S. Venkatesan Department of Computer Science Spring 2003 Operating Systems Principles Class 1

S. Venkatesan Department of Computer Science Spring 2003 Views of Operating Systems  Resource manager  Control program that controls all the devices  Interface between the machine and the user

S. Venkatesan Department of Computer Science Spring 2003 Overview of a Computer System  OS uses/manages HW resources to provide service to users  Main components are: (a) Processor, (b) Main memory, © I/O devices  Processor has program counter (PC), instruction register (IR), MAR, MDR, etc.

S. Venkatesan Department of Computer Science Spring 2003 Instruction execution  Fetch next instruction from memory  Decode instruction  Fetch operand from memory if needed  Execute instruction  Store result in memory if needed  Repeat this..

S. Venkatesan Department of Computer Science Spring 2003 Interrupts  A mechanism by which the normal flow of program execution is changed  Identify type of interrupt  Service interrupt  Resume original execution  Can disable interrupts (when? Why?)

S. Venkatesan Department of Computer Science Spring 2003 I/O devices  Store/generate data  Transfer data – Programmed I/O [CPU constantly checks status] – Interrupt driven: CPU waits for an interrupt – DMA (Direct Memory Access): Transfer large volume of data

S. Venkatesan Department of Computer Science Spring 2003 Evolution of Operating Systems  Today’s OS evolved over time  Many techniques used in old OS are very useful

S. Venkatesan Department of Computer Science Spring 2003 I. Early Systems (I generation)  No OS  Users operate like an open shop – sign up for slots (like renting) User has complete control of the machine Utilization is very low; set up time is high

S. Venkatesan Department of Computer Science Spring 2003 II. Early batch systems  Motivation: Increase CPU utilization  Use a slow computer to aggregate programs and feed into fast computer.  Same on the output side  Issues; – Job Control Cards – Protection of OS and Users

S. Venkatesan Department of Computer Science Spring 2003 III Sophisticated batch systems  Refine early batch systems  1. Buffering and double buffering  SPOOLing  Multipogramming

S. Venkatesan Department of Computer Science Spring 2003 IV. Time-sharing systems  Combination of early systems (I generation) and multiprogramming  Time-sharing=interactive programming+multiprogramming  Suitable for small jobs

S. Venkatesan Department of Computer Science Spring 2003 V. Multiprocessor Systems  Multiple CPUs; two kinds – All CPUs attached to common main memory (parallel processing) NCUBE, Hybercube – Separate memory and large distances between CPUs-- distributed processing; WWW

S. Venkatesan Department of Computer Science Spring 2003 Real-time systems  Programs with tight deadline constraints; not much of slack time..  Examples – Flight control – Process control in factories/nuclear reactors – Military situations

S. Venkatesan Department of Computer Science Spring 2003 Two approaches to implement OS  Monolithic – OS is one big program – When executing, disable all interrupts – After execution, enable interrupts  Kernal or Nucleus – A small portion of code executed in an interrupt-disabled mode; others?can be interrupted

S. Venkatesan Department of Computer Science Spring 2003 Characteristics of modern OS  Microkernal architecture – Few function to kernal (address space, IPC, scheduling, etc.) – Other services are provided by user processes in user mode – Customize servers, simplifies implementation, flexibility

S. Venkatesan Department of Computer Science Spring 2003 Characteristics of modern OS; Continued  Multithreading – light duty processes – DB server – web server  Symmetric multiprocessing – >1 identical CPUs – Same memory, I/O, etc. – Incremental growth