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Architecture of 8086
The CPU of 8086 is divided into two parts:
The function Of BIU is to send address to:
Various Sections of the BIU are given below
BIU has 4 segment registers of 16-bit each i.e. CS, DS, SS and ES.
The memory pointers are used to point or address the particular memory location in memory.
To increase the execution speed, BIU fetches as many as six instruction bytes ahead to time from memory.
All the six bytes are then held in first-in-first-out 6-byte register called instruction queue IQ. Then all bytes have to be given to EU one-by-one.
This pre-fetching operation of BIU may be in parallel with the execution operation of EU, which improves the speed of execution of instruction.
The functions of execution unit are:
The EU contains the control circuitry to perform various internal operations.
A decoder in EU decodes the instruction fetched from memory to generate different internal or external control signal required to perform the operations.
EU has 16 bit ALU, which can perform arithmetic and logical operations on 8-bit as well as 16-bit data.
Flag register in EU is of 16-bit. These register contain nine active flags.
Five flags in the lower byte of the register are similar to 8085 flag register.
8086 flags are divided in two parts Status flags and Control flags.
The programming model of the 8086 through the Pentium II’s considered to be program visible because its registers are used during application programming and are specified by the instruc-tions.
Other registers, detailed later in this chapter, are considered to be program invisible be¬cause they are not addressable directly during applications programming, but may be used indirectly during system programming.
Only the 80286 and above contain the program-invisible registers used to control and operate the protected memory system.
The programming model contains 8-, 16-, and 32-bit registers. The 8-bit registers are AH, AL, BH, BL, CH, CL, DH, and DL and are referred to when an instruction is formed using these two-letter designations. The 16-bit registers are AX, BX, CX, DX, SP, BP, DI, SI, IP, FLAGS, CS, DS, ES, SS, FS, and GS. The extended 32-bit registers are EAX, EBX, ECX, EDX, ESP, EBP, EDI, ESI, EIP, and EFLAGS.
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