Lecture 3: OS Functions and Design Approaches

1. Lecture 3: OS Functions and Design Approaches • OS duties • process management • memory management • disk/file management • protection & security • interaction with OS • dual-mode operation • system calls • API, system programs, UI • OS design approaches: • monolithic kernel • microkernel • virtual machine

2. Process Management • OS manages many kinds of activities: • user programs • system programs: printer spoolers, name servers, file servers, etc. • a running program is called a process • a process includes the complete execution context (code, data, PC, registers, OS resources in use, etc.) • a process is not a program • program - a sequence of instructions (passive) • process - one instance of a program in execution (active); • many processes can be running the same program and one program may cause to create multiple processes • from OS viewpoint process is a unit of work; OS must: • create, delete, suspend, resume, and schedule processes • support inter-process communication and synchronization, handle deadlocks

3. Memory Management • primary (main) memory (RAM) • provides direct access storage for CPU • processes must be in main memory to execute • OS must: • mechanics • keep track of memory in use • keep track of unused (“free”) memory • protect memory space • allocate, deallocate space for processes • swap processes: memory  disk • policies • decide when to load each process into memory • decide how much memory space to allocate to each process • decide when a process should be removed from memory

4. Disk Management • the size of the disk is much greater than main memory and, unlike main memory, disk is persistent (survives system failures and power outages) • OS hides peculiarities of disk usage by managing disk space at low level: • keeps track of used spaces • keeps track of unused (free) space • keeps track of “bad blocks” • OS handles low-level disk functions, such as: • schedules of disk operations • and head movement

5. File Management • disks provide long-term storage, but are awkward to use directly • file - a logical named persistent collection of data maintained by OS • file system - a logical structure that that is maintained by OS to simplify file manipulation; usually directory based • OS must: • create and delete files and directories • manipulate files and directories - read, write, extend, rename, copy, protect • provide general higher-level services - backups, accounting, quotas • note the difference between disk management and file system management

6. Protection & Security • protection – any mechanism for controlling access of processes or users to resources • disk, memory, CPU, • security – defense of the system against internal and external attacks • systems generally first distinguish among users, to determine who can do what • user identities and privileges associated with this identifier • user ID then associated with all files, processes of that user to determine access control

7. Dual-Mode Operation • to allow protection OS operates in dual-mode • user mode and kernel mode • mode bit provided by hardware • user (1), kernel (0) • enables OS to distinguish when system is running user code or kernel code • some instructions designated as privileged, only executable in kernel mode • changing modes • modifying timers • modifying interrupt service routines • I/O device access

8. System Call Definition • app. program can ask the OS to carry out service forit by invoking a system call • to the application the invocation is similar to an ordinary function call • example: Unix write system call description prompt% man -S 2 write WRITE(2) Linux Programmer's Manual WRITE(2) NAME write - write to a file descriptor SYNOPSIS #include <unistd.h> ssize_t write(int fd, const void *buf, size_t count); DESCRIPTION write() writes up to count bytes to the file referenced by the file descriptor fd from the buffer starting at buf. … RETURN VALUE On success, the number of bytes written are returned …

9. Mode Switch • mode switch – transition from user to kernel mode • system call generates a trap (software generatedinterrupt) • physical interrupt occurs (e.g. a timer interrupt) • control is transferred to the interrupt service routine, which may pass control to the OS (in kernel mode) return it back to the user process • in case OS needs to do extensive work, it needs to save the context (state) of the user program

10. Function Invocation TraceAcross Modes • Example function invocation in Solaris (Unix-like OS from Sun Microsystems) • node the mode (User or Kernel) for function invocation

11. System Call Example • a typical app. program invokes system calls repeatedly • example: copying a file

12. Application Program Interface • “raw” system calls tend to be difficult to use • application program interface (API) defines functions that are more convenient to use • API functions • can be invoked by app. programmer • run in user mode • directly invoke system calls • implemented in system libraries that come with the OS and are linked to the app. program • API examples: POSIX API (implemented by most Unix systems, MacOS X), Java API, Win 32 API – windows • ex: printf (C function that prints formatted output, part of POSIX API) repeatedly invokes write

13. System Programs and UIs • system programs come with the OS • they are designed for end users – the personfor whom the computer/OS/app. programs are designed • cf. application programmers, OS programmers • operate in user mode • typical tasks: file manipulation, status info, file modification (editors), programming language support (compilers/assemblers), configuration, communication, etc. • user interface (UI) is a way the end-user interacts with system programs (and through them with the OS), • command-line interface (CLI) – interaction through command interpreter (shell) – a text-based system program • graphical-user interface (GUI) – mouse-heavy with a lot of graphics • most popular OSes support both kinds of UI

14. app. processes usermode system processes user processes systemcalls kernel machine- signals files CPU scheduling kernelmode terminals swapping page replacement independent character I/O disk, tape virtual memory machine- terminal device memory drivers drivers drivers dependent commands/interrupts hardware device controllers Monolithic Kernel OS Design • advantages: speed and ease of operation (everything is at hand) • disadvantages: • hard to develop, maintain, modify and debug • kernel gets bigger as the OS develops critical OS data structures and device registers are protected from user programs

15. Modular Kernel • classic monolithic kernel requires all functionality to beinbuilt into kernel and loaded at boot-time – waste of RAM, limited flexibility • loadable kernel module – object file to be loaded at boot time or, possibly compile time • extends base kernel functionality, common functions of kernel modules • device drivers/bus drivers, filesystems, system calls, CPU scheduling classes, executable file formats • loaded to kernel address space has full access to all kernel memory • has defined programming interface, possibly protected • Linuxes, Windows, Solaris, Mac OS X support kernel modules

16. system processes user processes user mode thread file CPU system system scheduling commands/interrupts hardware network paging support kernelmode micro-kernel communication protection low-level VM processor control device controllers Microkernel • advantages: reliability, ease of development, modularity - parts can be replaced and tailored to the architecture, user requirements etc. • disadvantages: slow? • examples: MacOS X, Windows XP • small kernel implements communication (usually messages) • when system services are required microkernell calls other parts of OS running in user modes and passes the request there

17. Virtual Machine • OS’s system callsare considered an enhancement ofhardware’sinstruction set • extend further –virtual machine • each user task isprovided with an abstract (virtual machine) which OS + hardware implement • IBM – pioneered • modern examples: • Java VM – next slide • VMware Player – extra guest/host bit, guest OS executes limited set of instructions, if different instruction – trap to host OS adv. – portability at binary-level, security, greater language flexibility dis. – speed(?)

18. Java Virtual Machine (JVM) • Java source code is translated into an architectureindependent java bytecode • bytecode is executed by JVM • JVM can be implemented purely in software or in hardware • JVM verifies bytecode’s correctness and then either interprets (translates the code into machines instructions one by one) • or just-in-time (JIT) compiles to optimize, or both adv. – portability at binary-level, security, greater language flexibility dis. – speed(?)

19. Lecture Review • major OS duties are • process management • memory management • disk/file management • OS interacts with users through system calls, to ease interaction • API - for app. programmers • GUI or CLI for end-users • OS is a big and complex program; traditional monolithic kernel design approach yields OSes that are fast but hard to develop, modify and debug; other approaches have been suggested: • microkernel • virtual machine