Computer Organization : Introduction

COMPUTER ORGANIZATION

INTRODUCTION
In describing Computer systems a distinction is often made between Computer Organization and Computer Architecture.Although it is difficult to give precise definitions for these terms a consensus exists between the general areas covered by each.

Computer Architecture refers to those attributes of a system visible to a programmer,the other way those attributes that have direct impact on the logical execution of the program.

Computer Organization refers to the Operational Units and there interconnections that realize or recognize the specifications of Computer Architecture.

Examples of the Architectural attributes include the instruction set,the no of bits used to represent the data types,Input Output mechanism and technique for addressing memories.

Examples of Organizational attributes includes Hardware details transparent to the programmer such as control signals,Interface between computers and peripherals and the memory technology used.

Many computer manufacturers offer the family of computer models,all of the same architecture but with differences in organization.Computer Architecture may remain for several years but organization changes with changing technology.

BUS STRUCTURE
A Bus is an electronic pathway in a digital computer that provides a communication path for data to flow between CPU and its memory and between the various peripheral devices connected to the computer.
A Bus contains one wire for each bit needed to specify the address of a device or location in a memory;plus additional wires that distinguish among the various data transfer operations to be performed.A bus can transmit data in either direction between any two components of the computing system.Without a bus a computer would need separate wires for all possible operations.Clearly a not tolerable situation...

    A bus consist typically of ,from 50 to 100 separate lines.Each line is assigned a particular meaning or a function or some operation to refer.Although there are many different bus designs on any bus the lines can be classified into 3 particular categories or groups.
    1.Control Bus
    2.Address Bus
    3.Data Bus
                                In addition there may be Power distribution bus lines that supply power to the attached modules. Bus Structure

INSTRUCTIONS
A computer must have instructions capable of performing four types of operations:
    1.Data transfers between the main memory and the CPU registers
    2.Arithmetic and logic operations on data
    3.Program Sequencing and control
    4.I/O transfers

Since the registers allow faster processing and results in shorter instructions, they are used to store data temporarily in the CPU during processing.

The CPU contains a register called Program Counter (PC),which holds the address of the instruction to be executed next.To begin executing a program the address of its first instruction must be placed in the PC.As each instruction is executed the PC is incremented by one to point to the next instruction.

Executing a given instruction is a two phase procedure.In the first phase,called the Instruction Fetch,the instruction is placed in the instruction register in the CPU.At the start of the second phase called Instruction Execute,the instruction in the IR. is examined to determine which operation to be performed.The specified operation is then performed by the CPU.This may involve fetching operands from the main memory,performing an arithmetic or logical operation,storing a result in to the main memory or some combination of these basic operations.

    The processor keeps track of some information about the results of various operations ,for use by subsequent branch instructions.This is done by recording the required information into individual bits called condition code flags.These flags are grouped together in a special register called the condition code or status register.
The four commonly used flags are:
    N (negative)    Set to 1 if the result is negative;otherwise,cleared to 0
    Z(zero)                Set to 1 if the result is zero;otherwise,cleared to 0
    V(overflow)      Set to 1 if arithmetic overflow occurs;otherwise,cleared to 0
    C(carry)             Set to 1 if a carry out results from the operation;otherwise,cleared to 0

ADDRESSING MODES
The term addressing mode refers to the way in which the operand of an instruction is specified.

Register mode : The operand is the contents of a CPU register;the name of the register is given in the instruction.
Absolute mode : The operand is in a memory location;the address of this location is given explicitly in the instruction.
Immediate mode : The operand is given explicitly in the instruction.
Indirect mode : The effective address of the operand is the contents of a register or main memory location,whose address appears in the instruction.
Index mode : The effective address of the operand is generated by adding a constant value to the contents of a register.
Autoincrement mode : The effective address of the operand is the contents of a register specified in the instruction.After accessing the operand,the contents of this register are incremented to point to the next item in a list.
Autodecrement mode : The contents of a register specified in the instruction are decremented.These contents are then used as the effective address of the operand.

Addressing modes of 8086 Microprocessor

Other Links

History of Computers http://www.yahoo.com/computers_and_internet/history

Evolution of Computers http://www.computer-museum.org

Pioneers in Computing http://www.comlab.ox.ac.uk/archive/other/museums/computing.html

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