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This document discusses different types of addressing modes used in instruction set architectures, including immediate addressing where the operand is present in the instruction, register or direct addressing where the address field directly contains the operand's address, indirect addressing where the address field refers to a word containing the operand's address, and displacement addressing which combines direct and indirect addressing by using a base register value and displacement value to calculate the operand's address. Displacement addressing provides more flexibility than direct addressing alone by allowing access to a larger address space.
Arithmetic and logic unit Continue...
- 1. Unit 2 Continue…. Instruction set architecture of a CPU registers, instruction execution cycle M Indraja 1
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- 4. Instruction Execution and Format: M Indraja 4
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- 11. Immediate Addressing: The simplest form of addressing is immediate addressing, in which the operand is actually present in the instruction. Operand is part of instruction Operand = address field M Indraja 11
- 12. Register (Direct) Addressing: A very simple form of addressing is direct addressing, in which: Address field contains address of operand Effective address (EA) = address field (A) e.g. ADD A ; Add contents of cell A to accumulator The technique was common in earlier generations of computers but is not common on contemporary architectures. It requires only one memory reference and no special calculation. The obvious limitation is that it provides only a limited address space. M Indraja 12
- 13. Register (Direct) Addressing Conti….. M Indraja 13
- 14. Indirect Addressing: 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. M Indraja 14
- 15. Displacement Addressing A very powerful mode of addressing combines the capabilities of direct addressing and register indirect addressing. It is known by a variety of names depending on the context of its use but the basic mechanism is the same. We will refer to this as displacement addressing, address field hold two values: •A = base value •R = register that holds displacement M Indraja 15
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