02-OS_Structures_2spp

02-OS_Structures_2spp - CSE 421/521 - Operating Systems...

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1 CSE 421/521 - Operating Systems Fall 2011 Tevfik Ko ! ar University at Buffalo September 1 st , 2011 Lecture - II OS Structures 2 Roadmap • OS Design and Implementation Different Design Approaches • Major OS Components ! Processes ! Memory management ! CPU Scheduling ! I/O Management
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3 OS Design Approaches 4 Operating System Design and Implementation • Start by defining goals and specifications • Affected by choice of hardware, type of system Batch, time shared, single user, multi user, distributed User goals and System goals User goals operating system should be convenient to use, easy to learn, reliable, safe, and fast System goals operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error- free, and efficient No unique solution for defining the requirements of an OS ! Large variety of solutions ! Large variety of OS
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5 Operating System Design and Implementation (Cont.) • Important principle: to separate policies and mechanisms Policy : What will be done? Mechanism : How to do something? Eg. to ensure CPU protection Use Timer construct (mechanism) How long to set the timer (policy) The separation of policy from mechanism allows maximum flexibility if policy decisions are to be changed later 6 OS Design Approaches • Simple Structure (Monolithic) • Layered Approach • Microkernels • Modules
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7 Simple Structure • Monolithic • No well defined structure • Start as small, simple, limited systems, and then grow • No Layers, not divided into modules 8 Simple Structure " Example: MS-DOS Silberschatz, A., Galvin, P. B. and Gagne. G. (2003) Operating Systems Concepts with Java (6th Edition). MS-DOS pseudolayer structure # initially written to provide the most functionality in the least space # started small and grew beyond its original scope # levels not well separated: programs could access I/O devices directly # excuse: the hardware of that time was limited (no dual user/kernel mode)
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9 Layered Approach " Monolithic operating systems # no one had experience in building truly large software systems # the problems caused by mutual dependence and interaction were grossly underestimated # such lack of structure became unsustainable as O/S grew " Enter hierarchical layers and information abstraction # each layer is implemented exclusively using operations provided by lower layers # it does not need to know how they are implemented # hence, lower layers hide the existence of certain data structures, private operations and hardware from upper layers 10 Simple Layered Approach UNIX system structure " The original UNIX # enormous amount of functionality crammed into the kernel - everything below system call interface # “The Big Mess”: a collection of procedures that can call any of the other procedures whenever they need to # no encapsulation, total visibility across the system # very minimal layering made of thick, monolithic layers Silberschatz, A., Galvin, P. B. and Gagne. G. (2003) Operating Systems Concepts with Java (6th Edition).
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11 Layered Approach
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02-OS_Structures_2spp - CSE 421/521 - Operating Systems...

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