What is a register?
A register is one of a small set of data holding places that are part of
a computer processor .
A register may hold :
a computer instruction
a storage address
or any kind of
data (such as a bit sequence or individual characters)
About Registers
Rarely more than 64 registers in number.
Small in size
Typically
a register is less than 64 bits in size
General Purpose Registers Four general-purpose registers:
1. AX
2. BX
3. CX
4. DX
AX (Accumulator)
This is accumulator register. It is of 16 bits and is divided into two
8-bit registers AH and AL to also perform 8-bit instructions. It is used in
arithmetic, logic and data transfer instructions in 8086 microprocessors. In
manipulation and division, one of the numbers involved must be in AX or AL.
BX (Base Register)
This is the base register. It is of 16 bits and is divided into two
8-bit registers BH and BL. BX register is an address register. It usually
contains a data pointer used for based, based indexed or register indirect
addressing.
CX (Count register)
This is Count register. It is of 16 bits and is divided into two 8-bit
registers CH and CL to also perform 8-bit instructions. This serves as a loop
counter. Program loop constructions are facilitated by it. Count register can
also be used as a counter in string manipulation and shift/rotate
instruction.
DX (Data Register)
This is data register. It is of 16 bits and is divided into two 8-bit
registers DH and DL to also perform 8-bit instructions. Data register can be
used as a port number in I/O operations. It is also used in multiplication and
division.
SP (Stack Pointer)
This is stack pointer register pointing to program stack. It is used in
conjunction with SS for accessing the stack segment. It is of 16 bits.
It points to the topmost item of the stack. If the stack is empty the stack
pointer will be (FFFE)H. Its offset address is relative to the stack
segment.
BP (Base Pointer)
This is base pointer register pointing to data in stack segment. Unlike
SP, we can use BP to access data in the other segments. It is of 16 bits. It
is primarily used in accessing parameters passed by the stack. Its offset
address is relative to the stack segment.
SI (Source Index)
This is source index register which is used to point to memory locations
in the data segment addressed by DS. Thus, when we increment the contents of
SI, we can easily access consecutive memory locations. It is of 16 bits.
Its offset is relative to the data segment.
DI (Destination Index)
This is destination index register performs the same function as SI.
There is a class of instructions called string operations, that use DI to
access the memory locations addressed by ES.
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