ADDRESSING MODES OF 8086
Addressing mode indicates a way of locating data or operands. Depending
upon the data types used in the instruction and the memory addressing modes,
any instruction may belong to one or more addressing modes, or some instruction
may not belong to any of the addressing modes. Thus the addressing modes
describe the types of operands and the way they are accessed for executing an
instruction. Here, we will present the addressing modes of the instructions
depending upon their types.
According to the flow of
instruction execution, the instructions may be categorized as
(i)
Sequential control flow
instructions and
(ii)
Control transfer
instructions.
Sequential control flow instructions are the instructions, which after execution, transfer control to the
next instruction appearing immediately after it (in the sequence) in the
program. For example, the arithmetic, logical, data transfer and processor
control instructions are sequential control flow instructions.
- Immediate: In this type of addressing, immediate data is a
part of instruction, and appears in the form of successive byte or bytes.
Example: MOV AX, 0005H
In the above example, 0005H is the immediate data. The immediate data may
be 8-bit or 16-bit in size.
- Direct: In the direct addressing mode, a 16-bit memory
address (offset) is directly specified in the instruction as a part of it.
Example: MOV AX,
[5000H]
Here, data resides in a memory location in the data segment, whose Physical
address may be computed using 5000H as the offset address and content of DS as
segment address. The Physical address, here, is 10H*DS+5000H.
- Register: In register addressing mode, the data is stored
in a register and it is referred using the particular register. All the
registers, except IP, may be used in this mode.
Example: MOV BX, AX.
- Register
Indirect:
Sometimes,
the address of the memory location, which contains data or operand, is
determined in an indirect way, using the offset registers. This mode of
addressing is known as register indirect mode.
In this addressing mode, the offset address of data is in either BX or SI
or DI registers. The default segment is either DS or ES. The data is supposed
to be available at the address pointed to by the content of any of the above
registers in the default data segment.
Example: MOV AX, [BX]
Here, data is present in a memory location in DS whose offset address is
in BX. The Physical address of the data is given as 10H*DS+ [BX].
- Indexed: In this addressing mode, offset of the operand
is stored in one of the index registers. DS and ES are the default
segments for index registers SI and DI respectively. This mode is a
special case of the above discussed register indirect addressing mode.
Example: MOV AX, [SI]
Here, data is available at an offset address stored in SI in DS. The Physical
address, in this case, is computed as 10H*DS+ [SI].
- Register
Relative:
In this
addressing mode, the data is available at an Physical address formed by
adding an 8-bit or 16-bit displacement with the content of any one of the
registers BX, BP, SI and DI in the default (either DS or ES) segment. The
example given before explains this mode.
Example: MOV Ax, 50H [BX]
Here, Physical address is given as 10H*DS+50H+ [BX].
- Based
Indexed:
The Physical
address of data is formed, in this addressing mode, by adding content of a
base register (any one of BX or BP) to the content of an index register
(any one of SI or DI). The default segment register may be ES or DS.
Example: MOV AX, [BX]
[SI]
Here, BX is the base register and SI is the index register. The Physical
address is computed as 10H*DS+ [BX] + [SI].
- Relative
Based Indexed:
The Physical
address is formed by adding an 8-bit or 16-bit displacement with the sum
of contents of any one of the bases registers (BX or BP) and any one of
the index registers, in a default segment.
Example: MOV AX, 50H
[BX] [SI]
Here, 50H is an immediate displacement, BX is a base register and SI is
an index register. The Physical address of data is computed as 160H*DS+ [BX] +
[SI] + 50H.
The addressing modes depend upon whether the
destination location is within the same segment or a different one. It also
depends upon the method of passing the destination address to the processor.
Basically,
there are two addressing modes for the control transfer instructions, viz. inter-segment
and intra-segment addressing modes.
If the
location to which the control is to be transferred lies in a different segment
other than the current one, the mode is called inter-segment mode. If
the destination location lies in the same segment, the mode is called
intra-segment.
Inter-segment
Direct
Inter-segment
Indirect
Modes for
control
Transfer instructions
Intra-segment
Intra-segment Direct
Intra-segment
Indirect
Addressing Modes for
Control Transfer Instruction
In this mode, the address to which the control is to be transferred lies
in the same segment in which the control transfer instruction lies and appears
directly in the instruction as an immediate displacement value. In this
addressing mode, the displacement is computed relative to the content of the
instruction pointer IP.
The Physical
address to which the control will be transferred is given by the sum of 8 or 16
bit displacement and current content of IP. In case of jump instruction, if the
signed displacement (d) is of 8 bits (i.e. –128<d<+128), we term it as
short jump and if it is of
16 its
(i.e. –32768<+32768), it is termed as long jump.
In this mode, the
displacement to which the control is to be transferred, is in the same segment
in which the control transfer instruction lies, but it is passed to the
instruction indirectly. Here, the branch address is found as the content of a
register or a memory location. This addressing mode may be used in
unconditional branch instructions.
In this mode,
the address to which the control is to be transferred is in a different
segment. This addressing mode provides a means of branching from one code
segment to another code segment. Here, the CS and IP of the destination address
are specified directly in the instruction.
In this mode,
the address to which the control is to be transferred lies in a different
segment and it is passed to the instruction indirectly, i.e. contents of a
memory block containing four bytes, i.e. IP (LSB), IP (MSB), CS (LSB) and CS
(MSB) sequentially. The starting address of the memory block may be referred
using any of the addressing modes, except immediate mode.
Courtesy : internet sources
