4.4BSD操作系统设计与实现概述

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"Design And Implementation Of The 4.4 BSD Operating System" 本文档详细介绍了4.4BSD操作系统的设计与实现，这是UNIX系统家族的一个重要分支，特别是在学术和研究领域有着广泛的影响。4.4BSD是在先前的BSD版本基础上发展起来的，它在系统设计、稳定性和功能上进行了诸多改进。首先，文档概述了UNIX系统的悠久历史，从最初的起源到后来的研究版，再到AT&T的UNIX System III和System V。此外，还提到了其他组织对UNIX的贡献，特别是伯克利软件分布（BSD）对UNIX世界的重要影响。BSD不仅在用户社区中产生了深远影响，而且其设计理念和特性也被其他操作系统所采纳。 4.4BSD的设计目标被明确地列出，包括对4.2BSD和4.3BSD设计目标的继承和扩展。这些目标旨在提高系统的可移植性、性能、可靠性和安全性。4.4BSD在发布工程方面也有所强化，确保了系统的质量和一致性。在设计概览部分，文档深入讨论了4.4BSD内核及其设施。内核是操作系统的核心，负责管理硬件资源和提供服务给用户进程。内核组织结构清晰，包含了一系列关键组件，如进程管理、内存管理和I/O系统。进程管理部分涵盖了信号、进程组和会话等概念，这些都是进程交互和控制的基础。内存管理是4.4BSD的关键特性之一，文档详细阐述了BSD内存管理的设计决策以及内核中的内存管理机制。 I/O系统在4.4BSD中扮演着重要角色，使用了描述符进行I/O操作，支持多种设备、套接字IPC（进程间通信）以及scatter/gather I/O。多文件系统支持允许挂载不同类型的文件系统，增强了系统的灵活性。文件系统章节探讨了4.4BSD如何处理文件的存储和访问，包括基本的文件系统类型和更高级的文件存储技术。网络文件系统（NFS）章节则介绍了如何通过网络进行文件共享。终端部分详细讲解了终端设备和终端模拟器的工作原理。最后，文档还涉及了进程间通信、网络通信和网络实现，这些都是4.4BSD实现网络功能的基础。 4.4BSD操作系统的设计和实现文档提供了对这个系统全面而深入的理解，对于研究操作系统原理和UNIX历史的读者来说具有极高的价值。它详细解析了操作系统内核的各个组成部分，展示了4.4BSD如何通过创新和优化来满足现代计算需求。

System V

1985 Release 2 XENIX 3

1986

Eighth

SunOS Edition 4.2BSD 2.9BSD

1987

Chorus

1988

1989

2.11BSD

Chorus

2.10BSD

1990

1991

1992

Novell

1993 Linux UNIX

Ware

1994

1995

1996

BSDI 1.0

4.4BSD

BSDI2.

Figure 1.2 The UNIX system family tree, 1986-1996.

language, the use of C was a critical first step [Ritchie et al, 1978; Kernighan &

Ritchie, 1978; Kernighan & Ritchie, 1988]. Ritchie's C language is descended

[Rosier, 1984] from Thompson's B language, which was itself descended from

BCPL [Richards & Whitby-Strevens, 1980]. C continues to evolve [Tuthill, 1985;

X3J11, 1988], and there is a variant—C++—that more readily permits data

abstraction [Stroustrup, 1984; USENIX, 1987].

The second important distinction of UNIX was its early release from Bell Lab-

oratories to other research environments in source form. By providing source, the

system's founders ensured that other organizations would be able not only to use

the system, but also to tinker with its inner workings. The ease with which new

ideas could be adopted into the system always has been key to the changes that

have been made to it. Whenever a new system that tried to upstage UNIX came

along, somebody would dissect the newcomer and clone its central ideas into

UNIX. The unique ability to use a small, comprehensible system, written in a

high-level language, in an environment swimming in new ideas led to a UNIX sys-

tem that evolved far beyond its humble beginnings.

The third important distinction of UNIX was that it provided individual users

with the ability to run multiple processes concurrently and to connect these pro-

cesses into pipelines of commands. At the time, only operating systems running

on large and expensive machines had the ability to run multiple processes, and the

number of concurrent processes usually was controlled tightly by a system admin-

istrator.

Most early UNIX systems ran on the PDP-11, which was inexpensive and

powerful for its time. Nonetheless, there was at least one early port of Sixth Edi-

tion UNIX to a machine with a different architecture, the Interdata 7/32 [Miller,

1978]. The PDP-11 also had an inconveniently small address space. The introduc-

tion of machines with 32-bit address spaces, especially the VAX-11/780, provided

an opportunity for UNIX to expand its services to include virtual memory and net-

working. Earlier experiments by the Research group in providing UNIX-like facil-

ities on different hardware had led to the conclusion that it was as easy to move

the entire operating system as it was to duplicate UNIX's services under another

operating system. The first UNIX system with portability as a specific goal was

UNIX Time-Sharing System, Seventh Edition (V7), which ran on the PDP-11

and the Interdata 8/32, and had a VAX variety called UNIX/32V Time-Sharing,

System Version 1.0 (32V). The Research group at Bell Laboratories has also

developed UNIX Time-Sharing System, Eighth Edition (V8), UNIX Time-Shar-

ing System, Ninth Edition (V9), and UNIX Time-Sharing System, Tenth Edi-

tion (V10). Their 1996 system is Plan 9.

AT&T UNIX System III and System V

After the distribution of Seventh Edition in 1978, the Research group turned over

external distributions to the UNIX Support Group (USG). USG had previously dis-

tributed internally such systems as the UNIX Programmer's Work Bench (PWB),

and had sometimes distributed them externally as well [Mohr, 1985].

USG's first external distribution after Seventh Edition was UNIX System III

(System III), in 1982, which incorporated features of Seventh Edition, of 32V,

and also of several UNIX systems developed by groups other than the Research

group. Features of UNIX /RT (a real-time UNIX system) were included, as were

many features from PWB. USG released UNIX System V (System V) in 1983;

that system is largely derived from System III. The court-ordered divestiture of

the Bell Operating Companies from AT&T permitted AT&T to market System V

aggressively [Wilson, 1985; Bach, 1986].

USG metamorphosed into the UNIX System Development Laboratory (USDL),

which released UNIX System V, Release 2 in 1984. System V, Release 2, Ver-

sion 4 introduced paging [Miller, 1984; Jung, 1985], including copy-on-write and

shared memory, to System V The System V implementation was not based on the

Berkeley paging system. USDL was succeeded by AT&T Information Systems

(ATTIS), which distributed UNIX System V, Release 3 in 1987. That system

included STREAMS, an IPC mechanism adopted from V8 [Presotto & Ritchie,

1985]. ATTIS was succeeded by UNIX System Laboratories (USL), which was

sold to Novell in 1993. Novell passed the UNIX trademark to the X/OPEN consor-

tium, giving the latter sole rights to set up certification standards for using the

UNIX name on products. Two years later, Novell sold UNIX to The Santa Cruz

Operation (SCO).

Other Organizations

The ease with which the UNIX system can be modified has led to development

work at numerous organizations, including the Rand Corporation, which is

responsible for the Rand ports mentioned in Chapter 11; Bolt Beranek and New-

man (BBN), who produced the direct ancestor of the 4.2BSD networking imple-

mentation discussed in Chapter 13; the University of Illinois, which did earlier

networking work; Harvard; Purdue; and Digital Equipment Corporation (DEC).

Probably the most widespread version of the UNIX operating system, accord-

ing to the number of machines on which it runs, is XENIX by Microsoft Corpora-

tion and The Santa Cruz Operation. XENIX was originally based on Seventh

Edition, but later on System V More recently, SCO purchased UNIX from Novell

and announced plans to merge the two systems.

Systems prominently not based on UNIX include IBM's OS/2 and Microsoft's

Windows 95 and Windows/NT. All these systems have been touted as UNIX

killers, but none have done the deed.

Berkeley Software Distributions

The most influential of the non-Bell Laboratories and non-AT&T UNIX develop-

ment groups was the University of California at Berkeley [McKusick, 1985].

Software from Berkeley is released in Berkeley Software Distributions

(BSD)—for example, as 4.3BSD. The first Berkeley VAX UNIX work was the

addition to 32V of virtual memory, demand paging, and page replacement in 1979

by William Joy and Ozalp Babaoglu, to produce 3BSD [Babaoglu & Joy, 1981].

The reason for the large virtual-memory space of 3BSD was the development of

what at the time were large programs, such as Berkeley's Franz LISP. This mem-

ory-management work convinced the Defense Advanced Research Projects

Agency (DARPA) to fund the Berkeley team for the later development of a stan-

dard system (4BSD) for DARPA's contractors to use.

A goal of the 4BSD project was to provide support for the DARPA Internet

networking protocols, TCP/IP [Cerf & Cain, 1983]. The networking implementa-

tion was general enough to communicate among diverse network facilities, rang-

ing from local networks, such as Ethernets and token rings, to long-haul networks,

such as DARPA's ARPANET.

We refer to all the Berkeley VAX UNIX systems following 3BSD as 4BSD,

although there were really several releases—4.0BSD, 4.1BSD, 4.2BSD, 4.3BSD,

4.3BSD Tahoe, and 4.3BSD Reno. 4BSD was the UNIX operating system of choice

for VAXes from the time that the VAX first became available in 1977 until the

release of System V in 1983. Most organizations would purchase a 32V license,

but would order 4BSD from Berkeley. Many installations inside the Bell System

ran 4.1BSD (and replaced it with 4.3BSD when the latter became available). A

new virtual-memory system was released with 4.4BSD. The VAX was reaching

the end of its useful lifetime, so 4.4BSD was not ported to that machine. Instead,

4.4BSD ran on the newer 68000, SPARC, MIPS, and Intel PC architectures.

The 4BSD work for DARPA was guided by a steering committee that included

many notable people from both commercial and academic institutions. The cul-

mination of the original Berkeley DARPA UNIX project was the release of 4.2BSD

in 1983; further research at Berkeley produced 4.3BSD in mid-1986. The next

releases included the 4.3BSD Tahoe release of June 1988 and the 4.3BSD Reno

release of June 1990. These releases were primarily ports to the Computer Con-

soles Incorporated hardware platform. Interleaved with these releases were two

unencumbered networking releases: the 4.3BSD Netl release of March 1989 and

the 4.3BSD Net2 release of June 1991. These releases extracted nonproprietary

code from 4.3BSD; they could be redistributed freely in source and binary form to

companies that and individuals who were not covered by a UNIX source license.

The final CSRG release was to have been two versions of 4.4BSD, to be released

in June 1993. One was to have been a traditional full source and binary distrib-

ution, called 4.4BSD-Encumbered, that required the recipient to have a UNIX

source license. The other was to have been a subset of the source, called 4.4BSD-

Lite, that contained no licensed code and did not require the recipient to have a

UNIX source license. Following these distributions, the CSRG would be dis-

solved. The 4.4BSD-Encumbered was released as scheduled, but legal action by

USL prevented the distribution of 4.4BSD-Lite. The legal action was resolved

about 1 year later, and 4.4BSD-Lite was released in April 1994. The last of the

money in the CSRG coffers was used to produce a bug-fixed version 4.4BSD-Lite,

release 2, that was distributed in June 1995. This release was the true final

distribution from the CSRG.

Nonetheless, 4BSD still lives on in all modern implementations of UNIX, and

in many other operating systems.

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