MOV DX, TAX_RATE ; Register in first operand
MOV COUNT, CX ; Register in second operand
MOV EAX, EBX ; Both the operands are in registers

As processing data between registers does not involve memory, it provides fastest processing of data.

Immediate Addressing

An immediate operand has a constant value or an expression. When an instruction with two operands uses immediate addressing, the first operand may be a register or memory location, and the second operand is an immediate constant. The first operand defines the LENGTH of the data.

For example,

BYTE_VALUE DB 150 ; A byte value is defined
WORD_VALUE DW 300 ; A word value is defined
ADD BYTE_VALUE, 65 ; An immediate operand 65 is added
MOV AX, 45H; Immediate constant 45H is TRANSFERRED to AX

Direct Memory Addressing

When operands are specified in memory addressing mode, direct access to main memory, usually to the data segment, is required. This way of addressing results in slower processing of data. To locate the exact location of data in memory, we need the segment start address, which is typically found in the DS register and an offset value. This offset value is also called effective address.

In direct addressing mode, the offset value is specified directly as part of the instruction, usually indicated by the variable NAME. The assembler calculates the offset value and maintains a symbol table, which stores the offset values of all the variables used in the PROGRAM.

In direct memory addressing, one of the operands refers to a memory location and the other operand references a register.

The three basic modes of addressing are −

Register Addressing

In this addressing mode, a register contains the operand. Depending upon the instruction, the register may be the first operand, the second operand or both.

For example,

MOV DX, TAX_RATE ; Register in first operand
MOV COUNT, CX ; Register in second operand
MOV EAX, EBX ; Both the operands are in registers

As processing data between registers does not involve memory, it provides fastest processing of data.

Immediate Addressing

An immediate operand has a constant value or an expression. When an instruction with two operands uses immediate addressing, the first operand may be a register or memory location, and the second operand is an immediate constant. The first operand defines the length of the data.

For example,

BYTE_VALUE DB 150 ; A byte value is defined
WORD_VALUE DW 300 ; A word value is defined
ADD BYTE_VALUE, 65 ; An immediate operand 65 is added
MOV AX, 45H; Immediate constant 45H is transferred to AX

Direct Memory Addressing

When operands are specified in memory addressing mode, direct access to main memory, usually to the data segment, is required. This way of addressing results in slower processing of data. To locate the exact location of data in memory, we need the segment start address, which is typically found in the DS register and an offset value. This offset value is also called effective address.

In direct addressing mode, the offset value is specified directly as part of the instruction, usually indicated by the variable name. The assembler calculates the offset value and maintains a symbol table, which stores the offset values of all the variables used in the program.

In direct memory addressing, one of the operands refers to a memory location and the other operand references a register.