Addressing modes are the way to identify operands in machine language instructions. It defines how to calculate effective address of operand with the information held in registers and/or constants contained by a machine instruction.
Some commonly used addressing modes are as follows:
Implied Mode: In this addressing mode operands are specified implicitly in the definition of instruction. E.g. complement Accumulator. In this instruction operand in the accumulator is implicitly defined in the instruction.
Example: CLA
Immediate Mode: In this addressing mode operand is specified in the instruction itself. Hence instruction contains operand instead of Address field. These addressing modes are used for the initialization of registers to a constant value.
Example: MOV R2, 123
Register Mode: In this type of addressing mode operands are used from the registers of CPU. An n bit field can specify 2n Registers.
Example: MOV R3, R2; R3 = R2
Register Indirect Mode: In this type of addressing mode instruction specify the register from CPU which contains the address of operand. Hence CPU register contains the address of operand rather than operand itself.
Example: LD R3,[R2] ; R3=contents of memory, address in R2
Direct Addressing Mode: In this type of addressing mode defined operand is in the memory and instruction carries the address of the memory location that contains operand.
Example: MOV A, 30H
Indirect Addressing Mode: In this type of addressing mode address field of instruction defines the address of memory location that contains the effective address of the operand.
Example: MOV A,@R1
Auto increment or Auto Decrement Addressing Mode: In this type of addressing mode operand are in CPU registers. Instruction fetches one operand and in next fetch register value is either incremented or decremented.
Relative Addressing Mode: This type of addressing mode adds Program counter contents to address field of instruction to get effective address of the operand.
Indexed Addressing Mode: In this type of addressing mode contents of index register are added to address field of instruction to get the effective address of the operand.
Base Register Addressing Mode: In this type of addressing mode contents of base register are added to obtain the effective address of the operand.
Some commonly used addressing modes are as follows:
Implied Mode: In this addressing mode operands are specified implicitly in the definition of instruction. E.g. complement Accumulator. In this instruction operand in the accumulator is implicitly defined in the instruction.
Example: CLA
Immediate Mode: In this addressing mode operand is specified in the instruction itself. Hence instruction contains operand instead of Address field. These addressing modes are used for the initialization of registers to a constant value.
Example: MOV R2, 123
Register Mode: In this type of addressing mode operands are used from the registers of CPU. An n bit field can specify 2n Registers.
Example: MOV R3, R2; R3 = R2
Register Indirect Mode: In this type of addressing mode instruction specify the register from CPU which contains the address of operand. Hence CPU register contains the address of operand rather than operand itself.
Example: LD R3,[R2] ; R3=contents of memory, address in R2
Direct Addressing Mode: In this type of addressing mode defined operand is in the memory and instruction carries the address of the memory location that contains operand.
Example: MOV A, 30H
Indirect Addressing Mode: In this type of addressing mode address field of instruction defines the address of memory location that contains the effective address of the operand.
Example: MOV A,@R1
Auto increment or Auto Decrement Addressing Mode: In this type of addressing mode operand are in CPU registers. Instruction fetches one operand and in next fetch register value is either incremented or decremented.
Relative Addressing Mode: This type of addressing mode adds Program counter contents to address field of instruction to get effective address of the operand.
Indexed Addressing Mode: In this type of addressing mode contents of index register are added to address field of instruction to get the effective address of the operand.
Base Register Addressing Mode: In this type of addressing mode contents of base register are added to obtain the effective address of the operand.
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