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The study of structures and encoding model of 8086

Computer Programming

Buildings of 8086

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The CENTRAL PROCESSING UNIT of 8086 is divided into two parts:

  • BIU (Bus Interface Unit)
  • EU (Execution Unit)
  • BIU (Bus User interface Unit)
  • The function Of BIU is to send treat to:

    • Fetch the instruction or data from memory.
    • Write the data to memory space.
    • Write the data to the port.
    • Read data from the port.

    Various Parts of the BIU are given listed below

    Segment Signs up

    BIU offers 4 portion registers of 16-bit each i. at the. CS, DS, SS and ES.

  • Code Segment (CS): Is used to address a memory position in the code segment with the memory, where the op-code of program can be stored.
  • Data Portion (DS): Is the register which usually points to part of memory, where the data is kept.
  • Bunch Segment (SS): Is the sign-up used to speak about the bunch location in stack portion of the memory and accustomed to store info temporarily for the stack.
  • Extra Portion (ES): Is usually to address the segment which can be additional info segment accustomed to store data.
  • The memory pointers are used to level or address the particular storage location in memory.

    Instruction queue IQ (Queue)

    To increase the execution velocity, BIU brings as many as six instruction bytes ahead to time via memory.

    All the six bytes will be then saved in first-in-first-out 6-byte register named instruction line IQ. Then all octet have to be given to EU one-by-one.

    This kind of pre-fetching operation of BIU may be in parallel with the execution procedure of EUROPEAN UNION, which enhances the speed of execution of instruction.

  • Execution Device (EU)
  • The functions of execution product are:

    • To tell BIU where to get the training or data from.
    • To decode the guidance.
    • To execute the instruction.

    The EU provides the control circuitry to perform numerous internal procedures.

    A decoder in EU decodes the instruction fetched via memory to build different inner or external control sign required to execute the operations.

    EUROPEAN has 18 bit ALUMINE, which can carry out arithmetic and logical businesses on 8-bit as well as 16-bit data.

    Flag sign-up in EUROPEAN is of 16-bit. These signup contain nine active flags.

    Five flags inside the lower octet of the sign-up are similar to 8085 flag sign-up.

    8086 flags will be divided in two parts Status red flags and Control flags.

    Status Red flags:

  • Carry Banner (CF): It can be set to 1 if there is carry out of the MSB position i actually. e. resulting from an addition or if a borrow is required at MSB during substraction.
  • Auxillary Carry (AF): If an operation performed in ALU produces a carry/borrow from lower nibble (i. e. D0-D3) to uppr nibble (i. e. D4-D7), the AF flag is set i. e. carry given by D3bit to D4 is definitely AF flag.
  • Parity Flag (PF): This flag is used to point the parity of result. If reduce order 8-bits of response to an businesses contains actually number in the event that 1, the parity flag is set and then for odd range of 1, the parity flag is totally reset.
  • Zero Flag (ZF): It is set, if the result of arithmetic or perhaps logical procedure is actually zero else it will probably be reset.
  • Sign Banner (SF): In sign value format the sign of number can be indicated by MSB little. If the response to operation is negative signal flag is set. The sign flag is definitely replica of MSB piece of result.
  • Overflow Banner (OF): In case of the agreed upon arithmetic procedure, the flood flag is defined, if the end result is too huge to fit inside the numbers parts available to support it. The overflow flag has no relevance in unsigned arithmetic procedure.
  • Control Flags:

  • Capture Flag (TF): It is used for single step control. That allows customer to perform one instruction of a system at a time pertaining to debugging. When ever trap banner is set, this software can be manage in solitary step mode.
  • Disrupt Flag (IF): It is an interrupt enable/disable flag. If it is set, the maskable interrupts INTR of 8086 is enabled and if it is reset, the interrupts is definitely disable.
  • Direction Flag (DF): The direction banner is used in string operation. If DF is set, line bytes happen to be read or perhaps write coming from higher memory address to lower memory talk about. If DF is reset, the thread bytes happen to be read or perhaps write from lower memory space address to raised memory talk about.
  • Programming Model Of 8086

    The development model of the 8086 throughout the Pentium II’s considered to be program visible because its registers are used during application encoding and are particular by the instruc-tions.

    Various other registers, detailed later with this chapter, are considered to be system invisible because they are not really addressable straight during applications programming, yet may be used indirectly during program programming.

    Only the 80286 and previously mentioned contain the program-invisible registers accustomed to control and operate the protected recollection system.

    The development model is made up of 8-, 16-, and 32-bit registers. The 8-bit registers are AH, AL, BH, BL, CH, CL, DH, and DL and are reported when an instructions is formed using these two-letter designations. The 16-bit signs up are AX, BX, CX, DX, SP, BP, PADA, SI, IP, FLAGS, CS, DS, ES, SS, FS, and GS. The extended 32-bit signs up are EAX, EBX, ECX, EDX, ESP, EBP, EDI, ESI, EIP, and EFLAGS.

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