Lecture 2: OS Overview Part 2

COMP 3400 Lecture 2: OS Overview Part 2

Operating System Structures

We've looked at major computer system structures that support operating system functionality. Now let's consider major OS functions and design considerations.

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

All the management responsibilities need to be designed so that policy is separated from mechanism.

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.

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 2.8.

System programs

These are programs provided by the OS to the interactive user. Some simply invoke system calls on the user's behalf, others are much more complex.

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 2.12 through 2.16 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

This can be visualized with OS layers as concentric circles or boxes surrounding hardware. Each layer provides services to the layers that surround it and is implemented using only the services provided by the layers it encloses. This approach is highly modular and easier to maintain, but performance suffers due to the multiple layers and it is difficult to decide which OS services belong in which layer. OS/2 and the first version of Windows NT were highly layered.

Modern Unix: kernel modules

More object-oriented. Core kernel and dynamically loadable kernel modules. Modern Unix variations: Solaris, Linux, Mac OS X, employ this. Has advantage of microkernel w/o disadvantage of message passing. Has protection advantage of layered w/o disadvantage of strict hierarchy (modules can call each other).

VMWare: virtual machines

The other OS architectures all have a kernel layer directly interfacing with hardware. Another strategy is to place an additional OS layer called the virtual machine between the kernel and the hardware. The virtual machine does not provide any functionality to the kernel but instead provides an identical interface as that of the hardware itself. The kernel thinks it is interacting directly with bare hardware. Advantage is that each process can have its own kernel (or OS) and different processes can use different OS's concurrently on the same hardware - even simple single user OS's.

VMWare and Virtual PC (Microsoft) are application programs that provide this capability. Java's JVM (Java Virtual Machine) and .NET's CLR (Common Language Runtime) also follow the virtual machine concept to provide "managed execution".

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Peter Sanderson (PSanderson@otterbein.edu)