An Architectural View of an OS

An Architectural View of an OS ProcessControlSubsystem Inter-processcommunication Process Scheduling User programs Libraries User level System Call Interface File Subsystem Memory Mgt Subsystem Kernel level Device Drivers Hardware Control Hardware level Hardware

System Call • System call • A set of functions/procedures to interface user-level programs and the underlying kernel • Used to create, use and delete objects in a kernel. • e.g., Processes • fork(), exit(), kill(), etc. • Files • create(), open(), close(), write(), read(), mkdir(), chmod(), etc. • Memory space • Network connections

Process State Transition Main Memory Layout A B C 0 ….. x Process Scheduler Process Scheduler a Process A b Process B B C A process c Process C C B Program counter(hardware register) A t

Main Memory 0 a+0 x a+1 Process Scheduler A program consisting of N instructions in anassembly language a+2 a …… Process A a+N-1 b Process B Running an process= Executing a program on a CPU= Executing a sequence of assembly languageinstructions on a CPU c Process C

Main Memory CPU T Instructions executed on a CPU 0 • a+0 • a+1 • a+2 • a+3 • a+4 • x+0 • x+1 • b+0 • b+1 • b+2 • x+0 • x+1 • c+0 • c+1 • c+2 • c+3 • c+4 • x+0 • x+1 x x+0x+1 Process Scheduler Running A a a+0a+1 Time out Process A … b Running B b+0b+1 Process B I/O request Process switching … c c+0c+1 Running C Process C … Time out

a+0 • a+1 • a+2 • a+3 • a+4 • x+0 • x+1 • b+0 • b+1 • b+2 • x+0 • x+1 • c+0 • c+1 • c+2 • c+3 • c+4 • x+0 • x+1 Running A Time out Process switching Running B I/O request Process switching Running C Time out Process switching

How to Implement Process States? Dispatched to a CPU admitted New Ready Running exit Terminated interrupted I/O or eventcompletion I/O or event wait Waiting (Blocked)

Dispatched to a CPU admitted New Ready Running exit Terminated interrupted I/O or eventcompletion I/O or event wait Waiting (Blocked) Ready Queue CPU exit dispatched admitted interrupted I/O or eventcompletion Blocked Queue I/O or event wait

Memory Layout and Memory Main Memory Layout 0 a+0 x a+1 Process Scheduler a+2 a …… Process A a+N-1 b A program consisting of N instructions in anassembly language Process B Assumption: All processes exitin the main memory. What if the total size of processesexceeds main memory’s size? c Process C Program Counter

Process Swapping • Process swapping • Moving a part or all of a process from main memory to disk. • Virtual memory • Who to be swapped out from main memory? • Blocked processes • They cannot run until an event (I/O op) completes. Why not swap it out? • Memory space is available for other processes.

6-State Process Behavior Model • When the swap-out operation is performed, there are two choices to select a process to bring into main memory. • A new processes being admitted • A suspended process • A preference should be given on a suspended one • to execute and complete a partially-executed process, rather than increasing the total memory consumption. Dispatched to a CPU admitted New Ready Running exit Terminated interrupted activated I/O or eventcompletion I/O or event wait Waiting (Blocked) Suspended Suspended (swapped)

However… • All suspended processes were in the blocked state at the time of suspension. • It makes no sense to bring a blocked (and suspended) process back in main memory. • It is not ready for execution. • Need to know whether a suspended process is still waiting for an event or ready for execution. • New states needed • 2x2 combination of (blocked or not) and (suspended or not)

7-State Process Behavior Model New • Ready: The process is in main memory and available for execution. • Blocked: The process is in main memory and blocked on an event. • Blocked, suspended: The process is in virtual memory and blocked on an event. • Ready, suspended: The process is in virtual memory but is available for execution as soon as it is loaded to main memory. admitted Dispatched to a CPU activated Ready,Suspended Ready Running exit Terminated Suspended interrupted Event completion I/O or event wait Blocked Blocked,Suspended activated Suspended

activated • Blocked  Blocked, suspended • When there are no ready processes. • New processes can use freed memory. • When there are ready processes. • Ready processes (and currently-running processes) can use more main memory. Ready,Suspended Ready Suspended Event completion Blocked Blocked,Suspended activated Suspended

activated • Blocked, suspended  Ready, suspended • Upon an event completion. • Ready, suspended  Ready • When there are no ready processes in main memory. • Ready  Ready, suspended • When a currently-running process requires more memory. Ready,Suspended Ready Suspended Event completion Blocked Blocked,Suspended activated Suspended

HW1 • Explain the transition “interrupted” and “event wait.” • Describe how they are different. Dispatched to a CPU admitted New Ready Running exit Terminated interrupted I/O or eventcompletion I/O or event wait Waiting (Blocked)

HW2 Ready Queue CPU exit dispatched admitted interrupted Event 1occurs Event 1 Blocked Queue Event 1 wait Event 2occurs Event 2 Blocked Queue Event 2 wait Event 3occurs Event 2 Blocked Queue Event 3 wait • This process mgt scheme assumes that each process waits for an event. • A process may want to wait for multiple events at the same time. When can this make sense? Provide an example. • How do you design process queues?

HW3 • Choose an OS, and describe • Where the OS places virtual memory • How to configure (change) the total size of virtual memory on the OS.

HW4 • For the 7-state process behavior model • Draw a queuing diagram.

An Architectural View of an OS