Khasman Ji

1. Types of Addressing Modes
Each instruction of a computer specifies an operation on certain data. The are various ways of
specifying address of the data to be operated on. These different ways of specifying data are
called the addressing modes. The most common addressing modes are:
Immediate addressing mode
Direct addressing mode
Indirect addressing mode
Register addressing mode
Register indirect addressing mode
Displacement addressing mode
Stack addressing mode
To specify the addressing mode of an instruction several methods are used. Most often used are :
a) Different operands will use different addressing modes.
b) One or more bits in the instruction format can be used as mode field. The value of the mode
field determines which addressing mode is to be used.
The effective address will be either main memory address of a register.
Immediate Addressing:
This is the simplest form of addressing. Here, the operand is given in the instruction itself. This
mode is used to define a constant or set initial values of variables. The advantage of this mode is
that no memory reference other than instruction fetch is required to obtain operand. The
disadvantage is that the size of the number is limited to the size of the address field, which most
instruction sets is small compared to word length.
INSTRUCTION
OPERAND
Direct Addressing:
In direct addressing mode, effective address of the operand is given in the address field of the
instruction. It requires one memory reference to read the operand from the given location and
provides only a limited address space. Length of the address field is usually less than the word
length.
Ex : Move P, Ro, Add Q, Ro P and Q are the address of operand.
Indirect Addressing:

2. Indirect addressing mode, the address field of the instruction refers to the address of a word in
memory, which in turn contains the full length address of the operand. The advantage of this
mode is that for the word length of N, an address space of 2N can be addressed. He disadvantage
is that instruction execution requires two memory reference to fetch the operand Multilevel or
cascaded indirect addressing can also be used.
Register Addressing:
Register addressing mode is similar to direct addressing. The only difference is that the address
field of the instruction refers to a register rather than a memory location 3 or 4 bits are used as
address field to reference 8 to 16 generate purpose registers. The advantages of register
addressing are Small address field is needed in the instruction.
Register Indirect Addressing:
This mode is similar to indirect addressing. The address field of the instruction refers to a
register. The register contains the effective address of the operand. This mode uses one memory
reference to obtain the operand. The address space is limited to the width of the registers
available to store the effective address.
Displacement Addressing:
In displacement addressing mode there are 3 types of addressing mode. They are :
1) Relative addressing
2) Base register addressing
3) Indexing addressing.
This is a combination of direct addressing and register indirect addressing. The value contained
in one address field. A is used directly and the other address refers to a register whose contents
are added to A to produce the effective address.
Stack Addressing:
Stack is a linear array of locations referred to as last-in first out queue. The stack is a reserved
block of location, appended or deleted only at the top of the stack. Stack pointer is a register
which stores the address of top of stack location. This mode of addressing is also known as
implicit addressing.

3. 1. Immediate Addressing Mode
o Immediate addressing mode means that the value for a given instruction in
assembly programming is directly specified. This means the value is constant
and written immediately and immutably into the instruction.
1. Common Format of Immediate Addressing
o The following is an example of the format typically found with immediate
addressing mode: Operator Target, Value
In the example format, the value of the number Value is operated on the value
stored in Target. Value is a constant value, which does not change no matter
what occurs in the set of assembly instructions.
o
Direct Addressing Mode
o Direct addressing mode means that the value for a given instruction in assembly
programming is pointed to by a given value. This means the value is variable,
based on what is stored in memory at a given address.
Common Format of Direct Addressing
o The following is an example of the format typically found with direct addressing
mode: Operator Target, (Address)
In the example format, the value at the memory location of Address is a variable
value in memory, which can be written to and read from during the course of a
program. The location in memory is directly specified with a number, which will
never change over the course of instruction execution.
Immediate addressing supplies the actual value and is normally prefixed with a #.
LD Acc, #5 ;Load the actual value 5 into the accumulator
Ax becomes 5.
LD Acc, 5 ;Load the value in memory location 5 into the accumulator
Ax becomes 3.

4. Indirect addressing uses the value as a memory location that points to the memory
location that holds the value. In this case the number is usually enclosed with square
brackets.
LD Acc, [5] ;Load the value stored in the memory location pointed to by the operand into the
accumulator
Memory location 5 is accessed which contains 3. Memory location 3 is accessed which is 17.
Ax becomes 17