A segment register is a register that contains the base address, or something related to the base address, of a region of memory. In the 8086/8088, the four segment registers are multiplied by 16 and added to the effective address to form the physical address.
An index register, on the other hand, is a register that contains an address that is added to another address to form the effective address.
In the 8086/8088, four address components are involved; 1.) the displacement contained within the instruction, often called the offset, 2.) a base address specified by the r/m field, often the BP or BX register, 3.) an index address specified by the r/m field, often the SI or DI register, and 4.) the segment address specified by context or by a segment override prefix, often the CS, DS, SS, or ES register.
register in CPU are used to store information temporarily that information could be one or two byte of data to be processed or the address of data .
index register : are 16-bit register used to access data in data segment .these are SI(source index) and DI (destination index)
there are 14 registers in 8088 micro processor. All the 14 are 16 bit registers. They are4 segment registers viz - code segment register, stack segment register, data segment register, extra segment register.general registers are - accumulator register i.e. AX, base register i.e. BX, count register i.e. CX, data register i.e. DX and stack pointer (SP), base pointer (BP).index registers are - source index(SI), destination index(DI),and the other registers are instruction pointer and flags register.
The SI (Source Index) and DI (Destination Index) registers are useful in repeated string operations, such as copy. The DS (Data Segment) register is paired up with SI and the ES (Extra Segment) register is paired up with DI.
DI is the Index register in Data segment(16-bit, 64 KB) .Destination Index (DI) is a 16-bit register. DI is used for indexed, based indexed and register indirect addressing, as well as a destination data address in string manipulation 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. Data segment (DS) is a 16-bit register containing address of 64KB segment 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. Data segment (DS) is a 16-bit register containing address of 64KB segment 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.
The code segment (CS) register is used for access to program code. The data segment (DS) register is used for access to data. The extra segment (ES) register is used for access to data during certain string primitive operations. The stack segment (SS) register is used for access to stack data.Any of these implied uses can be overridden with a segment override prefix opcode.
extra segment is a 16-bit register containing address of 64KB segment, usually with program data. By default, the processor assumes that the DI register points to the ES segment in string manipulation instructions. ES register can be changed directly using POP and LES instructions. It is possible to change default segments used by general and index registers by prefixing instructions with a CS, SS, DS or ES prefix. any corrections most welcome
Give the effective address if the segment register is AA03 and the offset register is 0200.
In the 8086/8088 microprocessor, there are eight 8 bit general registers, AH, AL, BH, BL, CH, CL, and DH, DL. They can be used as four 16 bit registers AX, BX, CX, and DX. There are four 16 bit pointer/index registers, SP (stack pointer), BP (base pointer), SI (source index), and DI (destination index). There are four 16 bit segment registers, CS (code segment), DS (data segment), SS (stack segment), and ES (extra segment). There is the 16 bit instruction pointer, IP, and there is a 16 bit flags register.
In the 8086/8088, the logical address corresponds to a segment register, such as CS (Code Segment), DS (Data Segment), SS (Stack Segment) and ES (Extra Segment). The segment register is selected by context, or it is explicitly selected using a segment override prefix. The segment register is left shifted 4 bits into a 20-bit temporary register. This is the same as multiplying it by 16. Then the logical address is added to that result. The final result is the physical address.
stack segment register
The segment register in the 8086/8088 provide a base address for any memory access. There are four segment registers, CS - Code Segment, DS - Data Segment, SS - Stack Segment, and ES - Extra Segment. Each in used in the context of a particular instruction and contains the base address of the memory segment divided by 16. This allows the processor to access up to 1 MB of memory, though only in segments of 64 KB at a time.
The source index (SI) register is required for some string (character) operations. In this context the SI is associated with the DS register. The destination index (DI) register is also required for some string operations. In this context the DI is associated with the ES register.