.\"	$NetBSD: bioscall.9,v 1.10 2017/07/03 21:31:01 wiz Exp $
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.\" by John Kohl.
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.Dd May 5, 2010
.Dt BIOSCALL 9 i386
.Os
.Sh NAME
.Nm bioscall
.Nd call system BIOS function from real mode
.Sh SYNOPSIS
.In i386/bioscall.h
.Ft void
.Fn bioscall "int function" "struct bioscallregs *regs"
.Sh DESCRIPTION
The
.Fn bioscall
function switches the processor into real mode, calls the
.Tn BIOS
interrupt numbered
.Fa function ,
and returns to protected mode.
.Pp
This function is intended to be called during the initial system
bootstrap when necessary to probe devices or pseudo-devices.
.Pp
The register values specified by
.Fa *regs
(with one exception) are installed before the
.Tn BIOS
interrupt is called.
The processor flags are handled specially.
Only the following flags are passed to the
.Tn BIOS
from the registers in
.Fa regs
(the remainder come from the processor's flags register at the time
of the call):
.Ar PSL_C ,
.Ar PSL_PF ,
.Ar PSL_AF ,
.Ar PSL_Z ,
.Ar PSL_N ,
.Ar PSL_D ,
.Ar PSL_V .
.Pp
The
.Va bioscallregs
structure is defined to contain structures for each register, to allow
access to 32-, 16- or 8-bit wide sections of the registers.
Definitions are provided which simplify access to the union members.
.Sh RETURN VALUES
The
.Fn bioscall
function fills in
.Fa *regs
with the processor registers as returned from the
.Tn BIOS
call.
.Sh EXAMPLES
The Advanced Power Management driver calls
.Fn bioscall
by setting up a register structure with the
.Tn APM
installation check and device types in registers
.Fa ax
and
.Fa bx ,
then calls the
.Tn BIOS
to fetch the details for calling the
.Tn APM
support through a protected-mode interface.
The
.Tn BIOS
returns these details in the registers:
.Pp
.Bd -literal -offset indent
#include <i386/bioscall.h>
#include <i386/apmvar.h>
struct bioscallregs regs;

regs.AX = APM_BIOS_FN(APM_INSTALLATION_CHECK);
regs.BX = APM_DEV_APM_BIOS;
regs.CX = regs.DX = 0;
regs.ESI = regs.EDI = regs.EFLAGS = 0;
bioscall(APM_SYSTEM_BIOS, &regs);
.Ed
.Sh CODE REFERENCES
.Pa sys/arch/i386/i386/bioscall.s ,
.Pa sys/arch/i386/bioscall/biostramp.S
.Sh REFERENCES
.Xr apm 4
.Sh HISTORY
The
.Fn bioscall
function first appeared in
.Nx 1.3 .
.Sh BUGS
Not all
.Tn BIOS
functions are safe to call through the trampoline, as they
may depend on system state which has been disturbed or used for other
purposes once the
.Nx
kernel is running.