Electrical computers and digital processing systems: support – Reconfiguration
Reexamination Certificate
2000-03-31
2003-10-14
Heckler, Thomas M. (Department: 2185)
Electrical computers and digital processing systems: support
Reconfiguration
Reexamination Certificate
active
06633977
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to information processing technology. More particularly, the present invention relates to a system and method for simplifying the duplication user environment data in a computer system.
2. Description of the Related Art
The UNIX operating system is an interactive time-sharing operating system invented in 1969. The UNIX operating system is a multi-user operating system supporting serial and network connected terminals for multiple users. UNIX is a multitasking operating system allowing multiple users to use the same system simultaneously. The UNIX operating system includes a kernel, shell, and utilities. UNIX is a portable operating system, requiring only the kernel to be written in assembler, and supports a wide range of support tools including development, debuggers, and compilers.
As a multi-user operating system, UNIX allows multiple people to share the same computer system simultaneously. UNIX accomplishes this by time-slicing the computer's central processing unit, or “CPU,” into intervals. Each user gets a certain amount of time for the system to execute requested instructions. After the user's allotted time has expired, the operating system intervenes by interrupting the CPU, saving the user's program state (program code and data), restores the next user's program state and begins executing the next user's program (for the next user's amount of time). This process continues indefinitely cycling through all users using the system. When the last user's time-slice has expired, control is transferred back to the first user again and another cycle commences.
The UNIX operating system is both a multi-user operating system and a multi-tasking operating system. As the name implies, the multi-user aspect of UNIX allows multiple users to use the same system at the same time. As a multi-tasking operating system, UNIX permits multiple programs (or portions of programs called threads of execution) to execute at the same time. The operating system rapidly switches the processor between the various programs (or threads of execution) in order to execute each of the programs or threads. IBM's OS/2 and Microsoft's Windows 95/98/NT are examples of single-user multi-tasking operating systems while UNIX is an example of a multiuser multi-tasking operating system. Multi-tasking operating systems support both foreground and background tasks. A foreground task is a task that directly interfaces with the user using an input device and the screen. A background task runs in the background and does not access the input device(s) (such as the keyboard, a mouse, or a touch-pad) and does not access the screen. Background tasks include operations like printing which can be spooled for later execution.
The UNIX operating system keeps track of all programs running in the system and allocates resources, such as disks, memory, and printer queues, as required. UNIX allocates resources so that, ideally, each program receives a fair share of resources to execute properly. UNIX doles out resources using two methods: scheduling priority and system semaphores. Each program is assigned a priority level. Higher priority tasks (like reading and writing to the disk) are performed more regularly. User programs may have their priority adjusted dynamically, upwards or downwards, depending on their activity and the available system resources. System semaphores are used by the operating system to control system resources. A program can be assigned a resource by getting a semaphore by making a system call to the operating system. When the resource is no longer needed, the semaphore is returned to the operating system, which can then allocate it to another program.
Disk drives and printers are serial in nature. This means that only one request can be performed at any one time. In order for more than one user to use these resources at once, the operating system manages them using queues. Each serial device is associated with a queue. When a programs wants access to the device (i.e., a disk drive) it sends a request to the queue associated with the device. The UNIX operating system runs background tasks (called daemons), which monitor the queues and service requests for them. The requests are performed by the daemon process and the results are returned to the user's program.
Multi-tasking systems provide a set of utilities for managing processes. In UNIX, these are ps (list processes), kill (kill a process), and & at the end of a command line (run a process in the background). In UNIX, all user programs and application software use the system call interface to access system resources such as disks, printers, and memory. The system call interface in UNIX provides a set of system calls (C language functions). The purpose of the system call interface is to provide system integrity, as all low-level hardware access is under the control of the UNIX operating system and not the user-written programs. This prevents a program from corrupting the system.
Upon receiving a system call, the operating system validates its access permission, executes the request on behalf of the requesting program, and returns the results to the requesting program. If the request is invalid or the user does not have access permission, the operating system does not perform the request and an error is returned to the requesting program. The system call is accessible as a set of C language functions, as the majority of UNIX is written in the C language. Typical system calls are: _read—for reading from the disk; _write—for writing to the disk; _getch—for reading a character from a terminal; _putch—for writing a character to the terminal; and _ioctl—for controlling and setting device parameters.
The Kernel
As the name implies, the kernel is at the core of the UNIX operating system and is loaded each time the system is started, also referred to as a system “boot.” The kernel manages the resources of the system, presenting them to the users as a coherent system. The user does not have to understand much, if anything, about the kernel in order to use a UNIX system. The kernel provides various necessary functions in the UNIX environment. The kernel manages the system's memory and allocates it to each process. It takes time for the kernel to save and restore the program's state and switch from one program to the next (called dispatching). This action needs to execute quickly because time spent switching between programs takes away from the time available to actually run the users' programs. The time spent in the “system state” where the kernel performs tasks like switching between user programs is the system overhead and should be kept as low as possible. In a typical UNIX system, system overhead should be less than 10% of the overall time.
The kernel also schedules the work to be done by the central processing unit, or “CPU,” so that the work of each user is carried out efficiently. The kernel transfers data from one part of the system to another. Switching between user programs in main memory is also done by the kernel. Main system memory is divided into portions for the operating system and user programs. Kernel memory space is kept separate from user programs. When insufficient main memory exists to run a program, another program is written out to disk (swapped) to free enough main memory to run the first program. The kernel determines which program is the best candidate to swap out to disk based on various factors. When too many programs are being executed on the system at the same time, the system gets overloaded and the operating system spends more time swapping files out to disk and less time executing programs causing performance degradation. The kernel also accepts instructions from the “shell” and carries them out. Furthermore, the kernel enforces access permissions that are in place in the system. Access permissions exist for each file and directory in the system and determine whether other users can access, execute, or m
Hamilton, II Rick Allen
Lipton Steven Jay
Heckler Thomas M.
Leeuwen Joseph T. Van
Mims Jr. David A.
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