Programming Model or Register Organization of 8086 Microprocessor

Fig. : Register Organization of 8086 Microprocessor

 

1)      General purpose register:

8086 microprocessor contains 8 general purpose register, which are used to store data temporarily. These register are AH, AL, BH, BL, CH, CL, DH& DL. All registers are 8 bit register. It means that they can store 8 bit data to store 16 bit data these registers are combined as follows.

AX (16) =AH (8) +AL (8)

BX (16) =BH (8) +BL (8)

CX (16) =CH (8) +CL (8)

DX (16) =DH (8) +DL (8)

            These registers are programmable registers. It means that users can read & write data into these registers and they can be overwritten by users.

2) Pointing and Index register:

            There are 2 pointing and 2 index registers in 8086 up. The two pointing registers are sp (stack pointer), & BP (Base Pointer). These registers are used to store 16bit offset address of data in memory.

            The SP is used to store offset address of data stored in stack segment of memory. BP is also used to store offset address of data stored in stack segment of memory. The difference between these two are that SP stores TOP data’s offset address and BP stores Bottom data’s offset address.

            The 2 index register SI (source index) & DI (destination index). These two 16-bit register are used to point towards data stored in either data segment (DS) or in extra segment (ES) of memory.

 

3) Instruction pointer (IP):

            This 16 bit register is use to point towards an instruction stored in code segment of memory.

4) Segment Register:

            The 8086 microprocessor can address up to 1MB of memory. To access (read/write) the memory and I/O devices 8086 sends 20 bit physical address. The memory of 8086 is divided into 4 segments i.e. code segment, data segment, extra segment &stack segment. Each segment is of 64K bytes in size. The 4 segment register of 16 bits in BIU are used to store base address for respective segments. The 20 bit physical address is calculated by adding two 16 bit addresses i.e. base address & offset address.

5) Flag Register:

            8086 microprocessor contains a 16 bit flag register. Out of this 16 bit only 9 flags are active (used). The 9 flags are classified into two types.

A] Conditional Flag:              i.e. carry flag, Auxilary carry flag, sign flag, zero flag, polarity flag, overflow flag.

B] Control Flag:                     Interrupt flag, Trap flag, directions flag.