Microprocessor Unit II.ppt

7/29/2019 Microprocessor Unit II.ppt

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Introduction to 8086Microprocessor


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Introduction:

It is a 16 bit p.

8086 has a 20 bit address bus can access

upto 2power20

memory locations ( 1 MB) . It can support upto 64K I/O ports.

It provides 14, 16-bit registers.

It has multiplexed address and data busAD0- AD15 and A16 A19.


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It requires single phase clock with 33% duty

cycle to provide internal timing.

8086 is designed to operate in two modes,Minimum and Maximum.

It can prefetches upto 6 instruction bytes

from memory and queues them in order to

speed up instruction execution.

It requires +5V power supply.

A 40 pin dual in line package.


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Architecture of 8086

The architecture of 8086 includes

Arithmetic Logic Unit (ALU)

Flags

General registers

Instruction byte queue

Segment registers


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EU & BIU

The 8086 CPU logic has been partitioned into twofunctional units namely Bus Interface Unit (BIU) andExecution Unit (EU)

The major reason for this separation is to increase

the processing speed of the processor The BIU has to interact with memory and input and

output devices in fetching the instructions and datarequired by the EU

EU is responsible for executing the instructions ofthe programs and to carry out the requiredprocessing


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EU & BIU


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Architecture Diagram


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Execution Unit

The Execution Unit (EU) has

Control unit

Instruction decoder

Arithmetic and Logical Unit (ALU)

General registers

Flag register

Pointers

Index registers


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Execution Unit

Control unit is responsible for the co-ordination of all other units of the processor

ALU performs various arithmetic and logical

operations over the data The instruction decoder translates the

instructions fetched from the memory into aseries of actions that are carried out by theEU


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Execution Unit - Registers

General registers are used for temporarystorage and manipulation of data andinstructions

Accumulator register consists of two 8-bitregisters AL and AH, which can be combinedtogether and used as a 16-bit register AX

Accumulator can be used for I/O operationsand string manipulation


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Base register consists of two 8-bit registers BL andBH, which can be combined together and used as a16-bit register BX

BX register usually contains a data pointer used for

based, based indexed or register indirect addressing Count register consists of two 8-bit registers CL and

CH, which can be combined together and used as a16-bit register CX

Count register can be used as a counter in stringmanipulation and shift/rotate instructions


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Data register consists of two 8-bit registersDL and DH, which can be combined togetherand used as a 16-bit register DX

Data register can be used as a port numberin I/O operations

In integer 32-bit multiply and divideinstruction the DX register contains high-order word of the initial or resulting number


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Execution Unit - Flags


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Overflow Flag (OF) - set if the result is too largepositive number, or is too small negative number tofit into destination operand

Direction Flag (DF) - if set then string manipulation

instructions will auto-decrement index registers. Ifcleared then the index registers will be auto-incremented

Interrupt-enable Flag (IF) - setting this bit enablesmaskable interrupts

Single-step Flag (TF) - if set then single-stepinterrupt will occur after the next instruction


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Sign Flag (SF) - set if the most significant bit of theresult is set.

Zero Flag (ZF) - set if the result is zero.

Auxiliary carry Flag (AF) - set if there was a carryfrom or borrow to bits 0-3 in the AL register.

Parity Flag (PF) - set if parity (the number of "1" bits)in the low-order byte of the result is even.

Carry Flag (CF) - set if there was a carry from or

borrow to the most significant bit during last resultcalculation


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Execution Unit - Pointers

Stack Pointer (SP) is a 16-bit register pointing to program stack

Base Pointer (BP) is a 16-bit register pointing to data in stacksegment. BP register is usually used for based, based indexed orregister indirect addressing.

Source Index (SI) is a 16-bit register. SI is used for indexed,

based indexed and register indirect addressing, as well as asource data addresses in string manipulation instructions.

Destination Index (DI) is a 16-bit register. DI is used for indexed,based indexed and register indirect addressing, as well as adestination data addresses in string manipulation instructions.


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Execution Unit - Pointers


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Bus Interface Unit

The BIU has

Instruction stream byte queue

A set of segment registers

Instruction pointer


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BIU Instruction Byte Queue

8086 instructions vary from 1 to 6 bytes

Therefore fetch and execution are takingplace concurrently in order to improve the

performance of the microprocessor The BIU feeds the instruction stream to the

execution unit through a 6 byte prefetchqueue

This prefetch queue can be considered as aform of loosely coupled pipelining


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BIU Instruction Byte Queue

Execution and decoding of certain instructions donot require the use of buses

While such instructions are executed, the BIUfetches up to six instruction bytes for the following

instructions (the subsequent instructions) The BIU store these prefetched bytes in a first-in-

first out register by name instruction byte queue

When the EU is ready for its next instruction, itsimply reads the instruction byte(s) for theinstruction from the queue in BIU


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Segment: Offset Notation

The total addressable memory size is 1MB

Most of the processor instructions use 16-bitpointers the processor can effectively

address only 64 KB of memory To access memory outside of 64 KB the

CPU uses special segment registers tospecify where the code, stack and data 64KB segments are positioned within 1 MB ofmemory


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Segment: Offset Notation

A simple scheme would be to order the bytes in aserial fashion and number them from 0 (or 1) to theend of memory

The scheme used in the 8086 is calledsegmentation

Every address has two parts, a SEGMENT and anOFFSET (Segmnet:Offset )

The segment indicates the starting of a 64 kilobyteportion of memory, in multiples of 16

The offset indicates the position within the 64kportion

Absolute address = (segment * 16) + offset


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Segment Registers

The memory of 8086 is divided into 4

segments namely

Code segment (program memory)

Data segment (data memory)

Stack memory (stack segment)

Extra memory (extra segment)


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Different Areas in Memory

Program memory Program can be locatedanywhere in memory

Data memory The processor can access data in

any one out of 4 available segments Stack memory A stack is a section of the memory

set aside to store addresses and data while asubprogram executes

Extra segment This segment is also similar to datamemory where additional data may be stored andmaintained


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Segment Registers

Code Segment (CS) register is a 16-bit registercontaining address of 64 KB segment withprocessor instructions

The processor uses CS segment for all accesses toinstructions referenced by instruction pointer (IP)

register Stack Segment (SS) register is a 16-bit register

containing address of 64KB segment with programstack

By default, the processor assumes that all datareferenced by the stack pointer (SP) and basepointer (BP) registers is located in the stacksegment


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Segment Registers

Data Segment (DS) register is a 16-bit registercontaining address of 64KB segment with programdata

By default, the processor assumes that all datareferenced by general registers (AX, BX, CX, DX)

and index register (SI, DI) is located in the datasegment

Extra Segment (ES) register is a 16-bit registercontaining address of 64KB segment, usually withprogram data

By default, the processor assumes that the DIregister references the ES segment in stringmanipulation instructions


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Segment Registers


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Pin Diagram


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Addressing Modes

Implied Addressing The data value/data addressis implicitly associated with the instruction

Register Addressing The data is specified byreferring the register or the register pair in which the

data is present Immediate Addressing The data itself is provided

in the instruction

Direct Addressing The instruction operandspecifies the memory address where data is located


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Addressing Modes

Register indirect addressing The instructionspecifies a register containing an address, wheredata is located

Based - 8-bit or 16-bit instruction operand is addedto the contents of a base register (BX or BP), theresulting value is a pointer to location where dataresides

Indexed - 8-bit or 16-bit instruction operand is added

to the contents of an index register (SI or DI), theresulting value is a pointer to location where dataresides


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Addressing Modes

Based Indexed - the contents of a base register (BX

or BP) is added to the contents of an index register

(SI or DI), the resulting value is a pointer to location

where data resides Based Indexed with displacement - 8-bit or 16-bit

instruction operand is added to the contents of a

base register (BX or BP) and index register (SI or

DI), the resulting value is a pointer to location wheredata resides


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Data Transfer Instructions


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Data Transfer Instructions


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Arithmetic Instructions


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Arithmetic Instructions


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Number Representation


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Logical Instructions


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String Instructions


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Program Transfer Instructions


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Program Transfer Instructions


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Processor Control Instructions


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Assembler Directives

Assembler directives give instruction to theassembler where as other instructions discussed inthe above section give instruction to the 8086microprocessor

Assembler directives are specific for a particularassembler

However all the popular assemblers like the Intel8086 macro assembler, the turbo assembler and the

IBM macro assembler use common assemblerdirectives


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Important Directives

The ASSUME directive tell the assembler the name of the logicalsegment it should use for a specified segment

The DB directive is used to declare a byte-type variable or to setaside one or more storage locations of type byte in memory(Define Byte)

The DD directive is used to declare a variable of type doublewordor to reserve memory locations which can be accessed as typedoubleword (Define Doubleword)

The DQ directive is used to tell the assembler to declare avariable 4 words in length or to reverse 4 words of storage in

memory (Define Quadword)


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Important Directives

The ENDS directive is used with the name of

a segment to indicate the end of that logical

segment

The EQU is used to give a name to some

value or symbol


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Assembly Language Program

Writing assembly language programs for 8086 isslightly different from that of writing assemblylanguage programs for 8085

In addition to the instructions that are meant for

solving the problem, some additional instructions arerequired to complete the programs

The purpose of these additional programs is toinitialize various parts of the system, such assegment registers, flags and programmable port

devices Some of the instructions are to handle the stack of

the 8086 based system


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Another purpose of these additional instructions is tohandle the programmable peripheral devices suchas ports, timers and controllers

The programmable peripheral interfaces should beassigned suitable control words to make them tofunction in the way as we expect

The best way to approach the initialization task is tomake a checklist of all the registers, programmable

devices and flags in the system we are working on


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An 8086 assembly language program has

five columns namely

Address

Data or code

Labels

Mmnemonics

Operands Comments


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The address column is used for the addressor the offset of a code byte or a data byte

The actual code bytes or data bytes are put

in the data or code column A label is a name which represents an

address referred to in a jump or callinstruction

Labels are put in the labels column


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The operands column contains the registers,

memory locations or data acted upon by the

instructions

A comments column gives space to describe

the function of the instruction for future

reference

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Microprocessor Unit II.ppt