The document discusses various addressing modes used in computer instructions, explaining how operands are chosen during program execution. It outlines different addressing methods such as immediate, direct, indirect, register, and displacement addressing, including their characteristics and benefits for programming versatility and instruction efficiency. Each method has its own advantages and limitations, affecting the number of memory references and the size of address fields.

1. 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
1
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:

2. Addressing Modes
Computers use addressing mode techniques
for the purpose of accommodating one or both
of the following provisions:
2
1. 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.
2. To reduce the number of bits in the addressing
field of the instruction.

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

4. Immediate Addressing
Operand is part of instruction
Operand = address field
e.g. ADD 5
4
e.g. ADD 5
Add 5 to contents of accumulator
5 is operand
No memory reference to fetch data
Fast
Limited range

5. Immediate Addressing Diagram
OperandOpcode
Instruction
5

6. 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
6
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

7. Direct Addressing Diagram
Address AOpcode
Instruction
Memory
7
Operand

8. Indirect Addressing (1)
Memory cell pointed to by address field contains
the address of (pointer to) the operand
EA = (A)
8
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

9. Indirect Addressing (2)
Large address space
2n where n = word length
May be nested, multilevel, cascaded
9
May be nested, multilevel, cascaded
e.g. EA = (((A)))
Draw the diagram yourself
Multiple memory accesses to find operand
Hence slower

10. Indirect Addressing Diagram
Address AOpcode
Instruction
Memory
10
Operand
Pointer to operand

11. Register Addressing (1)
Operand is held in register named in
address field
EA = R
11
EA = R
Limited number of registers
Very small address field needed
Shorter instructions
Faster instruction fetch

12. Register Addressing (2)
No memory access
Very fast execution
Very limited address space
Multiple registers helps performance
12
Multiple registers helps performance
Requires good assembly programming or
compiler writing
N.B. C programming
register int a;
Like Direct addressing

13. Register Addressing Diagram
Register Address ROpcode
Instruction
Registers
13
Operand

14. Register Indirect Addressing
Like indirect addressing
EA = (R)
Operand is in memory cell pointed to by
14
Operand is in memory cell pointed to by
contents of register R
Large address space (2n)
One fewer memory access than indirect
addressing

15. Register Indirect Addressing Diagram
Register Address ROpcode
Instruction
Memory
15
OperandPointer to Operand
Registers

16. Displacement Addressing
EA = A + (R)
Address field hold two values
A = base value
16
A = base value
R = register that holds displacement
or vice versa

17. Displacement Addressing Diagram
Register ROpcode
Instruction
Memory
Address A
17
OperandPointer to Operand
Registers
+

18. Relative Addressing
A version of displacement addressing
R = Program counter, PC
EA = A + (PC)
18
EA = A + (PC)
i.e. get operand from A cells from current
location pointed to by PC

19. Base-Register Addressing
A holds displacement
R holds pointer to base address
R may be explicit or implicit
19
R may be explicit or implicit
e.g. segment registers in 80x86

20. Indexed Addressing
A = base
R = displacement
EA = A + R
20
EA = A + R
Good for accessing arrays
EA = A + R
R++

21. Combinations
Postindex
EA = (A) + (R)
21
Preindex
EA = (A+(R))
(Draw the diagrams)

22. Stack Addressing
Operand is (implicitly) on top of stack
e.g.
ADD Pop top two items from stack
22
ADD Pop top two items from stack
and add