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+.\" Copyright (c) 1985 The Regents of the University of California.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"	@(#)5.t	5.1 (Berkeley) 4/17/91
+.\"
+.ds RH Functional Extensions
+.NH
+Functional Extensions
+.PP
+Some of the facilities introduced in 4.2BSD were not completely
+implemented.  An important part of the effort that went into
+4.3BSD was to clean up and unify both new and old facilities.
+.NH 2
+Kernel Extensions
+.PP
+A significant effort went into improving
+the networking part of the kernel.
+The work consisted of fixing bugs,
+tuning the algorithms,
+and revamping the lowest levels of the system
+to better handle heterogeneous network topologies.
+.NH 3
+Subnets, Broadcasts and Gateways
+.PP
+To allow sites to expand their network in an autonomous
+and orderly fashion, subnetworks have been introduced in 4.3BSD [GADS85].
+This facility allows sites to subdivide their local Internet address
+space into multiple subnetwork address spaces that are visible
+only by hosts at that site.  To off-site hosts machines on a site's
+subnetworks appear to reside on a single network.  The routing daemon
+has been reworked to provide routing support in this type of
+environment.
+.PP
+The default Internet broadcast address is now specified with a host part
+of all one's, rather than all zero's.
+The broadcast address may be set at boot time on a per-interface basis.
+.NH 3
+Interface Addressing
+.PP
+The organization of network interfaces has been
+reworked to more cleanly support multiple
+network protocols.  Network interfaces no longer
+contain a host's address on that network; instead
+each interface contains a pointer to a list of addresses 
+assigned to that interface.  This permits a single
+interface to support, for example, Internet protocols
+at the same time as XNS protocols.
+.PP
+The Address Resolution Protocol (ARP) support
+for 10 megabyte/second Ethernet\(dg
+.FS
+\(dg Ethernet is a trademark of Xerox.
+.FE
+has been made more flexible by allowing hosts to
+act as an ``clearing house'' for hosts that do
+not support ARP.  In addition, system managers have
+more control over the contents of the ARP translation
+cache and may interactively interrogate and modify
+the cache's contents.
+.NH 3
+User Control of Network Buffering
+.PP
+Although the system allocates reasonable default amounts of buffering
+for most connections, certain operations such as file system dumps
+to remote machines benefit from significant increases in buffering [Walsh84].
+The \fIsetsockopt\fP system call has been extended to allow such requests.
+In addition, \fIgetsockopt\fP and \fIsetsockopt\fP,
+are now interfaced to the protocol level allowing protocol-specific
+options to be manipulated by the user.
+.NH 3
+Number of File Descriptors
+.PP
+To allow full use of the many descriptor based services available,
+the previous hard limit of 30 open files per process has been relaxed.
+The changes entailed generalizing \fIselect\fP to handle arrays of 
+32-bit words, removing the dependency on file descriptors from
+the page table entries,
+and limiting most of the linear scans of a process's file table.
+The default per-process descriptor limit was raised from 20 to 64,
+though there are no longer any hard upper limits on the number
+of file descriptors.
+.NH 3
+Kernel Limits
+.PP
+Many internal kernel configuration limits have been increased by suitable
+modifications to data structures.
+The limit on physical memory has been changed from 8 megabyte to 64 megabyte,
+and the limit of 15 mounted file systems has been changed to 255.
+The maximum file system size has been increased to 8 gigabyte,
+number of processes to 65536,
+and per process size to 64 megabyte of data and 64 megabyte of stack.
+Note that these are upper bounds,
+the default limits for these quantities are tuned for systems
+with 4-8 megabyte of physical memory.
+.NH 3
+Memory Management
+.PP
+The global clock page replacement algorithm used to have a single
+hand that was used both to mark and to reclaim memory.
+The first time that it encountered a page it would clear its reference bit.
+If the reference bit was still clear on its next pass across the page,
+it would reclaim the page.
+The use of a single hand does not work well with large physical
+memories as the time to complete a single revolution of the hand
+can take up to a minute or more.
+By the time the hand gets around to the marked pages,
+the information is usually no longer pertinent.
+During periods of sudden shortages,
+the page daemon will not be able to find any reclaimable pages until
+it has completed a full revolution.
+To alleviate this problem,
+the clock hand has been split into two separate hands.
+The front hand clears the reference bits,
+the back hand follows a constant number of pages behind
+reclaiming pages that still have cleared reference bits.
+While the code has been written to allow the distance between
+the hands to be varied, we have not found any algorithms
+suitable for determining how to dynamically adjust this distance.
+.PP
+The configuration of the virtual memory system used to require
+a significant understanding of its operation to do such
+simple tasks as increasing the maximum process size.
+This process has been significantly improved so that the most
+common configuration parameters, such as the virtual memory sizes,
+can be specified using a single option in the configuration file.
+Standard configurations support data and stack segments
+of 17, 33 and 64 megabytes.
+.NH 3
+Signals
+.PP
+The 4.2BSD signal implementation would push several words
+onto the normal run-time stack before switching to an
+alternate signal stack.
+The 4.3BSD implementation has been corrected so that
+the entire signal handler's state is now pushed onto the signal stack.
+Another limitation in the original signal implementation was
+that it used an undocumented system call to return from signals.
+Users could not write their own return from exceptions;
+4.3BSD formally specifies the \fIsigreturn\fP system call.
+.PP
+Many existing programs depend on interrupted system calls.
+The restartable system call semantics of 4.2BSD signals caused
+many of these programs to break.
+To simplify porting of programs from inferior versions of 
+.UX
+the \fIsigvec\fP system call has been extended so that
+programmers may specify that system calls are not to be
+restarted after particular signals.
+.NH 3
+System Logging
+.PP
+A system logging facility has been added
+that sends kernel messages to the
+syslog daemon for logging in /usr/adm/messages and possibly for 
+printing on the system console.
+The revised scheme for logging messages
+eliminates the time lag in updating the messages file,
+unifies the format of kernel messages,
+provides a finer granularity of control over the messages
+that get printed on the console,
+and eliminates the degradation in response during the printing of 
+low-priority kernel messages.
+Recoverable system errors and common resource limitations are logged
+using this facility.
+Most system utilities such as init and login,
+have been modified to log errors to syslog
+rather than writing directly on the console.
+.NH 3
+Windows
+.PP
+The tty structure has been augmented to hold
+information about the size
+of an associated window or terminal.
+These sizes can be obtained by programs such as editors that want
+to know the size of the screen they are manipulating.
+When these sizes are changed,
+a new signal, SIGWINCH, is sent the current process group.
+The editors have been modified to catch this signal and reshape
+their view of the world, and the remote login program and server
+now cooperate to propagate window sizes and window size changes
+across a network.
+Other programs and libraries such as curses that need the width
+or height of the screen have been modified to use this facility as well.
+.NH 3
+Configuration of UNIBUS Devices
+.PP
+The UNIBUS configuration routines have been extended to allow auto-configuration
+of dedicated UNIBUS memory held by devices.
+The new routines simplify the configuration of memory-mapped devices
+and correct problems occurring on reset of the UNIBUS.
+.NH 3
+Disk Recovery from Errors
+.PP
+The MASSBUS disk driver's error recovery routines have been fixed to
+retry before correcting ECC errors, support ECC on bad-sector replacements,
+and correctly attempt retries after earlier
+corrective actions in the same transfer.
+The error messages are more accurate.
+.NH 2
+Functional Extensions to Libraries and Utilities
+.PP
+Most of the changes to the utilities and libraries have been to
+allow them to handle a more general set of problems,
+or to handle the same set of problems more quickly.
+.NH 3
+Name Server
+.PP
+In 4.2BSD the name resolution routines (\fIgethostbyname\fP,
+\fIgetservbyname\fP,
+etc.) were implemented by a set of database files maintained on the
+local machine.
+Inconsistencies or obsolescence in these files resulted in inaccessibility of
+hosts or services.
+In 4.3BSD these files may be replaced by a network name server that can
+insure a consistent view of the name space in a multimachine environment.
+This name server operates in accordance with Internet standards
+for service on the ARPANET [Mockapetris83].
+.NH 3
+System Management
+.PP
+A new utility, \fIrdist\fP,
+has been provided to assist system managers in keeping
+all their machines up to date with a consistent set of sources and binaries.
+A master set of sources may reside on a single central machine,
+or be distributed at (known) locations throughout the environment.
+New versions of \fIgetty\fP, \fIinit\fP, and \fIlogin\fP
+merge the functions of several
+files into a single place, and allow more flexibility in the
+startup of processes such as window managers.
+.PP
+The new utility \fItimed\fP keeps the time on a group of cooperating machines
+(within a single LAN) synchronized to within 30 milliseconds.
+It does its corrections using a new system call that changes
+the rate of time advance without stopping or reversing the system clock.
+It normally selects one machine to act as a master.
+If the master dies or is partitioned, a new master is elected.
+Other machines may participate in a purely slave role.
+.NH 3
+Routing
+.PP
+Many bugs in the routing daemon have been fixed;
+it is considerably more robust,
+and now understands how to properly deal with
+subnets and point-to-point networks.
+Its operation has been made more efficient by tuning with the use
+of execution profiles, along with inline expansion of common operations
+using the kernel's \fIinline\fP optimizer.
+.NH 3
+Compilers
+.PP
+The symbolic debugger \fIdbx\fP has had many new features added,
+and all the known bugs fixed.  In addition \fIdbx\fP
+has been extended to work with the Pascal compiler.
+The fortran compiler \fIf77\fP has had numerous bugs fixed.
+The C compiler has been modified so that it can, optionally,
+generate single precision floating point instructions when operating
+on single precision variables.