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We have examined the types of operands and operations that may be specified by machine
instructions. Now we have to see how is the address of an operand specified, and how are the bits of
an instruction organized to define the operand addresses and operation of that instruction.
❍ Direct
❍ Indirect
❍ Register
❍ Register Indirect
❍ Displacement
❍ Stack
All computer architectures provide more than one of these addressing modes. The question arises as to how the control unit can determine which addressing mode is being used in a particular instruction. Several approaches are used. Often, different opcodes will use different addressing modes. Also, one or more bits in the instruction format can be used as a mode field. The value of the mode field determines which addressing mode is to be used. What is the interpretation of effective address. In a system without virtual memory, the effective address will be either a main memory address or a register. In a virtual memory system, the effective address is a virtual address or a register. The actual mapping to a physical address is a function of the paging mechanism and is invisible to the programmer. To explain the addressing modes, we use the following notation:
A = contents of an address field in the instruction that refers to a memoryR = contents of an address field in the instruction that refers to a register EA = actual (effective) address of the location containing the referenced operand (X) = contents of location X
Addressing Modes: The most common addressing techniques :
❍ Immediate❍ Direct
❍ Indirect
❍ Register
❍ Register Indirect
❍ Displacement
❍ Stack
All computer architectures provide more than one of these addressing modes. The question arises as to how the control unit can determine which addressing mode is being used in a particular instruction. Several approaches are used. Often, different opcodes will use different addressing modes. Also, one or more bits in the instruction format can be used as a mode field. The value of the mode field determines which addressing mode is to be used. What is the interpretation of effective address. In a system without virtual memory, the effective address will be either a main memory address or a register. In a virtual memory system, the effective address is a virtual address or a register. The actual mapping to a physical address is a function of the paging mechanism and is invisible to the programmer. To explain the addressing modes, we use the following notation:
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