C SC 340 Lecture 2: OS Overview Part 2

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OS Responsibilities

OS is very complex, its responsibilities can be grouped into logical categories. Each is introduced in turn.

Process management

• process == running program (not exactly accurate but OK for now)
• processor is the CPU
• processor management is just a matter of making sure the CPU has something useful to do all the time, and process management takes care of this
• includes switching CPU from one process to another
• includes scheduling, creating, suspending, resuming, terminating processes
• includes interprocess communication
• includes process synchronization
• includes deadlock detection, prevention, avoidance, recovery

Memory management

• main goal is to utilize main memory in a way that supports most effective use of CPU
• instructions have to be in main memory to be executed
• to keep CPU busy, it may be necessary to work concurrently with more processes than will fit into main memory
• includes deciding which processes should be allocated memory
• includes allocating memory to processes
• includes deallocating memory from processes

File System management

• file is a logical concept, since it can be stored different ways on different media
• thus file management addresses device-independent issues
• includes file creation, manipulation, deletion, protection
• includes directory creation, manipulation, deletion, protection
• includes file system backup

Mass Storage management

• refers primarily to disk management
• includes management of free disk space
• includes allocating disk space to files/directories
• includes deallocating disk space from files/directories
• includes scheduling of disk read/write operations

I/O device management

• issues common to all I/O devices and specific to each type
• includes interrupt handlers
• includes buffering and caching of data
• includes hardware specific device drivers

Network communication

• includes communication protocols
• includes network configuration
• includes message passing and routing
• includes forming coherent system image from heterogeneous network configuration
• includes network reliability and security

Overarching design issue: mechanism versus policy

• policy refers to deciding what is to be done
• mechanism refers to how it is done, once the decision is made
• process management example:
• mechanism: when a running process terminates, the CPU is given another process to run. The transition is called context switching and involves saving and restoring register values.
• policy: deciding which process to give to the CPU. e.g., scheduling.
• disk management example:
• mechanism: allocating a block of disk space to a file
• policy: deciding which block to allocate
• It is best to separate them because designer will want to change policy without changing mechanism.

Non-traditional systems

• distributed systems (network OS), chap 16
• distributed file systems, chap 17
• coordination in distributed systems (mutual exclusion, deadlock), chap 18
• real-time systems, chap 19
• multimedia systems, chap 20

User Interaction

• interface between interactive user and operating system services
• not considered part of the OS kernel
• includes command-line interpreters (Unix, DOS)
  In the Unix/Linux world, these are known as shells. Anyone (even you!) can write a new shell. It merely maps user commands to system services. The major shells (C shell, Bourne shell, Bourne-again shell, Korn shell) differ in relatively minor ways
• includes graphical user interfaces (Windows, Macintosh)
  Original Windows was simply a GUI layered on top of DOS. Macs were designed for GUI interaction from the start, with CLI available only with OS X overhaul based on Unix kernel. Many GUIs have been developed for Unix/Linux, such as X, CDE, KDE, Gnome.
• consider human-computer interaction issues
  You may laugh at "primitive" command interfaces, but suppose for example you have a directory filled with different types of files and wanted to put all the ".txt" files into a new directory called "text". With a command interface, you could do all that by typing: mkdir text; mv *.txt text How long would it take using a GUI?

System services, calls, and programs

System services

OS provides a number of basic and advanced services to its users: loading and running programs, storing programs and data, operating I/O devices, providing communication, providing protection.

OS also provides services that benefit mostly its own operation and system administration: efficient resource allocation, accounting, security.

System calls

Processes interface with the OS through system calls. Put another way, the OS provides sets of system calls through its API (Application Programmer Interface) to those who will develop software to run on it.

• examples of APIs are WIN32 for Windows, POSIX for Unix/Linux, Java API for JVM
• System calls are predefined and precompiled functions, usually written in C, C++ or assembly.
• typical (layered) structure: system calls are not part of the API itself but are called by the API functions. This is illustrated in Figure 2.6.
• System calls typically operate in kernel mode.
• The API function is thus programmer's interface to system calls and thus priviledged instructions.

There are many different system calls, grouped into the same categories as OS responsibilities listed above (process management, file management, etc). The textbook contains a nice outlined list in Figure 2.5.

System programs

• They are distinguished from system calls in that they are typically used interactively at the user interface.
• In a GUI environment, system programs are typically triggered by clicking on a representative icon.
• E.g. In Windows, double-clicking on a folder (directory) icon called bin makes it current and displays its contents.
• In a command-line interpreter environment, system programs are typically triggered by typing in their names.
• E.g. In Unix/Linux, the commands cd bin followed by ls would make the bin directory current then display its contents.

Ways of structuring an OS

All OS's are complex software systems and so need to be structured in some way. Modern OS's have a layered structure. Textbook figures 2.10 through 2.15 illustrate layered structures used by different OS's. Each is clearly distinct from the others, showing there is no consensus on how layers should be structured. Here are a few approaches.

Original Unix: kernel and system programs

There are a small number of layers. The first layer above the hardware is the OS kernel. The kernel refers to the most essential modules of the OS, those that need to be memory resident at all times. The kernel primarily fulfills process, memory and file responsibilities. System calls are the interface of the kernel to programmers. The next layer is the system programs. Criticized for being too monolithic with too much kernel functionality.

Mach, NT: microkernel

Idea is to keep the kernel as small as possible. This allows OS overall to be more extendable and portable. Also simplifies protection. Kernel may contain only partial process, file, and communication management. Communication is through message passing. NT uses a client-server approach in which both are user mode but communication between them must go through kernel. Thus significant portion of the OS itself operates in user mode.

OS/2: highly layered

Modern Unix: kernel modules

IBM VM: virtual machines