An OS is a program that controls the execution of application programs, and acts as an interface between applications and the computer hardware.

The most important collection of system programs comprises the OS. The OS masks the details of the hardware from the programmer, and provides the programmer with a convenient interface for using the system.

Briefly, the OS typically provides services in the following areas:

The OS is responsible ofr controlling the use of a computer’s resources, such as I/O, main and secondary memory, and processor execution time.

A user program executes in a user mode, in which certain areas of memory are protected from the user’s use, and in which certain instructions may not be executed. The monitor executes in a system mode, or what has come to be called kernel mode, in which privileged instructions may be executed, and in which protected areas of memory may be accessed.

Even with the automatic job sequencing provided ny a simple batch OS, the processor is often idle. The problem is I/O devices are slow comoared to the processor.

There must be enough memory to hold the OS (resident monitor) and one user program. When one job needs to wait for I/O, the processor can switch to the other job. Furthermore, we might expand memory to hold three, four, or more programs and switch among all of them. The approach is known as multiprogramming, or multitasking. It is the central theme of modern operating systems.

The most notable additional feature that is useful for multiprogramming is the hardware that supports I/O interrupts and DMA (direct memory access). With interrupt-driven I/O or DMA, the processor can issue an I/O command for one job and proceed with the execution of another job while the I/O is carried out by the device controller. When the I/O operation is complete, the processor is interrupted and control is passed to an interrupt-handling program in the OS. The OS will then pass control to another job after the interrupt is handled.

Processor time is shared among multiple users.--------Time sharing Multiple users simultaneously access the system through terminals, with the OS interleaving the execution of each user program in a short burst or quantum of computation.

Compatible Time-Sharing System (CTSS)

At each clock interrupt, the OS regained control and could assign the processor to another user. ---------- Time slicing

Thus, at regular time intervals, the current user would be preempted and another user loaded in.

To preserve the old user program status ofr later resumption, the old user programs and data were written out to disk before the new user programs and data were read in. 在读取新程序和数据前，旧程序将会被写入磁盘。 Subsequently, the old user program code and data were restored in main memory when that program was next given a turn. 旧程序和数据读回（恢复）主内存。

Multiprogramming/Multitasking was designed to keep the processor and I/O devices, including storage devices, simultaneously busy to achieve maximum efficiency.

The key mechanism is this: In response to signals indicating the completion of I/O transactions, the processor is switched among the various programs residing in main memory.

Programming errors whose effects could be observed only when certain relatively rare sequences of actions occurred. Four main causes:

A process as consisting of three components: 1. An executable program 2. The associated data needed by the program (variables, work space, buffers, etc.) 3. The execution context of the program

Execution context/Process state: the internal data by which the OS is able to supervise and control the process. The context includes all of the information the OS needs to manage the process, and the processor needs to execute the process properly. The context includes the contents of the various processor registers, such as the program counter and data registers. It also includes information of use to the OS, such as the priority of the process and whether the process is waiting for the completion of a particular I/O event.

A block of memory is allocated to each process that contains the program, data, and context information. Each process is recorded in a process list built and maintained by the OS. The process list contains one entry for each process, which includes a pointer to the location of the block of memory that contains the process. The remainder of the execution context is tored elsewhere, perhaps with the process itself or frequently in a separate region of memory. The program counter points to the next instruction in that process to be executed . The base and limit registers define the region in memory occupied by the process: The base register is the starting address of the region of memory, and the limit is the size of the region.

process switch: save & restore

The entire state of the process at any instant is contained in its context.

new features —> expanding the context

A single process —> broken up into multiple, concurrent threads

Five principal storage management responsibilities:

Virtual memory is a facility that allows programs to address memory from a logical point of view, without regrad to the amount of main memory physically available. Virtual memory was conceived to meet the requirement of having multiple user jobs concurrently reside in main memory, so there would not be a hiatus(中断) between the execution of successor processes while one process was written out to secondary store and the successor process was read in.

Paging systems Allow processes to be comprised if a number of fixed-size blocks, called pages.

monolithic kernel 宏内核 —> single process (all elements sharing the same address space)

microkernel 微内核

Other OS services are provided by processes, sometimes called servers, that run in user mode and are treated like any other aoolication by the microkernel.

Multithreading is a technique in which a process executing an application, is divided into threads that can run concurrently.

Thread: A dispatchable unit of work. It includes a processor context (which includes the program counter and stack pointer) and its own data area for a stack (to enable subroutine branching), A thread executes sequentially and is interruptible so the processor can turn to another thread.

Process: A collection of one or more threads and associated system resources (such as memory containing both code and data, open files, and devices).

hardware virtualization

Virtualization technology enables a single PC or sever to simultaneously run multiple OSs or multiple sessions of a single OS.

The host OS can support a number of virtual machines(VMs), each of that has the characteristics of a particular OS and, in some versions of virtualization, the characterictics of a particular hardware platform.

Virtual machine monitor (VMM)

A multiprocessor OS must provide all the functionality of a multiprogramming system, plus additional features to accommodate multiple processors.

Key design issues:

system calls

User programs can invoke OS services either directly, or through library programs.

The system call interface is the boundary with the user and allows higher-level software to gain access to specific kernel functions.

System is divided into two main parts: one concerned with process control, and the other concerned with file management and I/O.

The process control subsystem is responsible for memory management, the scheduling and dispatching of processes, and the synchronization and interprocess communication of processes.

The file system exchanges data between memory and external devices.

Linux is structured as a collection of modules, a number of which can be automatically loaded and unloaded on demand. These relatively independent blocks are referred to as loadable modules.

A module is an object file whose code can be linked to and unlinked from the kernel at runtime.

A module implements some specific function, such as a file system, a device driver, or some other feature of the kernel’s upper layer.

A module is executed in kernel mode on behalf of the current process.

Two characteristics:

All of the components execute on the processor.

The principal kernel components are the following: