Stallings William Computer Organization and Architecture 6th Edition Chapter 8 Operating System Support
(revised 10/28/02)
Objectives and Functions Convenience Efficiency
--Making the computer easier to use --Allowing better use of computer resources
Layers and Views of a Computer System
Operating System Services Program creation Program execution Access to I/O devices Controlled access to files System access Error detection and response Accounting
O/S as a Resource Manager
Types of Operating System Interactive Batch Single program (Uniprogramming) Multiprogramming (Multitasking)
Early Systems Late 1940s to mid 1950s No Operating System Programs interact directly with hardware Two main problems:
--Scheduling --Setup time
Simple Batch Systems Resident Monitor program Users submit jobs to operator Operator batches jobs Monitor controls sequence of events to process batch When one job is finished, control returns to Monitor which reads next job Monitor handles scheduling
Memory Layout for Resident Monitor
Job Control Language Instructions to Monitor Usually denoted by $ e.g.
--$JOB --$FTN Some Fortran instructions --... --$LOAD --$RUN Some data --... --$END
Other Desirable Hardware Features Memory protection Timer
--To protect the Monitor --To prevent a job monopolizing the system --Only executed by Monitor --e.g. I/O --Allows for relinquishing and regaining control
Privileged instructions Interrupts
Multi-programmed Batch Systems I/O devices very slow When one program is waiting for I/O, another can use the CPU
Single Program
Multi-Programming with Two Programs
Multi-Programming with Three Programs
Sample Program Execution Attributes
Utilization
Effects of Multiprogramming on Resource Utilization
Time Sharing Systems Allow users to interact directly with the computer Multiprogramming allows a number of users to interact with the computer
--i.e. Interactive
Scheduling Key to multiprogramming Long term Medium term Short term I/O
Long Term Scheduling Determines which programs are submitted for processing i.e. controls the degree of multiprogramming Once submitted, a job becomes a process for the short term scheduler (or it becomes a swapped out job for the medium term scheduler)
Medium Term Scheduling Part of the swapping function (more later...) Usually based on the need to manage multi programming If no virtual memory, memory management is also an issue
Short Term Scheduler Dispatcher Fine grained decisions of which job to execute next i.e. which job actually gets to use the processor in the next time slot
Five-State Process Model
Halted
Waiting
Process Control Block Identifier State Priority Program counter Memory pointers Context data I/O status Accounting information
PCB Diagram
Key Elements of O/S
Process Scheduling
Memory Management Uniprogram
--Memory split into two --One for Operating System (monitor) --One for currently executing program --"User" part is subdivided and shared among active processes
Multiprogram
Swapping Problem: I/O is so slow compared with CPU that even in multiprogramming system, CPU can be idle most of the time Solutions:
--Increase main memory --Swapping
Expensive Leads to larger programs
What is Swapping? Long term queue of processes stored on disk Processes "swapped" in as space becomes available As a process completes it is moved out of main memory If none of the processes in memory are ready (i.e. all I/O blocked)
--Swap out a blocked process to intermediate queue --Swap in a ready process or a new process --But swapping is an I/O process...
Partitioning Splitting memory into sections to allocate to processes (including Operating System) Fixedsized partitions
--May not be equal size --Process is fitted into smallest hole that will take it (best fit) --Some wasted memory --Leads to variable sized partitions
Fixed Partitioning
Variable Sized Partitions (1) Allocate exactly the required memory to a process This leads to a hole at the end of memory, too small to use When all processes are blocked, swap out a process and bring in another New process may be smaller than swapped out process Another hole
--Only one small hole less waste
Variable Sized Partitions (2) Eventually have lots of holes (fragmentation) Solutions:
--Coalesce Join adjacent holes into one large hole --Compaction From time to time go through memory and move all hole into one free block (c.f. disk de fragmentation)
Effect of Dynamic Partitioning
Relocation No guarantee that process will load into the same place in memory Instructions contain addresses address Logical relative to beginning of program Physical address actual location in memory (this time) Automatic conversion using base address
--Locations of data --Addresses for instructions (branching)
Paging Split memory into equal sized, small chunks page frames Split programs (processes) into equal sized small chunks pages Allocate the required number page frames to a process Operating System maintains list of free frames A process does not require contiguous page frames Use page table to keep track
Logical and Physical Addresses - Paging
Virtual Memory Demand paging Page fault
--Do not require all pages of a process in memory --Bring in pages as required --Required page is not in memory --Operating System must swap in required page --May need to swap out a page to make space --Select page to throw out based on recent history
Thrashing Too many processes in too little memory Operating System spends all its time swapping Little or no real work is done Disk light is on all the time
Solutions
--Good page replacement algorithms --Reduce number of processes running --Fit more memory
Bonus We do not need all of a process in memory for it to run We can swap in pages as required So we can now run processes that are bigger than total memory available! Main memory is called real memory User/programmer sees much bigger memory virtual memory
Inverted Page Table Structure
Translation Lookaside Buffer Every virtual memory reference causes two physical memory access Use special cache for page table
--TLB --Fetch page table entry --Fetch data
TLB Operation
TLB and Cache Operation
Segmentation Paging is not (usually) visible to the programmer Segmentation is visible to the programmer Usually different segments allocated to program and data May be a number of program and data segments
Advantages of Segmentation Simplifies handling of growing data structures Allows programs to be altered and recompiled independently, without relinking and reloading Lends itself to sharing among processes Lends itself to protection Some systems combine segmentation with paging
Pentium II
Hardware for segmentation and paging Unsegmented unpaged
-- virtual address = physical address -- Low complexity -- High performance
Unsegmented paged
Segmented unpaged
-- Memory viewed as paged linear address space -- Protection and management via paging -- Berkeley UNIX -- Collection of local address spaces -- Protection to single byte level -- Translation table needed is on chip when segment is in memory -- Segmentation used to define logical memory partitions subject to access control -- Paging manages allocation of memory within partitions -- Unix System V
Segmented paged
Pentium II Address Translation Mechanism
Pentium II Segmentation Each virtual address is 16bit segment and 32 bit offset 2 bits of segment are protection mechanism 14 bits specify segment Unsegmented virtual memory 232 = 4Gbytes Segmented 246=64 terabytes
--Can be larger depends on which process is active --Half (8K segments of 4Gbytes) is global --Half is local and distinct for each process
Pentium II Protection Protection bits give 4 levels of privilege
--0 most protected, 3 least --Use of levels software dependent --Usually level 3 for applications, level 1 for O/S and level 0 for kernel (level 2 not used) --Level 2 may be used for apps that have internal security e.g. database --Some instructions only work in level 0
Pentium II Paging Segmentation may be disabled Two level page table lookup
--First, page directory --In which case linear address space is used
--Use TLB holding 32 page table entries --Two page sizes available 4k or 4M
1024 entries max Splits 4G linear memory into 1024 page groups of 4Mbyte Each page table has 1024 entries corresponding to 4Kbyte pages Can use one page directory for all processes, one per process or mixture Page directory for current process always in memory
PowerPC Memory Management Hardware 32 bit paging with simple segmentation Or, both do block address translation
--64 bit paging with more powerful segmentation --Map 4 large blocks of instructions & 4 of memory to bypass paging --e.g. OS tables or graphics frame buffers --12 bit byte selector --16 bit page id
=4kbyte pages
32 bit effective address
--4 bits indicate one of 16 segment registers
Segment registers under OS control
64k pages per segment
PowerPC 32-bit Memory Management Formats
PowerPC 32-bit Address Translation
Recommended Reading Stallings, W. Operating Systems, Internals and Design Principles, Prentice Hall 1998 Loads of Web sites on Operating Systems