Operating System Foundations

1
Operating System Foundations

• Lecture 2

2
Operating System Functions
Application
Application
Application
Processor
Memory
Mouse
Sound-card
Hard-disk
Frame-buffer
Printer
3
Operating System Functions
Application
Application
Application
Operating System
Processor
Memory
Mouse
Sound-card
Hard-disk
Frame-buffer
Printer
4
OS Functions

• OS as layer of protection

Process 1

• Protect
• Memory
• I/O

Process 2
Process 3
5
OS Protection

• Dont let any single application hog the
  processor
• Only the OS manages the clock
• Set timers
• Kick out a process when the time expires
• Take control of the processor every so often when
  there are system-wide things to do (I/O, memory
  management, scheduling).

6
OS Protection

• Privileged Instructions
• Only access via a privilege mode
• Make a distinction between User and supervisor
  mode
• Only supervisor mode can mask interupts
• Only supervisor mode can access memory management
  structures
• Only supervisor mode can do I/O

7
Operating Systems Functions

• OS as Abstraction Layer
• System Call Interface
• Systems Calls
• For interacting with Hardware and other processes
• open(file, read_rights)
• read (file_id, block_number)
• alarm(up_call, seconds)
• send_message(socket_id, message)

8
Scheduling
Application
Application
Application
Operating System Scheduler
Processor
Main Memory
ALU
9
Scheduling
10
Context Switching
Process B
OS
Process A
Save state A
Restore state B
Save state B
Restore state A
11
Memory Management
0x0FFF
Process 1
0x00FF
Process 2
0x00F0
Process 3
0x0000
12
Memory Management

• An alternative way and intuitively easier to
  understand
• Process 1 Memory Segment 1,2
• Process 2 Segment 3
• Process 3 Memory segment 7,8,9
• Every memory access is checked in hardware to
  verify that you are only accessing your own
  memory segment
• OS provides IPC!

13
I/O

• Operating System as a multiplexer of I/O devices
• Protect Resources, yet let processes share them

Set User mode (privilges Instruction)
User Mode
Privilege Mode
System Calls, Interrupts
14
I/O

• Open(device, rights)
• Privileged Instruction
• Process -- gt Open Devices Table