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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.
•
Pentium supports three fundamental addressing modes:
*
Register mode
*
Immediate mode
*
Memory mode
•
Specification of operands located in memory can be
done in a variety of ways
*
Mainly to support high-level language constructs and
data structures
Pentium
Addressing Modes (32-bit Addresses)
1. Immediate Addressing Modes
2. Register Addressing Modes
3. Memory Addressing Modes
a. Direct(Displacement) Addressing Modes
b. Indirect Addressing Modes
· Register Indirect[ Base]
· Based[Base+Displacement]
· Indexed[(Index*Scale)+Displacement]
· Based Indexed
1.
Based-indexed
with no scale factor [Base+Index+Disp]
2.
Based-indexed
with scale factor
3.
[Base+(Index*scale)+Disp]
Differences
between 16- and 32-bit Modes
|
|
16-bit addressing
|
32-bit addressing
|
|
Base register
|
BX, BP
|
EAX, EBX, ECX,
EDX, ESI, EDI,
EBP, ESP
|
|
Index register
|
SI, DI
|
EAX, EBX, ECX,EDX, ESI,
EDI,
EBP
|
|
Scale factor
|
None
|
1, 2, 4, 8
|
|
Displacement
|
0, 8, 16 bits
|
0, 8, 32 bits
|
Immediate Mode In this type
of addressing, immediate data is a part of instruction, and appears in the form
of successive byte or bytes.
§
mov eax,567
§
mov ah, 09h
§
mov dx, offset Prompt
Register Addressing mode: 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.
Memory
Addressing mode
·
Direct Addressing mode
·
Indirect Addressing mode
Direct Addressing mode: 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 effective address may be computed
using 5000H as the offset address and content of DS as segment address. The
effective address, here, is 10H*DS+5000H.
Example: add eax, [value]
Indirect
Addressing mode
1.Register
Indirect: often used for addressing data
arrays inside programming loops,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 effective address of
the data is given as 10H*DS+ [BX].
Effective
address of operand contained in a register.
For
32-bit addressing, all 32-bit registers can be used.
For
16-bit addressing, the offset value can be in one of the three registers: BX, SI, or DI:
Example:add
ax, [bx] ; Register indirect addressing
- Square brackets [ BX ] indicate that BX is holding a memory offset.
- Operand [ BX ] serves as a pointer to data in memory.
- Register indirect can be used to implement arrays.
2.Indexed: In this addressing mode, offset of the operand is
stored in one of the index registers.
constant base + register.
- Fixed Base (address) + Variable Register Offset (operand field contains a constant base)
- Effective address is obtained by adding value of operand field to contents of register.
This
is known as array type addressing, also called displacement addressing.
mov eax, [ ebx + 5 ]
mov
eax, [ ebx + esi + 5 ]
There
are restrictions on the combinations of registers allowed within the brackets:
you can have ESI or EDI,
but not both, and you can have EBX or EBP, but not both.
Indexing
With Scaling:
Base + Register Offset * Scaling Factor
- Operand field contains base address.
- Useful for array calculations where size of component is multiple bytes long.
- Stack Addressing: PUSH and POP, a variant of register indirect with auto-increment/decrement using the ESP register implicitly.
- Jump relative addressing, EIP + offset
- Operand field contains a displacement.
- Used by near and short jump instructions
Based Indexed:
Based
Indexed is of two types
1. Based-indexed addressing with no scale factor
2. Based-indexed addressing with scale factor
Based-indexed addressing with no scale factor
•
Effective address
is computed as
Base +
Index + signed displacement
•
Useful in
accessing two-dimensional arrays
»
Displacement
==> points to the beginning of the array
»
Base and index
registers point to a row and an element within that row
•
Useful in
accessing arrays of records
»
Displacement
==> represents the offset of a field in a record
»
Base and index
registers hold a pointer to the base of the array and the offset of an element
relative to the base of the array
•
Useful in
accessing arrays passed on to a procedure
»
Base register
==> points to the beginning of the array
»
Index register
==> represents the offset of an element relative to the base of the array
Example
Assuming EBX points to
table1
mov EAX,[EBX+ESI]
cmp EAX,[EBX+ESI+4]
Based-indexed addressing with scale factor
•
Effective address
is computed as
Base +
(Index * scale factor) + signed displacement
•
Useful in
accessing two-dimensional arrays when the element size is 2, 4, or 8 bytes
»
Displacement
==> points to the beginning of the array
»
Base register
==> holds offset to a row (relative to start of array)
»
Index register
==> selects an element of the row
»
Scaling factor
==> size of the array element
compares two
successive elements of table1
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