Introduction:8086 Microprocessor is an upgraded form of the 8085Microprocessor. This 16-bit microprocessor provides a storage capacity of 1MBand comprises 20 address lines and 16 data lines. It can operate in twomodes; Maximum and Minimum modes respectively. Maximum mode is apt when thesystem has multiple processors and Minimum mode is for a single processorsystem.8086Microprocessor is divide into two: Execution Unit BusInterface Unit (EU) (BU) EUThe EU passesinstructions to the BIU on where to fetch data from, how to decode and furtherexecute those instructions. It controls all operations with the help of theinstruction decoder and the ALU (Arithmetic and Logic Unit). It is connected tothe system only through the BIU.
The ALU performsarithmetic and logical operations like addition, subtraction, OR, Not, etc.1 Flag Register: A16-bit register that relies on the accumulator result for its status. Itcontains 9 flags further divided into Conditional and Control flags.
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· Conditional flags: 1. Carryflag: Set to 1 when there is an overflow in arithmetic operations.2. Auxiliaryflag: Set to 1 when a carry/borrow is generated in the lower nibble and passedon to the higher nibble.3. Parityflag: Set to 1 when the lower order 8-bit result contains even number of 1’s.Set to 0 when there are odd number of 1’s.4.
Zeroflag: Set to 1 when an arithmetic or logical operation result is zero. 5. Signflag: Set to 1 when an operation results in a negative.6. Overflowflag: Set to 1 when system capacity is exceeded.· Control flags:1. Directionflag: Set to 1 when it is required access the higher memory address in stringoperations.
2. Interruptflag: Set to 1 when interrupt conditions needs enabling which means that theinterruption of a program has to be allowed.3.
Trapflag: Set to 1 when the program is to be run in single step mode which meansone instruction is executed at a time to help in debugging.2 General PurposeRegister: AH, AL, BH, BL, CH, CL, DH and DL are the 8 general purposeregisters. They can either be used individually (8-bit) or in pairs (16-bit).
AH and AL = AXBH and BL = BXCH and CL = CXDH and DL = DX· AX register: Accumulator register;stores operands for arithmetic operations· BX register: Base register; stores theinitial base address of memory within data segment· CX register: Counter register; storesloop counter in loop instructions· DX register: stores I/O port address forI/O instructions3 Stack PointerRegister: It stored the starting address of the segment top the memory locationin 16-bit form of where the most recent word was stored on the stack.