registersss
All the registers are 16 bit and are grouped into several categories
- General-purpose registers, • Segment registers, • Special purpose registers, and • Flag register
These registers perform the following functions
Accumulator (AX) Register, Base (BX) Register, Counter (CX) Register, Data (DX) Register
• Base (BX) Register
For reading data from memory or writing data into memory, 8086 microprocessor has to select one memory location, 16-bit effective address of this memory location can be stored in register BX.
• Data (DX) Register
In multiplication and division instruction, if data or result is of 32-bits, then register DX is used to store 16 MSB,
• Counter (CX) Register
In rotate and shift instructions, register CL is used as 8 bit counter i.e., it used to store 8-bit count. In some instructions like REP (repeat), Loop, register CX is used as 16-bit implicit counter.
Stack Pointer (SP)
It is used to hold the address of the stack top. Stack top is the uppermost filled memory location in stack memory. There is no access to these register directly, the modifications are done depending on the contents of the stack.
Code Segment Register (CS)
It is used to point to the base address of the code segment.
Data Segment Register (DS)
It is used to point to the base address of the data segment.
Source Index (SI) and Destination Index (DI)
SI and DI act as memory pointers relative to segment register DS. The microprocessor will take the effective address of the data from SI and store it in DI.
Segment Registers
The 8086 microprocessor has a 20-bit wide physical address to access 1MB memory location. But the registers of the 8086 microprocessor that holds the logical address are only 16-bits wide. Thus 8086 microprocessor implements memory segmentation for 1MB physical memory where the memory is divided into sections or segments as shown below. So that each memory location in the segment can be accessed by the 16-bit logical address.
Instruction Pointer (IP)
The IP gives the offset address of the next instruction to be executed i.e., IP stores the address of the next instruction to be fetched from the code segment. When reset is activated, the instruction pointer is set to the address of the first instruction to be fetched.
Flag Register
The flag register of 8086 is a flip-flop. It is 16-bit wide with a collection of 1-bit values that indicates the current state of the execution of arithmetic or logical instruction in the processor. Out of 16 bits, 9 bits are used in the 8086 as shown below.
programmer's model
The register organization of the 8086 microprocessor is also known
Base Pointer (BP)
The use of BP as a pointer to a memory location is similar to the use of SI and DI registers. BP register acts as a memory pointer to the stack segment register. The BP register is mainly used to access any location directly in the stack.
Extra Segment Register (ES)
This register is used to store the base address of the extra memory segment.
Stack Segment Register (SS)
This register is used to store the base address of the stack memory segment.
• Accumulator (AX) Register
When the microprocessor performs any arithmetic or logical operations, the accumulator provides one of the operands and it also holds the result of the operation. The user can access this register. It is denoted by A. The accumulator register AX can collect a total of 16-bit data .
Special Purpose Registers
pointer and index registers, instruction pointer, and program counter. These are 16-bit registers used by the CPU for specific purposes. Since they are 16-bit, one or more registers are associated with each segment register in order to generate a 20-bit physical address on the address bus. The special-purpose registers are used as memory pointers and are belongs to the pointer and index group shown below.
General
purpose registers - used for holding variables or data. They can be also used as counters or as temporary storage for intermediate results during any operation.
The four
segment registers and their functions
