Program Execution-Programming and Computer Architecture-Lecture Slides, Slides of Computer Architecture and Organization

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Data ManipulationProgram Execution

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Program Execution

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A computer follows a program stored inits memory by copying the instructionsfrom memory into the CPU as needed.

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Once in the CPU, each instruction isdecoded and obeyed.

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The order in which the instructions arefetched from the memory corresponds tothe order in which the instructions arestored in the memory unless there is aJUMP instruction.

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Program Execution(Contd.)

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Fetch:

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Each instruction is two bytes long, fetchprocess involves retrieving the contents of thetwo memory cells from the main memory.

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CPU places the instruction received from thememory in its instruction register and thenincrements the program counter by two.

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The counter contains the address of the nextinstruction stored in the memory.

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Thus the program counter will be ready forthe next fetch.

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Program Execution(Contd.)

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Decode:

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The instruction now in the instructionregister, the CPU decodes the instruction,which involves breaking the operand field intoits proper components based on theinstruction’s op-code.

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Execute:

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CPU then executes the instruction byactivating the appropriate circuitry toperform the requested task.

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Example B

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B258:

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JUMP to the instruction at address 58 if the contentsof register 2 is the same as that of register 0.

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We will start by comparing the values in register 2and 0.

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If the contents are different bits, the execution stepterminates and the next machine cycle begins.

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If the contents are equal than the machine places thevalue 58 in its program counter during the executestep.

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Next fetch step finds 58 in the program counter, soinstruction at that address will the next instruction tobe fetched and executed.

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Decoding the instruction B

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An example of ProgramExecution(Contd.)

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We first need to put the programsomewhere in the memory.

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For our example, suppose the program isstored in consecutive addresses, startingat address A0(hexadecimal).

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With the program stored in this manner, we can cause the machine to execute itby placing the address(A0) of the firstinstruction in the program counter andstarting the machine.

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The program stored in main memory ready forexecution

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Performing the fetch step of the machinecycle

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An example of ProgramExecution(Contd.)

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Notice that, in our machine, instructionsare 16bits(two bytes) long.

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Thus entire instruction to be fetchedoccupies the memory cells at bothaddress A0 and A1.

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CPU is designed to take this into accountso it retrieves the content of both cellsand places the bit pattern received in theinstruction register which is 16 bit long.

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An example of ProgramExecution(Contd.)

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At the end of the fetch step of the firstmachine cycle, the program counter andinstruction register contain the followingdata:

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Program Counter:A

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Instruction Register: 156C

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Operating SystemsOperating System Architecture

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Operating System Architecture

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To understand the composition of atypical operating system we need to firstconsider the complete spectrum ofsoftware found within a typical computersystem.

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Then we will concentrate on theoperating system itself.

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A Software Survey

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Microsoft Window’s operating systems

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Group of programs called “Accessories” and“Administrative Tools”.

These are called

application and utility classes.

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Machine software categories:

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Application Software

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System Software

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