source: bootcd/isolinux/syslinux-6.03/gpxe/src/arch/i386/firmware/pcbios/pnpbios.c

/*
 * Copyright (C) 2007 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

FILE_LICENCE ( GPL2_OR_LATER );

#include <stdint.h>
#include <string.h>
#include <errno.h>
#include <realmode.h>
#include <pnpbios.h>

/** @file
 *
 * PnP BIOS
 *
 */

/** PnP BIOS structure */
struct pnp_bios {
        /** Signature
         *
         * Must be equal to @c PNP_BIOS_SIGNATURE
         */
        uint32_t signature;
        /** Version as BCD (e.g. 1.0 is 0x10) */
        uint8_t version;
        /** Length of this structure */
        uint8_t length;
        /** System capabilities */
        uint16_t control;
        /** Checksum */
        uint8_t checksum;
} __attribute__ (( packed ));

/** Signature for a PnP BIOS structure */
#define PNP_BIOS_SIGNATURE \
        ( ( '$' << 0 ) + ( 'P' << 8 ) + ( 'n' << 16 ) + ( 'P' << 24 ) )

/**
 * Test address for PnP BIOS structure
 *
 * @v offset            Offset within BIOS segment to test
 * @ret rc              Return status code
 */
static int is_pnp_bios ( unsigned int offset ) {
        union {
                struct pnp_bios pnp_bios;
                uint8_t bytes[256]; /* 256 is maximum length possible */
        } u;
        size_t len;
        unsigned int i;
        uint8_t sum = 0;

        /* Read start of header and verify signature */
        copy_from_real ( &u.pnp_bios, BIOS_SEG, offset, sizeof ( u.pnp_bios ));
        if ( u.pnp_bios.signature != PNP_BIOS_SIGNATURE )
                return -EINVAL;

        /* Read whole header and verify checksum */
        len = u.pnp_bios.length;
        copy_from_real ( &u.bytes, BIOS_SEG, offset, len );
        for ( i = 0 ; i < len ; i++ ) {
                sum += u.bytes[i];
        }
        if ( sum != 0 )
                return -EINVAL;

        DBG ( "Found PnP BIOS at %04x:%04x\n", BIOS_SEG, offset );

        return 0;
}

/**
 * Locate Plug-and-Play BIOS
 *
 * @ret pnp_offset      Offset of PnP BIOS structure within BIOS segment
 *
 * The PnP BIOS structure will be at BIOS_SEG:pnp_offset.  If no PnP
 * BIOS is found, -1 is returned.
 */
int find_pnp_bios ( void ) {
        static int pnp_offset = 0;

        if ( pnp_offset )
                return pnp_offset;

        for ( pnp_offset = 0 ; pnp_offset < 0x10000 ; pnp_offset += 0x10 ) {
                if ( is_pnp_bios ( pnp_offset ) == 0 )
                        return pnp_offset;
        }

        pnp_offset = -1;
        return pnp_offset;
}