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+.\" Copyright (c) 1983, 1993
+.\"	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. 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.
+.\"
+.sh "Memory management\(dg
+.NH 3
+Text, data and stack
+.PP
+.FS
+\(dg This section represents the interface planned for later
+releases of the system.  Of the calls described in this section,
+only \fIsbrk\fP and \fIgetpagesize\fP are included in 4.3BSD.
+.FE
+Each process begins execution with three logical areas of memory
+called text, data and stack.  
+The text area is read-only and shared, while the data and stack
+areas are private to the process.  Both the data and stack areas may
+be extended and contracted on program request.  The call
+.DS
+addr = sbrk(incr);
+result caddr_t addr; int incr;
+.DE
+changes the size of the data area by \fIincr\fP bytes and
+returns the new end of the data area, while
+.DS
+addr = sstk(incr);
+result caddr_t addr; int incr;
+.DE
+changes the size of the stack area.
+The stack area is also automatically extended as needed.
+On the VAX the text and data areas are adjacent in the P0 region,
+while the stack section is in the P1 region, and grows downward.
+.NH 3
+Mapping pages
+.PP
+The system supports sharing of data between processes
+by allowing pages to be mapped into memory.  These mapped
+pages may be \fIshared\fP with other processes or \fIprivate\fP
+to the process.
+Protection and sharing options are defined in \fI<sys/mman.h>\fP as:
+.DS
+.ta \w'#define\ \ 'u +\w'MAP_HASSEMAPHORE\ \ 'u +\w'0x0080\ \ 'u
+/* protections are chosen from these bits, or-ed together */
+#define	PROT_READ	0x04	/* pages can be read */
+#define	PROT_WRITE	0x02	/* pages can be written */
+#define	PROT_EXEC	0x01	/* pages can be executed */
+.DE
+.DS
+.ta \w'#define\ \ 'u +\w'MAP_HASSEMAPHORE\ \ 'u +\w'0x0080\ \ 'u
+/* flags contain mapping type, sharing type and options */
+/* mapping type; choose one */
+#define MAP_FILE	0x0001	/* mapped from a file or device */
+#define MAP_ANON	0x0002	/* allocated from memory, swap space */
+#define MAP_TYPE	0x000f	/* mask for type field */
+.DE
+.DS
+.ta \w'#define\ \ 'u +\w'MAP_HASSEMAPHORE\ \ 'u +\w'0x0080\ \ 'u
+/* sharing types; choose one */
+#define	MAP_SHARED	0x0010	/* share changes */
+#define	MAP_PRIVATE	0x0000	/* changes are private */
+.DE
+.DS
+.ta \w'#define\ \ 'u +\w'MAP_HASSEMAPHORE\ \ 'u +\w'0x0080\ \ 'u
+/* other flags */
+#define MAP_FIXED	0x0020	/* map addr must be exactly as requested */
+#define MAP_INHERIT	0x0040	/* region is retained after exec */
+#define MAP_HASSEMAPHORE	0x0080	/* region may contain semaphores */
+#define MAP_NOPREALLOC	0x0100	/* do not preallocate space */
+.DE
+The cpu-dependent size of a page is returned by the
+\fIgetpagesize\fP system call:
+.DS
+pagesize = getpagesize();
+result int pagesize;
+.DE
+.LP
+The call:
+.DS
+maddr = mmap(addr, len, prot, flags, fd, pos);
+result caddr_t maddr; caddr_t addr; int *len, prot, flags, fd; off_t pos;
+.DE
+causes the pages starting at \fIaddr\fP and continuing
+for at most \fIlen\fP bytes to be mapped from the object represented by
+descriptor \fIfd\fP, starting at byte offset \fIpos\fP.
+The starting address of the region is returned;
+for the convenience of the system,
+it may differ from that supplied
+unless the MAP_FIXED flag is given,
+in which case the exact address will be used or the call will fail.
+The actual amount mapped is returned in \fIlen\fP.
+The \fIaddr\fP, \fIlen\fP, and \fIpos\fP parameters
+must all be multiples of the pagesize.
+A successful \fImmap\fP will delete any previous mapping
+in the allocated address range.
+The parameter \fIprot\fP specifies the accessibility
+of the mapped pages.
+The parameter \fIflags\fP specifies
+the type of object to be mapped,
+mapping options, and
+whether modifications made to
+this mapped copy of the page
+are to be kept \fIprivate\fP, or are to be \fIshared\fP with
+other references.
+Possible types include MAP_FILE,
+mapping a regular file or character-special device memory,
+and MAP_ANON, which maps memory not associated with any specific file.
+The file descriptor used for creating MAP_ANON regions is used only
+for naming, and may be given as \-1 if no name
+is associated with the region.\(dd
+.FS
+\(dd The current design does not allow a process
+to specify the location of swap space.
+In the future we may define an additional mapping type, MAP_SWAP,
+in which the file descriptor argument specifies a file
+or device to which swapping should be done.
+.FE
+The MAP_INHERIT flag allows a region to be inherited after an \fIexec\fP.
+The MAP_HASSEMAPHORE flag allows special handling for
+regions that may contain semaphores.
+The MAP_NOPREALLOC flag allows processes to allocate regions whose
+virtual address space, if fully allocated,
+would exceed the available memory plus swap resources.
+Such regions may get a SIGSEGV signal if they page fault and resources
+are not available to service their request;
+typically they would free up some resources via \fIunmap\fP so that
+when they return from the signal the page
+fault could be successfully completed.
+.PP
+A facility is provided to synchronize a mapped region with the file
+it maps; the call
+.DS
+msync(addr, len);
+caddr_t addr; int len;
+.DE
+writes any modified pages back to the filesystem and updates
+the file modification time.
+If \fIlen\fP is 0, all modified pages within the region containing \fIaddr\fP
+will be flushed;
+if \fIlen\fP is non-zero, only the pages containing \fIaddr\fP and \fIlen\fP
+succeeding locations will be examined.
+Any required synchronization of memory caches
+will also take place at this time.
+Filesystem operations on a file that is mapped for shared modifications
+are unpredictable except after an \fImsync\fP.
+.PP
+A mapping can be removed by the call
+.DS
+munmap(addr, len);
+caddr_t addr; int len;
+.DE
+This call deletes the mappings for the specified address range,
+and causes further references to addresses within the range
+to generate invalid memory references.
+.NH 3
+Page protection control
+.PP
+A process can control the protection of pages using the call
+.DS
+mprotect(addr, len, prot);
+caddr_t addr; int len, prot;
+.DE
+This call changes the specified pages to have protection \fIprot\fP\|.
+Not all implementations will guarantee protection on a page basis;
+the granularity of protection changes may be as large as an entire region.
+.NH 3
+Giving and getting advice
+.PP
+A process that has knowledge of its memory behavior may
+use the \fImadvise\fP call:
+.DS
+madvise(addr, len, behav);
+caddr_t addr; int len, behav;
+.DE
+\fIBehav\fP describes expected behavior, as given
+in \fI<sys/mman.h>\fP:
+.DS
+.ta \w'#define\ \ 'u +\w'MADV_SEQUENTIAL\ \ 'u +\w'00\ \ \ \ 'u
+#define	MADV_NORMAL	0	/* no further special treatment */
+#define	MADV_RANDOM	1	/* expect random page references */
+#define	MADV_SEQUENTIAL	2	/* expect sequential references */
+#define	MADV_WILLNEED	3	/* will need these pages */
+#define	MADV_DONTNEED	4	/* don't need these pages */
+#define	MADV_SPACEAVAIL	5	/* insure that resources are reserved */
+.DE
+Finally, a process may obtain information about whether pages are
+core resident by using the call
+.DS
+mincore(addr, len, vec)
+caddr_t addr; int len; result char *vec;
+.DE
+Here the current core residency of the pages is returned
+in the character array \fIvec\fP, with a value of 1 meaning
+that the page is in-core.
+.NH 3
+Synchronization primitives
+.PP
+Primitives are provided for synchronization using semaphores in shared memory.
+Semaphores must lie within a MAP_SHARED region with at least modes
+PROT_READ and PROT_WRITE.
+The MAP_HASSEMAPHORE flag must have been specified when the region was created.
+To acquire a lock a process calls:
+.DS
+value = mset(sem, wait)
+result int value; semaphore *sem; int wait;
+.DE
+\fIMset\fP indivisibly tests and sets the semaphore \fIsem\fP.
+If the previous value is zero, the process has acquired the lock
+and \fImset\fP returns true immediately.
+Otherwise, if the \fIwait\fP flag is zero,
+failure is returned.
+If \fIwait\fP is true and the previous value is non-zero,
+\fImset\fP relinquishes the processor until notified that it should retry.
+.LP
+To release a lock a process calls:
+.DS
+mclear(sem)
+semaphore *sem;
+.DE
+\fIMclear\fP indivisibly tests and clears the semaphore \fIsem\fP.
+If the ``WANT'' flag is zero in the previous value,
+\fImclear\fP returns immediately.
+If the ``WANT'' flag is non-zero in the previous value,
+\fImclear\fP arranges for waiting processes to retry before returning.
+.PP
+Two routines provide services analogous to the kernel
+\fIsleep\fP and \fIwakeup\fP functions interpreted in the domain of
+shared memory.
+A process may relinquish the processor by calling \fImsleep\fP
+with a set semaphore:
+.DS
+msleep(sem)
+semaphore *sem;
+.DE
+If the semaphore is still set when it is checked by the kernel,
+the process will be put in a sleeping state
+until some other process issues an \fImwakeup\fP for the same semaphore
+within the region using the call:
+.DS
+mwakeup(sem)
+semaphore *sem;
+.DE
+An \fImwakeup\fP may awaken all sleepers on the semaphore,
+or may awaken only the next sleeper on a queue.