资源介绍
The Design and Implementation of the 4.4BSD Operating System
Marshall Kirk McKusick
Keith Bostic
Michael J. Karels
John S. Quarterman
Copyright © 1996 Addison-Wesley Longman, Inc
The second chapter of the book, The Design and Implementation of the 4.4BSD Operating System is excerpted here with the permission of the publisher. No part of it may be further reproduced or distributed without the publisher's express written permission. The rest of the book explores the concepts introduced in this chapter in incredible detail and is an excellent reference for anyone with an interest in BSD UNIX. More information about this book is available from the publisher, with whom you can also sign up to receive news of related titles. Information about BSD courses is available from Kirk McKusick.
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Table of Contents
2 Design Overview of 4.4BSD
2.1 4.4BSD Facilities and the Kernel
2.1.1 The Kernel
2.2 Kernel Organization
2.3 Kernel Services
2.4 Process Management
2.4.1 Signals
2.4.2 Process Groups and Sessions
2.5 Memory Management
2.5.1 BSD Memory-Management Design Decisions
2.5.2 Memory Management Inside the Kernel
2.6 I/O System
2.6.1 Descriptors and I/O
2.6.2 Descriptor Management
2.6.3 Devices
2.6.4 Socket IPC
2.6.5 Scatter/Gather I/O
2.6.6 Multiple Filesystem Support
2.7 Filesystems
2.8 Filestores
2.9 Network Filesystem
2.10 Terminals
2.11 Interprocess Communication
2.12 Network Communication
2.13 Network Implementation
2.14 System Operation
References
List of Tables
2-1. Machine-independent software in the 4.4BSD kernel
2-2. Machine-dependent software for the HP300 in the 4.4BSD kernel
List of Figures
2-1. Process lifecycle
2-2. A small filesystem
Chapter 2 Design Overview of 4.4BSD
2.1 4.4BSD Facilities and the Kernel
The 4.4BSD kernel provides four basic facilities: processes, a filesystem, communications, and system startup. This section outlines where each of these four basic services is described in this book.
Processes constitute a thread of control in an address space. Mechanisms for creating, terminating, and otherwise controlling processes are described in Chapter 4. The system multiplexes separate virtual-address spaces for each process; this memory management is discussed in Chapter 5.
The user interface to the filesystem and devices is similar; common aspects are discussed in Chapter 6. The filesystem is a set of named files, organized in a tree-structured hierarchy of directories, and of operations to manipulate them, as presented in Chapter 7. Files reside on physical media such as disks. 4.4BSD supports several organizations of data on the disk, as set forth in Chapter 8. Access to files on remote machines is the subject of Chapter 9. Terminals are used to access the system; their operation is the subject of Chapter 10.
Communication mechanisms provided by traditional UNIX systems include simplex reliable byte streams between related processes (see pipes, Section 11.1), and notification of exceptional events (see signals, Section 4.7). 4.4BSD also has a general interprocess-communication facility. This facility, described in Chapter 11, uses access mechanisms distinct from those of the filesystem, but, once a connection is set up, a process can access it as though it were a pipe. There is a general networking framework, discussed in Chapter 12, that is normally used as a layer underlying the IPC facility. Chapter 13 describes a particular networking implementation in detail.
Any real operating system has operational issues, such as how to start it running. Startup and operational issues are described in Chapter 14.
Sections 2.3 through 2.14 present introductory material related to Chapters 3 through 14. We shall define terms, mention basic system calls, and explore historical developments. Finally, we shall give the reasons for many major design decisions.
2.1.1 The Kernel
The kernel is the part of the system that runs in protected mode and mediates access by all user programs to the underlying hardware (e.g., CPU, disks, terminals, network links) and software constructs (e.g., filesystem, network protocols). The kernel provides the basic system facilities; it creates and manages processes, and provides functions to access the filesystem and communication facilities. These functions, called system calls appear to user processes as library subroutines. These system calls are the only interface that processes have to these facilities. Details of the system-call mechanism are given in Chapter 3, as are descriptions of several kernel mechanisms that do not execute as the direct result of a process doing a system call.
A kernel in traditional operating-system terminology, is a small nucleus of software that provides only the minimal facilities necessary for implementing additional operating-system services. In contemporary research operating systems -- such as Chorus Rozier et al, 1988, Mach Accetta et al, 1986, Tunis Ewens et al, 1985, and the V Kernel Cheriton, 1988 -- this division of functionality is more than just a logical one. Services such as filesystems and networking protocols are implemented as client application processes of the nucleus or kernel.
The 4.4BSD kernel is not partitioned into multiple processes. This basic design decision was made in the earliest versions of UNIX. The first two implementations by Ken Thompson had no memory mapping, and thus made no hardware-enforced distinction between user and kernel space Ritchie, 1988. A message-passing system could have been implemented as readily as the actually implemented model of kernel and user processes. The monolithic kernel was chosen for simplicity and performance. And the early kernels were small; the inclusion of facilities such as networking into the kernel has increased its size. The current trend in operating-systems research is to reduce the kernel size by placing such services in user space.
Users ordinarily interact with the system through a command-language interpreter, called a shell, and perhaps through additional user application programs. Such programs and the shell are implemented with processes. Details of such programs are beyond the scope of this book, which instead concentrates almost exclusively on the kernel.
Sections 2.3 and 2.4 describe the services provided by the 4.4BSD kernel, and give an overview of the latter's design. Later chapters describe the detailed design and implementation of these services as they appear in 4.4BSD.
2.2 Kernel Organization
In this section, we view the organization of the 4.4BSD kernel in two ways:
As a static body of software, categorized by the functionality offered by the modules that make up the kernel
By its dynamic operation, categorized according to the services provided to users
The largest part of the kernel implements the system services that applications access through system calls. In 4.4BSD, this software has been organized according to the following:
Basic kernel facilities: timer and system-clock handling, descriptor management, and process management
Memory-management support: paging and swapping
Generic system interfaces: the I/O, control, and multiplexing operations performed on descriptors
The filesystem: files, directories, pathname translation, file locking, and I/O buffer management
Terminal-handling support: the terminal-interface driver and terminal line disciplines
Interprocess-communication facilities: sockets
Support for network communication: communication protocols and generic network facilities, such as routing