Software Layers 2 Introduction to unix 2

How the OS allows simultaneously running of programs

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How the OS controls main memory

How the OS allows simultaneously running of programs

Computers usually have only one CPU (note: more CPUs  more problems...)

note: for programs to run simultaneously  must use CPU at the same time.

the OS schedules small amounts of time (milliseconds) for programs to use the CPU
the Fetch-Decode-Execute Cycle may proceed during these small amounts of time on a given program  constant switching of programs gives the impression of all programs running together

eg: 3 programs are running: pine, pico, tcsh  the OS may schedule these programs like this:

Time (ms)	Running Program	Waiting Program(s)
0.000	tcsh	pine, pico
0.001	pine	pico, tcsh
0.002	pico	tcsh, pine

Round-Robin Scheduling  This kind of scheduling continuously cycles over the waiting programs

Some different way of scheduling programs: first-come first-served, shortest job first, priority scheduling, round-robin, etc.

How the OS controls main memory

programs execute in a block of main memory (the blocks are separate from each other)
one program for one block of main memory
when a program ends, the OS reclaims the memory block

note: If there is not enough memory  programs wait until the OS can provide memory and the OS re-organises the blocks so that waiting is reduced

The OS uses the MMU (memory management unit) of a CPU:

the MMU allows communication between CPU & memory
it makes sure memory requests are valid
typically, programs have "logical" memory addresses from which "physical" memory addresses are computed by the MMU (& vice versa)
this allows the OS to relocate programs in memory if necessary!

eg: Two programs need to be allocated memory: xgdb needs 3MB pine needs 0.5MB

Memory currently contains:

Address (MB)	Contents
0 to 1	OS
1 to 2	Graphics Buffer
2 to 4	Device Drivers
4 to 8	free

The OS allocates xgdb & then pine

(biggest program first):

Address (MB)	Contents
0 to 1	OS
1 to 2	Graphics Buffer
2 to 4	Device Drivers
4 to 7	xgdb
7 to 7.5	pine
7.5 to 8	free

Then xgdb finishes, & the OS reclaims the 3MB:

Address (MB)	Contents
0 to 1	OS
1 to 2	Graphics Buffer
2 to 4	Device Drivers
4 to 7	free
7 to 7.5	pine
7.5 to 8	free

note: If a new program requests 2.5MB

pine moved to 4-4.5
new program occupies 4.5-7

How does the OS handle file requests?

Files are stored in secondary storage (eg: disk drives)

Files change over time and occupy various parts of a disk:
A file look-up table records where the file is stored.

Potential problems arise when:

2 programs try to change the same file at the same time
a program does not have correct access privileges

Directory: student
student -> How to Create a Student Recital Program
student -> Tracks, Mixes, Masters, and More!: Analyzing Preservation Priorities for Multitrack Recordings
student -> A chronology 1660-1832 The Restoration Settlement
student -> Introduction
student -> Disaster planning and recovery: post-katrina lessons for mixed media collections
student -> Comparing Hurricanes and Hurricane Seasons
student -> Storm Surge: a rising Danger
student -> Dr. Jeff Masters' WunderBlog
student -> April 9, 2008 Hurricanes (Continued)
student -> Instructions for Writing Hurricane Term Paper

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