OS - software that manages the computer's resources

OS - software that manages the computer's resources

1. Concurrency & synchronization

   • Concurrency/multiprogramming - having a process run on the CPU while one or more are doing I/O

2. Intro

   • Referee (manages shared resources, protects processes from one another, allows processes to work together), illusionist (illusion of resources), glue (interface w hw - fs, vm, networks)
   • Evaluations of OS
     • Reliability - do what its supposed to
     • Security - only permitted actions allowed
     • Portability - OS can live forever, doesn't change when hw changes
       • AMI - API + mem access model + executable instructions
       • API - function calls to APPS
       • HAL - Abstraction of hw
     • Performance
       • Efficiency/overhead, Fairness, response time, throughput, predictability
   • History of OS
     • phases
       • 1) one user, one process at a time - low utilization of expensive components
       • 2) batch processing, load program, run output to tape, print, repeat - no protections, computer idle during i/o, better use of hw, debugging is hard
       • 3) overlap of i/o and computations, interrupts - concurrency within process
       • 4) Multiprogramming - one job runs until i/o then another runs, requires mem protection and program mngmt
       • 5) interactive timesharing - lets multiple users interact w the system, req: more sharing, protection, concurrency, probs: response time, thrashing
       • 6) personal computing - computers are cheap now
       • 7) parallel/distributed computing - increased performance and reliability, sharing of resources
   • Dual mode exec - support by hw
     • Why?
       • to enforce restricted rights - malicious apps could: crash apps, violate privacy of other apps, hog all resources, change OS, crash OS
         • hw should support:
           • privileged instructions, timer interrupts, mem protection
     • Processes - executing program = program + execution state
       • Needs: memory and CPU registers
       • Process sstate - state: at least the code, PC, SP, static data, heap, HP, values of CPU reg, PID, execution state
       • PCB (how OS keeps track of a processes data, PCB pointer in ready queue - dynamic kernel data structure): PC, SP, contents of gen purpose registers, mem mgmnt info (HP), usrname of owner, list of open files, PID, parent/child relationships
       • Creating a process: the loader reads and interprets the executable file, sets up the processes' memory (process' data), pushes argc & argv to the stack, sets cpu registers properlly, program starts running
         • process can create another process
           • fork () - parents and child are exact copies
             • parent - it is child process id
             • child (0)
             • same PC, same variables (not shared), have their own memory
           • exec () - overwrtites the program (PID doesnt change)
           • wait () - parent waits for child to terminate, able to get child's return value, parent is asleep, exit() returns child's return, can deallocate the zombie
           • exit () - deallocates and closes files, if parent is alive, it becomes a zombie, else it gets completely deallocated
       • zombies: process terminated but parent not collected status
         • kill () - sends a signal to a specified process
       • signals: user level interrupts
       • Orphaned processes - parent terminates before child, sometimes children are killed
     • User mode: access is restricted
       • may not: address I/O directly , use instructions that manipulate OS memory (e.g., page tables) , set the mode bits that determine user or kernel mode , disable and enable interrupts, halt the machine
         • causes a processor exception
     • Kernel: access is unrestricted
       • Entering the kernel
         • 1) exceptions -(synchronous) doing something silly (divisiion by 0), trying to do kernel stuff
         • 2) interrupts - (asynchronous) timer, i/o
         • 3) system calls/taps - (synchronous) user program needs OS things
           • interface bts the application and the oS API, hw calls the OS at the sys call handler
         • Deets::
           • 1) Save state of user program
             • hw register points to exception stack,
           • 2) hw identifies why its going to kernel
           • 3) hw selects entry from interrupt vector
           • 4) handler is invoked

3. Memory management

4. File Systems: use and implementation

5. Networked systems

6. Parallel and distributed computing

7. security