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General purpose of registers

By Ashik

 

General purpose of registers:

General purpose registers are basically used to hold temporarily data and intermediately result.
example: ax,bx,cx,dx each of 16 bits. whereas special purpose register are primly used for memory access.

It is of two types:

 1. segment register and 

2. index register/ pointer



Data Segment: (DS) is a 16-bit register containing address of 64KBsegment with program data. By default, the processor assumes that all data referenced by general registers (AX, BX, CX, DX) and index register (SI, DI)is located in the data segment. DS register can be changed directly using POP and LDS instructions.

Code segment: (CS) is a 16-bit register containing address of 64 KB
segment with processor instructions. The processor uses CS segment for all
accesses to instructions referenced by instruction pointer (IP) register. CS
register cannot be changed directly. The CS register is automatically updated
during far jump, far call and far return instructions.


Role of segment register:
In the In the x86 processor architecture, memory addresses are
specified in two parts called the segment and the offset.
One usually thinks of the segment as specifying the
beginning of a block of memory allocated by the system and
the offset as an index into it. Segment values are stored
in the segment registers. There are four or more segment
registers: CS contains the segment of the current
instruction (IP is the offset), SS contains the stack
segment (SP is the offset), DS is the segment used by
default for most data operations, ES (and, in more recent
Processors, FS and GS) is an extra segment register. Most memory operations accept a segment override prefix that allows use of a segment register other than the default one.

Purpose of segment register of
microprocessor:
The segment register in the 80806/8088 microprocessor contains the base address (divided by 16) of a region of memory. Since the register is 16 bits in size, there are 65,536 possible segment base
addresses, ranging from 00000H to FFFF0H, in increments of 00010H.
After address translation at the instruction level, the generated 16 bit offset is added to the selected segment register times 16 to generate a physical address between 00000H and FFFFFH. (If the offset and base go past FFFFFH, they wrap around back to 00000H.) Since the offset is also 16 bits in size, and since the overlap is only 4 bits (times 16), then each 64 kb segment overlaps by 16 bytes.
There are four segment registers; CS, DS, ES, and SS, standing for Code Segment, Data Segment, Extra Segment, and Stack Segment.
CS is used for opcode fetches. DS is used for normal data. ES is used for certain string operations as the destination address. SS is used for stack and frame (BP) data.
The segment registers can be implicitly selected by context, or they can be explicitly selected with a  segment prefix opcode.

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