Addressing Modes
Addressing modes are the techniques used by the CPU to identify
where the data needed for an operation is stored. They provide rules
for interpreting or modifying the address field in an instruction
before accessing the operand.
Types of Addressing Modes in Computer Architecture
Addressing Modes used by 8086 microprocessor are discussed below:
Implied mode
In implied addressing the operand is specified in the instruction itself. In this
mode the data is 8 bits or 16 bits long and data is the part of instruction. Zero
address instruction are designed with implied addressing mode.
Example: CLC
(used to reset Carry flag to 0)
Immediate addressing mode (symbol #)
In this mode data is present in address field of instruction .Designed like one
address instruction format. Note: Limitation in the immediate mode is that
the range of constants are restricted by size of address field.
Example: MOV AL, 35H
(move the data 35H into AL register)
Register mode
In register addressing the operand is placed in one of 8 bit or 16 bit general
purpose registers. The data is in the register that is specified by the
instruction. Here one register reference is required to access the data.
Example: MOV AX,CX
(move the contents of CX register to AX register)
Register Indirect mode
In this addressing the operand’s offset is placed in any one of the registers
BX,BP,SI,DI as specified in the instruction. The effective address of the data
is in the base register or an index register that is specified by the instruction.
Here two register reference is required to access the data.
The
8086 CPUs let you access memory indirectly through a register using the
register indirect addressing modes.
MOV AX, [BX]
(move the contents of memory location addressed by the register BX to the
register AX)
Auto Indexed (increment mode)
Effective address of the operand is the contents of a register specified in the
instruction. After accessing the operand, the contents of this register are
automatically incremented to point to the next consecutive memory location.
(R1)+. Here one register reference, one memory reference and one ALU
operation is required to access the data. Example:
Add R1, (R2)+ // OR
R1 = R1 +M[R2]
R2 = R2 + d
Useful for stepping through arrays in a loop. R2 – start of array d – size of an
element
Auto indexed ( decrement mode)
Effective address of the operand is the contents of a register specified in the
instruction. Before accessing the operand, the contents of this register are
automatically decremented to point to the previous consecutive memory
location. –(R1)Here one register reference, one memory reference and one
ALU operation is required to access the data. Example:
Add R1,-(R2) //OR
R2 = R2-d
R1 = R1 + M[R2]
Auto decrement mode is same as auto increment mode. Both can also be
used to implement a stack as push and pop . Auto increment and Auto
decrement modes are useful for implementing “Last-In-First-Out” data
structures.
Direct addressing/ Absolute addressing Mode (symbol [ ])
The operand’s offset is given in the instruction as an 8 bit or 16 bit
displacement element. In this addressing mode the 16 bit effective address
of the data is the part of the instruction. Here only one memory reference
operation is required to access the data.
Example: ADD AL,[0301] //add the contents of offset address 0301 to AL
Indirect addressing Mode (symbol @ or () )
In this mode address field of instruction contains the address of effective
address. Here two references are required. 1st reference to get effective
address. 2nd reference to access the data. Based on the availability of
Effective address, Indirect mode is of two kind:
Register Indirect: In this mode effective address is in the register, and
corresponding register name will be maintained in the address field of an
instruction. Here one register reference, one memory reference is
required to access the data.
Example : MOV A, @R0
Memory Indirect: In this mode effective address is in the memory, and
corresponding memory address will be maintained in the address field of
an instruction. Here two memory reference is required to access the data.
Example : MOV AX, [[5000H]]
Indexed addressing mode
The operand’s offset is the sum of the content of an index register SI or DI
and an 8 bit or 16 bit displacement.
Example: MOV AX, [SI +05]
Based Indexed Addressing
The operand’s offset is sum of the content of a base register BX and an
index register SI or DI.
Example: ADD AX, [BX+SI]
Based on Transfer of control, addressing modes are:
PC relative addressing mode
PC relative addressing mode is used to implement intra segment transfer of
control, In this mode effective address is obtained by adding displacement to
PC.
EA= PC + Address field value
PC= PC + Relative value.
Base register addressing mode
Base register addressing mode is used to implement inter segment transfer
of control. In this mode effective address is obtained by adding base register
value to address field value.
EA= Base register + Address field value.
PC= Base register + Relative value
Note :
PC relative and based register both addressing modes are suitable for
program relocation at runtime.
Based register addressing mode is best suitable to write position
independent codes.
Advantages of Addressing Modes
Enable advanced programming techniques like pointers and counters for
loops.
Simplify memory access for arrays and complex data structures.
Allow program relocation during runtime.
Reduce the size of the instruction field, making the program more
efficient.
