OS Structure, Processes

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OS Structure, Processes Philosophizing

• Vivek Pai / Kai Li
• Princeton University

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But First, A Word From Our Sponsor

• Project 1 you have to do two things
• bootblock.s
• createimage.c
• Both are conceptually simple
• Just a simple matter of programming
• Fred Brooks Mythical Man-Month

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What you need to do

• Create bootblock.s
• Compile it
• Using createimage, append dummy kernel
• Transfer image to floppy disk
• Boot from floppy disk

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Monolithic
User program
User program

• All kernel routines are together
• A system call interface
• Examples
• Linux
• Most Unix OS
• NT

return
call
entry
Kernel many many things
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Monolithic Pros and Cons

• Pros
• Relatively few crossings
• Shared kernel address space
• Performance
• Cons
• Flexibility
• Stability
• Experimentation

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Layered Structure

• Hiding information at each layer
• Develop a layer at a time
• Examples
• THE (6 layers)
• MS-DOS (4 layers)

Level N
. . .
Level 2
Level 1
Hardware
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Layering Pros and Cons

• Pros
• Separation of concerns
• Simplicity / elegance
• Cons
• Boundary crossings
• Performance?

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Microkernel
User program
Services

• Micro-kernel is micro
• Services are implemented as regular process
• Micro-kernel get services on behalf of users by
  messaging with the service processes
• Example Taos, Mach

return
call
entry
m-kernel
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Microkernel Pros and Cons

• Pros
• Easier to develop services
• Fault isolation
• Customization
• Smaller kernel gt easier to optimize
• Cons
• Lots of boundary crossings
• Really poor performance

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Virtual Machine

• Virtual machine monitor
• provide multiple virtual real hardware
• run different OS codes
• Example
• IBM VM/370
• virtual 8086 mode
• Java

user
user
OS1
OSn
. . .
VM1
VMn
Small kernel
Bare hardware
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Hardware Support

• What is the minimal support?
• Can virtual machine be protected without such
  support?
• Hint what is a Turing machine?

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Time For Philosophizin

• I can out-learn you,
• I can out-read you,
• I can out-think you,
• I can out-philosophize you

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What Is a Program?

• Idea / desire / specification
• Source code
• Compiled executable
• But what brings it to life?

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Programs Have a Time Component

• What is the program doing now?
• Execution state
• Program state
• Can two people do different things with the same
  program?
• Can one person? How?

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Resource Virtualization

• CPU
• Memory
• Devices

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Concurrency and Process

• Problem
• A shared CPU, many I/O devices and lots of
  interrupts
• Users feel they have machine to themselves
• Answer
• Decompose hard problems into simple ones
• Deal with one at a time
• Process is such a unit

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Process

• Sequential execution
• No concurrency inside a process
• Everything happens sequentially
• Process state
• Registers
• Main memory
• Files in UNIX
• Communication ports

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Program and Process
main() ... foo() ... foo()
... Program
main() ... foo() ... foo()
... Process
heap
stack main foo
registers PC
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Process vs. Program

• Process gt program
• Program is just part of process state
• Example many users can run the same program
• Process lt program
• A program can invoke more than one process
• Example Fork off processes to lookup webster

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Process State Transition
terminate
Scheduler dispatch
Running
Wait for resource
Create a process
Ready
Blocked
Resource becomes available
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Process Control Block(Process Table)

• What
• Process management info
• State (ready, running, blocked)
• Registers, PSW, parents, etc
• Memory management info
• Segments, page table, stats, etc
• I/O and file management
• Communication ports, directories, file
  descriptors, etc.

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Discussion Question

• What need to be saved and restored on a context
  switch and how?
• What are the implications?