Assembly Language Registers Explain

What are registers ? What are the registers in assembly language?

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  • Processor operations mostly involve processing data. This data can be stored in memory and accessed. However, reading data from and storing data into memory slows down the processor, as it involves complicated processes of sending the data request across the control bus and into the memory storage unit and getting the data through the same channel.To speed up the processor operations, the processor includes some internal memory storage locations, called registers.The registers store data elements for processing without having to access the memory. 
    A limited number of registers are built into the processor chip.They are:

    01:Processor Registers
    There are ten 32-bit and six 16-bit processor registers in IA-32 architecture. The registers are grouped into three categories −
    General registers,
    Control registers, and
    Segment registers.
    The general registers are further divided into the following groups −
    Data registers,
    Pointer registers, and
    Index registers.
    *Data Registers:Four 32-bit data registers are used for arithmetic, logical, and other operations.Lower and higher halves of the above-mentioned four 16-bit registers can be used as eight 8-bit data registers: AH, AL, BH, BL, CH, CL, DH, and DL.
    Some of these data registers have specific use in arithmetical operations--
    AX is the primary accumulator; 
    BX is known as the base register,
    CX is known as the count register,
    DX is known as the data register. It is also used in input/output operations. It is also used with AX register along with DX for multiply and divide operations involving large values.

    *Pointer Registers:The pointer registers are 32-bit EIP, ESP, and EBP registers and corresponding 16-bit right portions IP, SP, and BP. There are three categories of pointer registers −
    Instruction Pointer (IP) − The 16-bit IP register stores the offset address of the next instruction to be executed.
    Stack Pointer (SP) − The 16-bit SP register provides the offset value within the program stack.
    Base Pointer (BP) − The 16-bit BP register mainly helps in referencing the parameter variables passed to a subroutine.

    *Index Registers:The 32-bit index registers, ESI and EDI, and their 16-bit rightmost portions. SI and DI, are used for indexed addressing and sometimes used in addition and subtraction. There are two sets of index pointers −
    Source Index (SI) − It is used as source index for string operations.
    Destination Index (DI) − It is used as destination index for string operations.

    02:Control Registers
    The 32-bit instruction pointer register and the 32-bit flags register combined are considered as the control registers.The common flag bits are:
    *Overflow Flag (OF) − It indicates the overflow of a high-order bit (leftmost bit) of data after a signed arithmetic operation.
    *Direction Flag (DF) − It determines left or right direction for moving or comparing string data. When the DF value is 0, the string operation takes left-to-right direction and when the value is set to 1, the string operation takes right-to-left direction.
    *Interrupt Flag (IF) − It determines whether the external interrupts like keyboard entry, etc., are to be ignored or processed. It disables the external interrupt when the value is 0 and enables interrupts when set to 1.
    *Trap Flag (TF) − It allows setting the operation of the processor in single-step mode. The DEBUG program we used sets the trap flag, so we could step through the execution one instruction at a time.
    *Sign Flag (SF) − It shows the sign of the result of an arithmetic operation. This flag is set according to the sign of a data item following the arithmetic operation. The sign is indicated by the high-order of leftmost bit. A positive result clears the value of SF to 0 and negative result sets it to 1.
    *Zero Flag (ZF) − It indicates the result of an arithmetic or comparison operation. A nonzero result clears the zero flag to 0, and a zero result sets it to 1.
    03:Segment Registers
    Segments are specific areas defined in a program for containing data, code and stack. There are three main segments −
    *Code Segment − It contains all the instructions to be executed. A 16-bit Code Segment register or CS register stores the starting address of the code segment.
    *Data Segment − It contains data, constants and work areas. A 16-bit Data Segment register or DS register stores the starting address of the data segment.
    *Stack Segment − It contains data and return addresses of procedures or subroutines. It is implemented as a 'stack' data structure. The Stack Segment register or SS register stores the starting address of the stack.
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