1. Computer Architecture and the Fetch-Execute Cycle
Memory Addressing Techniques

2. Learning Objectives
Explain the concepts of direct, indirect, indexed, relative addressing and immediate addressing of memory when referring to low-level languages.

3. Memory Addressing Techniques
Direct Addressing
Indirect Addressing
Indexed Addressing
Relative Addressing
Immediate Addressing

4. Direct Addressing

5. Direct Addressing
Means that the value in the address part of a machine code instruction is the address of the data.
Simple to use but does not allow access to all memory addresses as there are memory addresses larger than can be held in the address part of an instruction.
e.g. A 32-bit memory location may use 12 bits for the instruction code and 20 bits for the address of the data.
This would allow for 2^12 (= 4096) instruction codes and 2^20 (= ) memory addresses.
However, a computer using 32-bit memory locations will have 2^32 (= ) memory addresses.
So direct addressing does not allow reference to all addresses e.g. for a computer using 32-bit memory locations, addresses from 2^20 – 2^32 cannot be referred to.

6. Assembly Language Instructions
Explanation
Op Code
Operand
LDD
<address>
Load using direct addressing

7. Assembly Language Instructions
Main memory
LDD 117
115
116
117
118
119
120
121
122 ~
200
Accumulator

8. Assembly Language Instructions
Main memory
LDD 117
115
116
117
118
119
120
121
122 ~
200
Accumulator

9. Assembly Language Instructions
Main memory
LDD 117
115
116
117
118
119
120
121
122 ~
200
Accumulator

10. Assembly Language Instructions
Main memory
LDD 117
115
116
117
118
119
120
121
122 ~
200
Accumulator

11. 2. Indirect Addressing

12. Indirect Addressing
Means that the value in the address part of a machine code instruction is the address of the address of the data.

13. Why use indirect addressing?
Allows more memory to be accessed than direct addressing as the full size of register is used for an address.
If this value points to a location which holds nothing but an address then 2^32 locations in memory can be addressed.

14. Assembly Language Instructions
Explanation
Op Code
Operand
LDD
<address>
Load using direct addressing
LDI
Load using indirect addressing

15. Assembly Language Instructions
Main memory
LDI 117
115
116
117
118
119
120
121
122 ~
200
Accumulator

16. Assembly Language Instructions
Main memory
LDI 117
115
116
117
118
119
120
121
122 ~
200
Accumulator

17. Assembly Language Instructions
Main memory
LDI 117
115
116
117
= 200
118
119
120
121
122 ~
200
Accumulator

18. Assembly Language Instructions
Main memory
LDI 117
115
116
117
= 200
118
119
120
121
122 ~
200
Accumulator

19. Assembly Language Instructions
Main memory
LDI 117
115
116
117
= 200
118
119
120
121
122 ~
200
Accumulator

20. Assembly Language Instructions
Main memory
LDI 117
115
116
117
= 200
118
119
120
121
122 ~
200
Accumulator

21. 3. Indexed Addressing

22. Indexed Addressing Index Register (IR)
A special register which holds an integer value, which is added to the base address in the instruction currently in the CIR, to give a new effective address of the data.

23. Indexed Addressing can be used once during one Fetch-Decode-Execute-Reset Cycle:
To reach a particular element in an array or field in a record.
IR = Offset of the required element or field from the start of the array or record.
Instruction address = Address of the start of the array or record.
Address
Data:
100
…..
101
102
103
104
105

24. Assembly Language Instructions
Explanation
Op Code
Operand
LDD
<address>
Load using direct addressing
LDI
Load using indirect addressing
LDX
Load using indexed addressing

25. Assembly Language Instructions
Main memory
LDX 117
115
116
117
118
119
120
121
122 ~
200
Accumulator
Index Register

26. Assembly Language Instructions
Main memory
LDX 117
115
116
117
118
119
120
121
122 ~
200
Accumulator
Index Register
= 3

27. Assembly Language Instructions
Main memory
LDX = 120
115
116
117
118
119
120
121
122 ~
200
Accumulator
Index Register
= 3

28. Assembly Language Instructions
Main memory
LDX = 120
115
116
117
118
119
120
121
122 ~
200
Accumulator
Index Register
= 3

29. Assembly Language Instructions
Main memory
LDX = 120
115
116
117
118
119
120
121
122 ~
200
Accumulator
Index Register
= 3

30. Assembly Language Instructions
Main memory
LDX = 120
115
116
117
118
119
120
121
122 ~
200
Accumulator
Index Register
= 3

31. Indexed Addressing can be used repeatedly
To allow a number of fields in contiguous records or contiguous elements in an array to be accessed by incrementing the Index Register (IR) between each successive access instruction.

32. Indexed Addressing e.g. to add 5 memory locations Instr. ADD 100 IR: 4
- 1
Instr. ADD 100
IR: 4
Address
Data:
100
…..
101
102
103
104

33. Indexed Addressing Instr. ADD 100 IR: 3 Address Data: 100 ….. 101 102
103
104

34. Indexed Addressing Instr. ADD 100 IR: 2 Address Data: 100 ….. 101 102
103
104

35. Indexed Addressing Instr. ADD 100 IR: 1 Address Data: 100 ….. 101 102
103
104

36. Indexed Addressing Instr. ADD 100 IR: Address Data: 100 ….. 101 102
Address
Data:
100
…..
101
102
103
104
STOP

37. Why use Indexed Addressing?
Why not do something similar to the high-level instruction?
NewAddress = OriginalAddress + Offset
Add NewAddress
Doing it this way would involve:
Reserving two memory addresses:
Address & Offset.
8 Fetch-Decode-Execute-Reset cycles:
Set the OriginalAddress, set the Offset, load the OriginalAddress, add Offset, store the NewAddress, access the contents of this “indirect” NewAddress, load offset, check if offset is < or > than a specified condition (number of elements required) and either loop back or stop.

38. Why use Indexed Addressing?
Indexed addressing avoids this:
Note that it is usual to decrement instead of incrementing as most computers have a command to test if a register is below 0, which provides a convenient method of stopping the loop.
Set the IR = Number of elements required - 1
Access (Address of 1st element + IR)
Decrement IR by 1
Test if IR < 0 otherwise loop back to step 2.
This only involves only 4 Fetch-Decode-Execute-Reset cycles and one register (IR).
Note the IR is in the processor, not memory so no memory locations are used in indexed addressing.

39. Why use Indexed Addressing?
This means that the original access instruction as written in the code does not need to be modified .
Also avoids 4 additional cycles and using 2 memory locations.
Note the IR is in the processor, not memory so no memory locations are used in indexed addressing.

40. Why use Indexed Addressing?
Note for fields in a record then:
Set the IR = (No of records required – 1)*offset
Address in access instruction = Address of 1st record
Decrement IR by offset.

41. For more research:

42. 4. Relative Addressing

43. Relative Addressing
Address in the instruction is the displacement from the current instruction’s address + 1 (due to the PC being incremented) already in the PC.
Used mainly with jump instructions.
The address in the instruction (via the MAR) is added to the value in the PC.
e.g.
The current instruction is in address 3000 and is Jump *45*, using relative addressing this means that the line wants to jump to an instruction 45 lines ahead of the next instruction (due to the PC being automatically incremented by 1 during any Fetch-Decode-Execute-Reset Cycle this means 46 lines ahead of the current instruction).
The PC will have been incremented to 3001 (ready for the next instruction) so the PC is changed to = 3046.
The next cycle retrieves the next instruction from there.
The program knows where the line is relative to the current instruction but not the true address – see Memory Management Presentation or next 2 slides.
Therefore allows for relocatable code (code that be moved anywhere without jump instructions etc… being affected).

44. Index Table + Physical Address Displacement e.g. 3001+45 = 3046
Physical Page Number
Logical Page Number
Base Address 8 5
3000
Displacement
e.g. 45 +
Physical Address
e.g = 3046

45. Segment Index Table Physical Address + e.g. 3501+131=3632 Displacement
Base Address
Segment Size
Segment
3500
1500 3

46. Relative Addressing
Can also be used to access data in addresses relative to the current instruction.
The only difference is that the new “effective” address would be loaded into the MAR (contents of PC + operand of instruction in CIR) and used from there, instead of the going on to change the PC.

47. Relative Vs Indexed Addressing
Similar as they both involve changing an address by addition but:
Indexed Addressing:
Changes the address in the instruction in the CIR.
Address limited to the size of the address field in the CIR, as in direct addressing.
Effective address = address in the CIR + contents of IR
Relative Addressing:
Changes the address in the MAR and for Jump instructions the PC as well, not the address in the instruction in the CIR.
Address can be full value of the register as modified effective address is on its own in the PC or MAR.
Effective address = current instruction’s address + 1
+ value in instruction

48. For more research

49. 5. Immediate Addressing

50. Immediate Addressing
Immediate addressing is so-named because the value to be stored in memory immediately follows the operation code in memory.
e.g. “Load 20” where 20 refers to a value and is loaded directly into the accumulator.
You could argue that there is no address being used at all, but there you go!
Immediate addressing is very fast since the value to be loaded is included in the instruction.
However, since the value to be loaded is fixed at compile-time it is not very flexible.

51. Immediate Addressing Advantage: Disadvantage:
No memory reference other than instruction fetch is required to obtain the data to be used.
Disadvantage:
The size of the number is limited to the size of the address field, which most instruction sets is small compared to word length.
This is similar to the problem “direct addressing” has.

52. For more research:

53. Plenary
State the 5 methods of memory addressing and explain why they are used.