.TH REALEMU 8
.SH NAME
realemu \- software emulation of /dev/realmode
.SH SYNOPSIS
.B aux/realemu
[
.B -Dpt
] [
.B -s
.I srvname
] [
.B -m
.I mountpoint
]
.SH DESCRIPTION
.PP
Originally, kernel provided 
.B /dev/realmode
files with the
.IR arch (3)
device to access and call the
.SM BIOS.
.PP
Interrupts had to be disabled and the processor was switched in the
legacy 16-bit
.SM realmode
with memory protection disabled to execute
.SM BIOS
code.
.PP
This is problematic in case the
.SM BIOS
reprograms hardware currently
used by the operating system or when it reenables interrupts or just
crashes. This will freeze or reboot the machine with no way to
recover or diagnose the problem.
.PP
To avoid this,
.I realemu
is used to emulate the execution of the
.SM BIOS
routines by interpreting the machine instructions and intercepting
dangerous actions that would compromise the systems stability.
.PP
Running
.I realemu
with no arguments, it mounts itself before
.B /dev
and
replaces the original
.B /dev/realmode
file in the current namespace.
.PP
Then programs like
.IR vga (8)
can use it to make their
.SM BIOS
calls.
.PP
The
.B D
flag will enable debug messages for 9P.  The
.B p
and
.B t
flags
control tracing of i/o port access and cpu instructions to
stderr (fd 2).
.PP
When a
.I srvname
is given with the
.B s
argument, the default
.I mountpoint
is ignored and a
.SM 9P
channel is created in
.B /srv
that can be used to mount
the filesystem from another namespace. If a
.I mountpoint
is given before
the
.I srvname
argument then it is ignored, otherwise it will be used.
.SH EXAMPLES
The
.I realemu
process is only needed when accessing
.B /dev/realmode.
To invoke a subshell so that
.I realemu
exits normally after
.B aux/vga
completes:
.IP
.EX
% @{rfork n; aux/realemu; aux/vga -m vesa -l $vgasize}
.EE
.SH SOURCE
.B /sys/src/cmd/aux/realemu
.SH "SEE ALSO"
.IR vga (8),
.IR arch (3)
.SH HISTORY
.I Realemu
first appeared in 9front (April, 2011).