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Objectives

To look at the stored program concept employed

in modern computer system

To learn the difference between program,

address and data

To study the software model of Intel

8088/8086 MPU

To look at the data format in 8088/8086

To study the memory organization in 8088/8086

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Stored Program Concept

A microprocessor is capable of performing numerousoperations (normally over 100 different operations)

However, it will only carry out what it has been told to do

The operations which the microprocessor can be told to

perform are called instructions A group of instructions arranged in a logical order is known

as a program

A computer is often compared to a calculator, which is toldwhat to do by the operator via the keypad

By pressing the right keys, we can instruct a calculator toadd, subtract, multiply, divide etc (instructions).

We must also key in the numbers to be added, subtractedetc (data).

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Stored Program Concept-Cont.

With a calculator, we can add a list of numbers as quicklyas we can enter the numbers and instructions

Therefore, the operation is limited by the speed andaccuracy of the operator

Computer designers recognized that it was the humanoperator that slowed down the computation process

To overcome this, the stored program concept wasintroduced

Using this approach, the instructions (program) and data

are stored in the computer memory For e.g., we have 20 numbers to be manipulated by a

program with 100 instructions and 10 answers will beproduced in the process

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Stored Program Concept-Cont.

Before any computation begins, the 100-instructionprogram plus the 20 numbers are loaded into thecomputer memory

Furthermore, 10 memory locations are reserved for 10answers

Only then the computer is allowed to execute theprogram

The actual computation time depends on the speed ofthe computer (normally less than 1ms)

This automatic operation is one of the features thatdistinguishes the computer from the calculator

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Program, Address, Data

Microprocessor performs operations on dataheld in the memory where program is alsolocated.

Thus, program and data used by computerare located in the same memory, but indifferent region

A computer that keeps instructions and datain the same memory is called a von Neumannmachine

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Program, Address, Data-Cont.

*Named after the mathematician, John von Neumann

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Program, Address, Data-Cont.

Memory is normally regarded as array of

storage locations (drawers)

Each drawer contains some data Each drawercan be identified with a unique number known

as address

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Example Figure below demonstrates how a program (instructions) and

data coexists in the same memory:

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Example-Cont.

In this example, the memory has only seven locations, withaddress from 0 to 6

The program occupies memory locations at address 0 to 3,and the data at address 4 to 6

The first instruction Get [4] means get the contents ofmemory location at address 4 from memory

Square brackets is used to denote the contents of theaddress they enclose, so [4] = 2

Second instruction Add it to [5], means add the number

brought by the previous instruction to the contents ofaddress 5

Therefore, the microprocessor adds 2 to 7 to get 9

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Example-Cont.

Third instruction Put result in [6], tells the

computer to put the result in memory at

address 6

The 1 that was in address 6 before the

instruction is replaced by 9

Finally, the last instruction in address 3 tells

the computer to stop

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Software Model of 8088/8086

Microprocessor

The software model helps the programmer inunderstanding the operation of a microprocessorfrom a software point of view

To program a microprocessor, there is no needto know all its hardware architectural features

A programmer should know the variousregisters and understand their purpose,functions, operating capabilities and limitations

The programmer should also know howinformation is arranged in registers, memory,input/output, and how memory and I/O areaddressed to obtained instructions and data

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The software model above illustrates the software architecture of the 8088/086 microprocessor

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Software Model of 8088/8086

Microprocessor- Cont.

The model includes 13 16-bit registers:i. Instruction pointers

ii. Four data registers (AX, BX, CX and DX)

iii. Two pointer registers (BP and SP)

iv. Two index registers (SI and DI)

v. Four segment registers (CS, DS, SS and ES)

In addition, there is another register called Status Register(SR), with nine of its bits implemented as status and controlflags

The 8088/8086 architecture implements independentmemory and input/output address spaces

The memory address space is 1,048,576 bytes (1Mbytes) insize and the I/O address space is 65,536 bytes (64Kbytes) insize

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Memory Address Space & Data

Organization

The 8088/8086 supports 1Mbyte of externalmemory

The memory space is organized as individual

bytes of data stored at consecutive addressesover the address range 0000016 to FFFFF16 (or00000H to FFFFFH)

Two consecutive bytes can be accessed as oneword (16 bits) of data

The lower-addressed byte is the least significantbyte of the word, and the higher-addressed byteis its most significant byte (little endian format)

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Example

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Example-Cont.

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Example-Cont.

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Example

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Solution

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Example

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Example

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Example

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Solution

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Data TypesThe 8088/8086 can store data in 3 formats: byte (8 bits), word (16 bits) and doubleword (32 bits)

Data types that can be stored as byte:

i. Unsigned integer (010 to 25510)

ii. Signed integer (-12810 to 12710)

iii. ASCII characteriv. Unpacked BCD (one digit)

v. Packed BCD (two digits)

Data types that can be stored as word:

i. Unsigned integer (010 to 6553510)

ii.S

igned integer (-3276810 to 3276710)iii. Instruction code

Data types that can be stored as double word:

i. Pointer (segment:offset)

ii. IEEE single-precision floating point number

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Extra Example

1. How would -204710 be stored in an 8086

based computer in memory starting at

address 0A000H?

2. How would the unpacked BCD 29 be stored

in memory starting at address 0B000H?

(assume that the least significant digit is

stored at the lower address).