1. CS149D Elements of Computer Science
Ayman Abdel-Hamid
Department of Computer Science
Old Dominion University
Lecture 9: 9/24/2002
Lecture 9: 9/24/2002
CS149D Fall 2002

2. Outline File Manager Memory Manager and Virtual Memory
Coordinating a machine activities (The Process concept)
Should cover sections 3.2 and 3.3
Lecture 9: 9/24/2002
CS149D Fall 2002

3. Kernel Components revisited
File Manager
Coordinate use of machine’s mass storage
Device drivers
Control operation of machine’s peripheral devices (printer, disk drive, monitor, …)
Memory Manager
Coordinate machine’s use of main memory
Scheduler
Which activity is executed next
Dispatcher
Controls allocation of time slices to activities
Lecture 9: 9/24/2002
CS149D Fall 2002

4. File Manager Keep records of files stored in secondary storage
Location of each file
Which users are allowed to access which files
Available space in secondary storage
Group files into a bundle called a directory or folder
A directory may contain other directories called subdirectories
A chain of directories within directories is called a directory path
Provides means to access files (open and manipulate files)
Lecture 9: 9/24/2002
CS149D Fall 2002

5. Memory Manager One task at a time
Load, execute, replace by program for performing next task
Multi-user or Multi-tasking environments
Many programs and blocks of data reside in main memory
Allocate areas of memory for each program
Fulfill memory requirements for programs
Keep track of memory areas no longer occupied
Lecture 9: 9/24/2002
CS149D Fall 2002

6. Virtual Memory
When total main memory required exceeds actually available?
Memory manager creates illusion of additional memory space by swapping programs and data back and forth between main memory and secondary storage
Illusionary memory space called Virtual Memory
Memory requirements are allocated in units called pages ( page size < 4 KB)
Main memory is 64 MB, but only 32 MB is available?
Memory manager stores contents of pages in secondary storage
When a page in secondary storage is needed for access, load it in main memory
If no space in main memory, swap some other page no longer required to secondary storage
Lecture 9: 9/24/2002
CS149D Fall 2002

7. Concept of a Process
Distinction between a program and the activity of executing a program
Program  static set of directions
Activity of executing a program  dynamic activity  known as a process
Process state
Current position in the program being executed (value of PC)
Values in other registers and associated memory cells
A single program can be associated with more than one process
In a time-sharing computer, processes compete for time slices
Operating System coordinates among processes
Lecture 9: 9/24/2002
CS149D Fall 2002

8. Process Administration
Process coordination handled by scheduler and dispatcher within OS kernel
Scheduler
maintains a record of processes present in computer
Maintains a block of information in main memory called process table
When a new task is assigned to machine, creates a process for that task by placing a new entry in the process table
Memory area assigned to process (from memory manager)
Process priority
Process status (ready, waiting for some event (I/O for example))
Dispatcher
Ensures that scheduled processes are actually executed
Divide time into time slices (or quantum)
Process switching
Lecture 9: 9/24/2002
CS149D Fall 2002

9. Process Administration
At interrupt
CPU completes current machine cycle, saves current process state, and transfers control to an interrupt handler
Interrupt handler is part of the dispatcher
Dispatcher
selects a ready process from the process table (highest priority)
Restart timer circuit for the next interrupt
Allow selected process to begin execution
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Sometimes, the process time slice is terminated before the timer expires
For example when executing I/O
Lecture 9: 9/24/2002
CS149D Fall 2002