Addressing Modes

The instruction format for a general purpose computer was described in  Instruction Types  states that the OP-Code field specifies the operation to be performed, data is required to be supplied to ALU for this operation.

Addressing Mode

The way the data or the operands are chosen during the execution of a program depends on how the data is to be supplied since data may come from input devices, from memory or from registers etc.

So, the addressing mode specifies the rule or the methods for interpreting the address fields of the instruction before the operand is actually referred.

Purpose of the addressing modes:

• To give programming versatilities to the user by providing different ways of addressing or referencing to data such as naming variables, pointers (*) and address (&) etc , in assembly they are referred are contents in registers, in memory at location given direct or indirect etc.
• To reduce the number of bits in the address field of the instruction.
• To give programmers flexibility in writing programs.

Type of addressing modes

1. Implied addressing Mode
2. Immediate addressing Mode
3. Register Addressing Mode
4. Register Indirect addressing Mode
5. Auto increment/Auto decrement addressing mode
6. Direct Addressing Mode
7. Indirect addressing Mode
8. Relative Addressing Mode
9. Indexed Addressing Mode
10. Base Register Addressing Mode

These are explain below:

1. Implied addressing Mode:

Operands are specified implicitly in the instruction. These type of instruction are zero address instructions. CLC, CLA are example. Programming example of zero address instruction is:

PUSH A,

PUSH B

ADD                  :  TOS <–(A+B) ; this instruction does not use address field

2. Immediate addressing Mode :

Operand is specified in the instruction itself, the operand is specified in the address part of the field.

3. Register Addressing Mode :

Address part of the instruction specifies a processor register rather than the address of operands. In these type of instructions the operand is specified in the registers.

4. Register Indirect addressing Mode :

Here the content of the register specified in the instruction give the address of the operand residing in memory.

ADD A, M[R]

5. Auto increment/Auto decrement addressing mode

In this type of instructions, the register is incremented or decremented after or before its value is used to access the memory i.e. after or before execution of the instruction.

6. Direct Addressing Mode:

The address of the operand in memory is given in the address part instruction.

7. Indirect addressing Mode:

The address part instruction gives the information about another address where the operand is to be found in the memory. Some instructions require the addressing part of the instruction to be added to the content of a register to find the effective address of the operand.

i.e.  Effective address = address part of the instruction + content of specified register

8. Relative Addressing Mode :

Here the content of the PC is added to the address part of the instruction to get to the operand.

i.e.  Effective address = address part of the instruction + content of the PC+1

1 is added because when an instruction at address is fetched, its content increments by 1.

Example is PC=500 and the address part of instruction which will be fetched contains 50 then the effective address of the operand will be:

i.e.  Effective address = 50 + content of the 501 =             551

 

9. Indexed Addressing Mode

In this case content of an index register is added to the address part of instruction to get the effective address of the operand

i.e.  Effective address = address part of the instruction + content of the index register

10. Base Register Addressing Mode

This type of mode is similar to the above (indexed addressing mode) except that the register now is a base register.

i.e.  Effective address = address part of the instruction + content of the base  register

Example:

An instruction Load AC, 500 is stored in memory at address 200 and 201, opcode at address 200 and address part at 201 address.

Content of  PC = 200                       R1= 400, Index Register XR =100

AC receives the operand after execution of instruction

Show the effective address of the operand and the content of the AC after the instruction is executed using different addressing mode.

Solution: We can use the above data to draw the following figure

Table below lists the effective address and the content of the accumulator when the instruction is executed using different addressing modes.

Addressing Mode	Effective Address	Content of Accumulator (AC)
Implied	–	–
Immediate Addressing Mode	201	500
Direct Memory Addressing	500	800
Indirect Addressing	800	300
Relative Addressing	PC(202) + addr (500) =702 Since PC incremented twice once for Load PC, then after reading address part of instruction which is at address 201	325
Indexed Addressing	600	900
Register Addressing	–	400
Register Indirect	400	700
Auto Increment	400	700
Auto Decrement	399	450

** Auto Increment Mode is same as Register Indirect mode except that R1 is incremented to 401 after the execution of the instruction

** Auto Increment Mode is same as Register Indirect mode except that R1 is decremented to 399  prior to the execution of the instruction .

Exercise:

1. An instruction is stored at location 300 with its address field at location 301. The address field has the value 400. A processor register R1 contains the number 200. Evaluate the effective address if the addressing mode of instruction is:

i. Direct

ii. Immediate

iii. Relative

iv. Register Indirect

v. Indexed with R1 as the index register

2. A two-word instruction is stored in memory at address designated by symbol W. The address field of the instruction (stored at W+1) is designated by symbol Y. The operand used during the execution of the instruction is stored at an address symbolized by Z. An index register contains the value X. State how Z is calculated from the other addresses if the addressing mode of the instruction is:

direct

indirect

relative

indexed