Lesson 1 How a Program Executes 2015

7/21/2019 Lesson 1 How a Program Executes 2015

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Operating system

• What is an operating system? Some definitions:

• A program that is executed by the processor thatfrequently relinquishes control and must dependon the processor to regain control.

• A program that mediates between application

programs and the hardware

• A set of procedures that enable a group of peopleto use a computer system.A program that controls the execution of

application programs• An interface between applications and hardware



What is a program?

• A sequence of steps

• For each step an arithmetic or logicaloperation is done

• For each operation a different set of

control signals is needed



Function of Control Unit

• For each operation a unique code ispro!ided

"e.g. A## $%&'

• A hardware segment accepts the code and

issues the control signals

• We ha!e a computer(



Components

• )he *ontrol +nit and the Arithmetic and,ogic +nit constitute the *entral-rocessing +nit

• #ata and instructions need to get into the

system and results out"nput/output

• )emporary storage of code and results isneeded

"$ain memory



Computer Components:

Top Level View



How a program wors

• #ata and instructions are stored in asingle read0write memory

• )he contents of this memory areaddressable by location

• 'xecution occurs in a sequentialfashion1normally2 from one instruction tothe next.



!nstruction Cycle

• )wo steps:

"Fetch

"'xecute



Fetch Cycle

• -rogram *ounter 1-*2 holds address ofnext instruction to fetch

• -rocessor fetches instruction frommemory location pointed to by -*

• ncrement -*"+nless told otherwise

• nstruction loaded into nstruction3egister 132

• -rocessor interprets instruction andperforms required actions



Four main categories of instructions

• -rocessor0memory

"data transfer between *-+ and main memory

• -rocessor /%

"#ata transfer between *-+ and /% module

• #ata processing"Some arithmetic or logical operation on data

• *ontrol

"Alteration of sequence of operations"e.g. 4ump



"#ample of $rogram "#ecution



!nterrupts

• $echanism by which other modules 1e.g./%2 may interrupt normal sequence ofprocessing 1fetch 5 execute cycle2

• -rogram

"e.g. o!erflow di!ision by 6ero• )imer

"7enerated by internal processor timer

"+sed in pre0empti!e multi0tas8ing

• /%

"from /% controller

• 9ardware failure

"e.g. memory parity error



$rogram Flow Control



!nterrupt Cycle

• Added to instruction cycle

• -rocessor chec8s for interrupt

"ndicated by an interrupt signal

• f no interrupt fetch next instruction

• f interrupt pending:"Suspend execution of current program

"Sa!e context

"Set -* to start address of interrupt handlerroutine

"-rocess interrupt

"3estore context and continue interrupted

program



Transfer of Control via !nterrupts



!nstruction Cycle with !nterrupts



$rogram Timing

%hort !&O Wait



!nstruction Cycle 'with !nterrupts( )

%tate *iagram





Changes in Memory and Registers for anInterrupt



Changes in Memory and Registers for anInterrupt



+ultiple !nterrupts

• #isable interrupts

"-rocessor will ignore further interrupts whilstprocessing one interrupt

"nterrupts remain pending and are chec8edafter first interrupt has been processed

"nterrupts handled in sequence as they occur

• #efine priorities

",ow priority interrupts can be interrupted by

higher priority interrupts"When higher priority interrupt has been

processed processor returns to pre!iousinterrupt



+ultiple !nterrupts ) %e,uential

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Lesson 1 How a Program Executes 2015