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Operating Systems
Lecture # 3
Recap
Application
System Call
Trap
Operating System
Processor
Hardware
Trap
Hardware
Time-sharing OS
• OS emulates that all processes are making
simultaneous progress
• Batch mode does not lie all that well
• Executes one process to completion, then runs the next
one
• OS is simpler since no pre-emptive context switching
• Time-sharing OS puts up a better pretence
• Every process gets a time quantum periodically -- e.g.
every 2 msec -- so that everyone makes some progress
simultaneously
• The OS must include support for pre-emptive contextswitching
Concept of a Task in an
Operating System
• Some way to allocate resources on the
hardware for the application
• Space (memory)
• Data structures for running computation
• Some way to execute the application
• May be more than execution paths in the
application code
• Typically called an “execution thread”
What is a process?
Frame1
Stack
Frame2
Heap
Static
Variables
Code
Corresponding to
each subroutine
Invocation
Hello(a, b) --> push frame
..
Return; --> pop frame
What is a thread?
Shared “Heap”
Static Variables
•Different PC
Stack1
•Different
Registers
Stack2
Process Code
•Almost same
“memory”
Context-switching
• Capture the state of the running process
• Freeze it
• Such that you can re-incarnate it later on
• Simplest context-switch
• Subroutine “return”
• Can implement yourself in user space
• Called “Cooperative scheduling” or co-routine
scheduling
Inside the OS
• When to context-switch
• When make a system call
• User to Kernel
• When make a blocking system call
• Read from disk or keyboard
• Process time quantum runs out
• Pre-emptive scheduling
• Non-text book detail
• Scheduling is done when returning from a system
call
Process States
Scheduling
• FCFS
• Round-robin
• Priority