From NetBSD: merge of 960317

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+.\"	$OpenBSD: i386_vm86.2,v 1.1 1996/03/19 23:15:02 niklas Exp $
+.\"	$NetBSD: i386_vm86.2,v 1.1 1996/02/21 00:21:52 jtk Exp $
+.\"
+.\" Copyright (c) 1996 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" This code is derived from software contributed to The NetBSD Foundation
+.\" by John Kohl.
+.\"
+.\" 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 NetBSD 
+.\"	  Foundation, Inc. and its contributors.
+.\" 4. Neither the name of The NetBSD Foundation 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 NETBSD FOUNDATION, INC. 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.
+.\"
+.\"     from: @(#)fork.2	6.5 (Berkeley) 3/10/91
+.\"
+.Dd February 20, 1996
+.Dt I386_VM86 2
+.Os NetBSD
+.Sh NAME
+.Nm i386_vm86
+.Nd set virtual 8086 processor registers and mode
+.Sh SYNOPSIS
+.Fd #include <sys/types.h>
+.Fd #include <signal.h>
+.Fd #include <machine/segments.h>
+.Fd #include <machine/sysarch.h>
+.Fd #include <machine/vm86.h>
+.Ft int
+.Fn i386_vm86 "struct vm86_struct *vmcp"
+.Sh DESCRIPTION
+.Fn i386_vm86
+will set the process into virtual 8086 mode using the registers and
+selectors specified by the context pointed to by
+.Ar vmcp .
+The processor registers are set from 
+.Ar vmcp->substr.regs ,
+and the emulated processor type from 
+.Ar vmcp->substr.ss_cpu_type .
+.Pp
+The kernel keeps a pointer to the context, and uses the tables stored at
+.Ar vmcp->int_byuser
+and
+.Ar vmcp->int21_byuser
+for fast virtual interrupt handling.  If the 
+.Ar n th
+bit is clear in the first of
+these arrays, then the kernel may directly emulate the real-mode x86 INT
+.Ar n
+instruction handling.  If the
+.Ar n
+th bit is set, then the process is
+delivered a signal when an INT instruction is executed.
+.Pp
+Since MS-DOS puts many DOS functions onto interrupt 21, it is handled
+specially:  the
+.Ar k
+th bit in the
+.Ar vmcp->int21_byuser
+array is checked when INT
+.Ar 21
+is requested and the
+.Ar ah
+register is
+.Ar k .
+.Sh RETURN VALUES
+This routine does not normally return: 32-bit mode will be restored by
+the delivery of a signal to the process.  In case of an error in setting
+the VM86 mode, a value of -1 is returned and the global variable
+.Va errno
+is set to indicate the error.
+.Sh ERRORS
+.Fn i386_vm86
+will fail if:
+.Bl -tag -width [EINVAL]
+.It Bq Er EFAULT
+The state at
+.Ar vmcp
+was not readable to the user process.
+.Sh REFERENCES
+i386 Microprocessor Programmer's Reference Manual, Intel