Addressing Modes
The way the operands are chosen during program execution is dependent on the addressing mode of the instructions. The addressing mode specifies a rule for interpreting or modifying the address field of the instruction before the operand is actually referenced. Computers use addressing mode techniques for the purpose of accommodating one or both of the following provisions:

1

Addressing Modes
Computers use addressing mode techniques for the purpose of accommodating one or both of the following provisions:
1.

2.

to give programming versatility to the user by providing such facilities as pointers to memory, counters for loop control, indexing of data and program relocation. To reduce the number of bits in the addressing field of the instruction.
2

Addressing Modes
Immediate Direct Indirect Register Register Indirect Displacement (Indexed) Stack
3

Immediate Addressing
Operand is part of instruction Operand = address field e.g. ADD 5
Add 5 to contents of accumulator 5 is operand

No memory reference to fetch data Fast Limited range
4

Immediate Addressing Diagram
Instruction Opcode Operand

5

Direct Addressing
Address field contains address of operand Effective address (EA) = address field (A) e.g. ADD A
Add contents of cell A to accumulator Look in memory at address A for operand Single memory reference to access data

No additional calculations to work out effective address Limited address space
6

Direct Addressing Diagram
Instruction Opcode Address A Memory

Operand

7

Indirect Addressing (1)
Memory cell pointed to by address field contains the address of (pointer to) the operand EA = (A)
Look in A, find address (A) and look there for operand e.g. ADD (A) Add contents of cell pointed to by contents of A to accumulator

8

Indirect Addressing (2)
Large address space 2n where n = word