int13.c

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/*
 * Copyright (C) 2006 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., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 *
 * You can also choose to distribute this program under the terms of
 * the Unmodified Binary Distribution Licence (as given in the file
 * COPYING.UBDL), provided that you have satisfied its requirements.
 */

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );

#include <stdint.h>
#include <stdlib.h>
#include <limits.h>
#include <byteswap.h>
#include <errno.h>
#include <assert.h>
#include <ipxe/blockdev.h>
#include <ipxe/io.h>
#include <ipxe/acpi.h>
#include <ipxe/sanboot.h>
#include <ipxe/device.h>
#include <ipxe/pci.h>
#include <ipxe/eltorito.h>
#include <realmode.h>
#include <bios.h>
#include <biosint.h>
#include <bootsector.h>
#include <int13.h>

/** @file
 *
 * INT 13 emulation
 *
 * This module provides a mechanism for exporting block devices via
 * the BIOS INT 13 disk interrupt interface.  
 *
 */

/** INT 13 SAN device private data */
struct int13_data {
        /** BIOS natural drive number (0x00-0xff)
         *
         * This is the drive number that would have been assigned by
         * 'naturally' appending the drive to the end of the BIOS
         * drive list.
         *
         * If the emulated drive replaces a preexisting drive, this is
         * the drive number that the preexisting drive gets remapped
         * to.
         */
        unsigned int natural_drive;

        /** Number of cylinders
         *
         * The cylinder number field in an INT 13 call is ten bits
         * wide, giving a maximum of 1024 cylinders.  Conventionally,
         * when the 7.8GB limit of a CHS address is exceeded, it is
         * the number of cylinders that is increased beyond the
         * addressable limit.
         */
        unsigned int cylinders;
        /** Number of heads
         *
         * The head number field in an INT 13 call is eight bits wide,
         * giving a maximum of 256 heads.  However, apparently all
         * versions of MS-DOS up to and including Win95 fail with 256
         * heads, so the maximum encountered in practice is 255.
         */
        unsigned int heads;
        /** Number of sectors per track
         *
         * The sector number field in an INT 13 call is six bits wide,
         * giving a maximum of 63 sectors, since sector numbering
         * (unlike head and cylinder numbering) starts at 1, not 0.
         */
        unsigned int sectors_per_track;

        /** Address of El Torito boot catalog (if any) */
        unsigned int boot_catalog;
        /** Status of last operation */
        int last_status;
};

/** Vector for chaining to other INT 13 handlers */
static struct segoff __text16 ( int13_vector );
#define int13_vector __use_text16 ( int13_vector )

/** Assembly wrapper */
extern void int13_wrapper ( void );

/** Dummy floppy disk parameter table */
static struct int13_fdd_parameters __data16 ( int13_fdd_params ) = {
        /* 512 bytes per sector */
        .bytes_per_sector = 0x02,
        /* Highest sectors per track that we ever return */
        .sectors_per_track = 48,
};
#define int13_fdd_params __use_data16 ( int13_fdd_params )

/**
 * Equipment word
 *
 * This is a cached copy of the BIOS Data Area equipment word at
 * 40:10.
 */
static uint16_t equipment_word;

/**
 * Number of BIOS floppy disk drives
 *
 * This is derived from the equipment word.  It is held in .text16 to
 * allow for easy access by the INT 13,08 wrapper.
 */
static uint8_t __text16 ( num_fdds );
#define num_fdds __use_text16 ( num_fdds )

/**
 * Number of BIOS hard disk drives
 *
 * This is a cached copy of the BIOS Data Area number of hard disk
 * drives at 40:75.  It is held in .text16 to allow for easy access by
 * the INT 13,08 wrapper.
 */
static uint8_t __text16 ( num_drives );
#define num_drives __use_text16 ( num_drives )

/**
 * Calculate SAN device capacity (limited to 32 bits)
 *
 * @v sandev            SAN device
 * @ret blocks          Number of blocks
 */
static inline uint32_t int13_capacity32 ( struct san_device *sandev ) {
        uint64_t capacity = sandev_capacity ( sandev );
        return ( ( capacity <= 0xffffffffUL ) ? capacity : 0xffffffff );
}

/**
 * Test if SAN device is a floppy disk drive
 *
 * @v sandev            SAN device
 * @ret is_fdd          SAN device is a floppy disk drive
 */
static inline int int13_is_fdd ( struct san_device *sandev ) {
        return ( ! ( sandev->drive & 0x80 ) );
}

/**
 * Parse El Torito parameters
 *
 * @v sandev            SAN device
 * @v scratch           Scratch area for single-sector reads
 * @ret rc              Return status code
 *
 * Reads and parses El Torito parameters, if present.
 */
static int int13_parse_eltorito ( struct san_device *sandev, void *scratch ) {
        struct int13_data *int13 = sandev->priv;
        static const struct eltorito_descriptor_fixed boot_check = {
                .type = ISO9660_TYPE_BOOT,
                .id = ISO9660_ID,
                .version = 1,
                .system_id = "EL TORITO SPECIFICATION",
        };
        struct eltorito_descriptor *boot = scratch;
        int rc;

        /* Read boot record volume descriptor */
        if ( ( rc = sandev_read ( sandev, ELTORITO_LBA, 1,
                                  virt_to_user ( boot ) ) ) != 0 ) {
                DBGC ( sandev->drive, "INT13 drive %02x could not read El "
                       "Torito boot record volume descriptor: %s\n",
                       sandev->drive, strerror ( rc ) );
                return rc;
        }

        /* Check for an El Torito boot catalog */
        if ( memcmp ( boot, &boot_check, sizeof ( boot_check ) ) == 0 ) {
                int13->boot_catalog = boot->sector;
                DBGC ( sandev->drive, "INT13 drive %02x has an El Torito boot "
                       "catalog at LBA %08x\n", sandev->drive,
                       int13->boot_catalog );
        } else {
                DBGC ( sandev->drive, "INT13 drive %02x has no El Torito boot "
                       "catalog\n", sandev->drive );
        }

        return 0;
}

/**
 * Guess INT 13 hard disk drive geometry
 *
 * @v sandev            SAN device
 * @v scratch           Scratch area for single-sector reads
 * @ret heads           Guessed number of heads
 * @ret sectors         Guessed number of sectors per track
 * @ret rc              Return status code
 *
 * Guesses the drive geometry by inspecting the partition table.
 */
static int int13_guess_geometry_hdd ( struct san_device *sandev, void *scratch,
                                      unsigned int *heads,
                                      unsigned int *sectors ) {
        struct master_boot_record *mbr = scratch;
        struct partition_table_entry *partition;
        unsigned int i;
        unsigned int start_cylinder;
        unsigned int start_head;
        unsigned int start_sector;
        unsigned int end_head;
        unsigned int end_sector;
        int rc;

        /* Read partition table */
        if ( ( rc = sandev_read ( sandev, 0, 1, virt_to_user ( mbr ) ) ) != 0 ) {
                DBGC ( sandev->drive, "INT13 drive %02x could not read "
                       "partition table to guess geometry: %s\n",
                       sandev->drive, strerror ( rc ) );
                return rc;
        }
        DBGC2 ( sandev->drive, "INT13 drive %02x has MBR:\n", sandev->drive );
        DBGC2_HDA ( sandev->drive, 0, mbr, sizeof ( *mbr ) );
        DBGC ( sandev->drive, "INT13 drive %02x has signature %08x\n",
               sandev->drive, mbr->signature );

        /* Scan through partition table and modify guesses for
         * heads and sectors_per_track if we find any used
         * partitions.
         */
        *heads = 0;
        *sectors = 0;
        for ( i = 0 ; i < 4 ; i++ ) {

                /* Skip empty partitions */
                partition = &mbr->partitions[i];
                if ( ! partition->type )
                        continue;

                /* If partition starts on cylinder 0 then we can
                 * unambiguously determine the number of sectors.
                 */
                start_cylinder = PART_CYLINDER ( partition->chs_start );
                start_head = PART_HEAD ( partition->chs_start );
                start_sector = PART_SECTOR ( partition->chs_start );
                if ( ( start_cylinder == 0 ) && ( start_head != 0 ) ) {
                        *sectors = ( ( partition->start + 1 - start_sector ) /
                                     start_head );
                        DBGC ( sandev->drive, "INT13 drive %02x guessing "
                               "C/H/S xx/xx/%d based on partition %d\n",
                               sandev->drive, *sectors, ( i + 1 ) );
                }

                /* If partition ends on a higher head or sector number
                 * than our current guess, then increase the guess.
                 */
                end_head = PART_HEAD ( partition->chs_end );
                end_sector = PART_SECTOR ( partition->chs_end );
                if ( ( end_head + 1 ) > *heads ) {
                        *heads = ( end_head + 1 );
                        DBGC ( sandev->drive, "INT13 drive %02x guessing "
                               "C/H/S xx/%d/xx based on partition %d\n",
                               sandev->drive, *heads, ( i + 1 ) );
                }
                if ( end_sector > *sectors ) {
                        *sectors = end_sector;
                        DBGC ( sandev->drive, "INT13 drive %02x guessing "
                               "C/H/S xx/xx/%d based on partition %d\n",
                               sandev->drive, *sectors, ( i + 1 ) );
                }
        }

        /* Default guess is xx/255/63 */
        if ( ! *heads )
                *heads = 255;
        if ( ! *sectors )
                *sectors = 63;

        return 0;
}

/** Recognised floppy disk geometries */
static const struct int13_fdd_geometry int13_fdd_geometries[] = {
        INT13_FDD_GEOMETRY ( 40, 1, 8 ),
        INT13_FDD_GEOMETRY ( 40, 1, 9 ),
        INT13_FDD_GEOMETRY ( 40, 2, 8 ),
        INT13_FDD_GEOMETRY ( 40, 1, 9 ),
        INT13_FDD_GEOMETRY ( 80, 2, 8 ),
        INT13_FDD_GEOMETRY ( 80, 2, 9 ),
        INT13_FDD_GEOMETRY ( 80, 2, 15 ),
        INT13_FDD_GEOMETRY ( 80, 2, 18 ),
        INT13_FDD_GEOMETRY ( 80, 2, 20 ),
        INT13_FDD_GEOMETRY ( 80, 2, 21 ),
        INT13_FDD_GEOMETRY ( 82, 2, 21 ),
        INT13_FDD_GEOMETRY ( 83, 2, 21 ),
        INT13_FDD_GEOMETRY ( 80, 2, 22 ),
        INT13_FDD_GEOMETRY ( 80, 2, 23 ),
        INT13_FDD_GEOMETRY ( 80, 2, 24 ),
        INT13_FDD_GEOMETRY ( 80, 2, 36 ),
        INT13_FDD_GEOMETRY ( 80, 2, 39 ),
        INT13_FDD_GEOMETRY ( 80, 2, 40 ),
        INT13_FDD_GEOMETRY ( 80, 2, 44 ),
        INT13_FDD_GEOMETRY ( 80, 2, 48 ),
};

/**
 * Guess INT 13 floppy disk drive geometry
 *
 * @v sandev            SAN device
 * @ret heads           Guessed number of heads
 * @ret sectors         Guessed number of sectors per track
 * @ret rc              Return status code
 *
 * Guesses the drive geometry by inspecting the disk size.
 */
static int int13_guess_geometry_fdd ( struct san_device *sandev,
                                      unsigned int *heads,
                                      unsigned int *sectors ) {
        unsigned int blocks = sandev_capacity ( sandev );
        const struct int13_fdd_geometry *geometry;
        unsigned int cylinders;
        unsigned int i;

        /* Look for a match against a known geometry */
        for ( i = 0 ; i < ( sizeof ( int13_fdd_geometries ) /
                            sizeof ( int13_fdd_geometries[0] ) ) ; i++ ) {
                geometry = &int13_fdd_geometries[i];
                cylinders = INT13_FDD_CYLINDERS ( geometry );
                *heads = INT13_FDD_HEADS ( geometry );
                *sectors = INT13_FDD_SECTORS ( geometry );
                if ( ( cylinders * (*heads) * (*sectors) ) == blocks ) {
                        DBGC ( sandev->drive, "INT13 drive %02x guessing "
                               "C/H/S %d/%d/%d based on size %dK\n",
                               sandev->drive, cylinders, *heads, *sectors,
                               ( blocks / 2 ) );
                        return 0;
                }
        }

        /* Otherwise, assume a partial disk image in the most common
         * format (1440K, 80/2/18).
         */
        *heads = 2;
        *sectors = 18;
        DBGC ( sandev->drive, "INT13 drive %02x guessing C/H/S xx/%d/%d "
               "based on size %dK\n", sandev->drive, *heads, *sectors,
               ( blocks / 2 ) );
        return 0;
}

/**
 * Guess INT 13 drive geometry
 *
 * @v sandev            SAN device
 * @v scratch           Scratch area for single-sector reads
 * @ret rc              Return status code
 */
static int int13_guess_geometry ( struct san_device *sandev, void *scratch ) {
        struct int13_data *int13 = sandev->priv;
        unsigned int guessed_heads;
        unsigned int guessed_sectors;
        unsigned int blocks;
        unsigned int blocks_per_cyl;
        int rc;

        /* Guess geometry according to drive type */
        if ( int13_is_fdd ( sandev ) ) {
                if ( ( rc = int13_guess_geometry_fdd ( sandev, &guessed_heads,
                                                       &guessed_sectors )) != 0)
                        return rc;
        } else {
                if ( ( rc = int13_guess_geometry_hdd ( sandev, scratch,
                                                       &guessed_heads,
                                                       &guessed_sectors )) != 0)
                        return rc;
        }

        /* Apply guesses if no geometry already specified */
        if ( ! int13->heads )
                int13->heads = guessed_heads;
        if ( ! int13->sectors_per_track )
                int13->sectors_per_track = guessed_sectors;
        if ( ! int13->cylinders ) {
                /* Avoid attempting a 64-bit divide on a 32-bit system */
                blocks = int13_capacity32 ( sandev );
                blocks_per_cyl = ( int13->heads * int13->sectors_per_track );
                assert ( blocks_per_cyl != 0 );
                int13->cylinders = ( blocks / blocks_per_cyl );
                if ( int13->cylinders > 1024 )
                        int13->cylinders = 1024;
        }

        return 0;
}

/**
 * Update BIOS drive count
 */
static void int13_sync_num_drives ( void ) {
        struct san_device *sandev;
        struct int13_data *int13;
        uint8_t *counter;
        uint8_t max_drive;
        uint8_t required;

        /* Get current drive counts */
        get_real ( equipment_word, BDA_SEG, BDA_EQUIPMENT_WORD );
        get_real ( num_drives, BDA_SEG, BDA_NUM_DRIVES );
        num_fdds = ( ( equipment_word & 0x0001 ) ?
                     ( ( ( equipment_word >> 6 ) & 0x3 ) + 1 ) : 0 );

        /* Ensure count is large enough to cover all of our SAN devices */
        for_each_sandev ( sandev ) {
                int13 = sandev->priv;
                counter = ( int13_is_fdd ( sandev ) ? &num_fdds : &num_drives );
                max_drive = sandev->drive;
                if ( max_drive < int13->natural_drive )
                        max_drive = int13->natural_drive;
                required = ( ( max_drive & 0x7f ) + 1 );
                if ( *counter < required ) {
                        *counter = required;
                        DBGC ( sandev->drive, "INT13 drive %02x added to "
                               "drive count: %d HDDs, %d FDDs\n",
                               sandev->drive, num_drives, num_fdds );
                }
        }

        /* Update current drive count */
        equipment_word &= ~( ( 0x3 << 6 ) | 0x0001 );
        if ( num_fdds ) {
                equipment_word |= ( 0x0001 |
                                    ( ( ( num_fdds - 1 ) & 0x3 ) << 6 ) );
        }
        put_real ( equipment_word, BDA_SEG, BDA_EQUIPMENT_WORD );
        put_real ( num_drives, BDA_SEG, BDA_NUM_DRIVES );
}

/**
 * Check number of drives
 */
static void int13_check_num_drives ( void ) {
        uint16_t check_equipment_word;
        uint8_t check_num_drives;

        get_real ( check_equipment_word, BDA_SEG, BDA_EQUIPMENT_WORD );
        get_real ( check_num_drives, BDA_SEG, BDA_NUM_DRIVES );
        if ( ( check_equipment_word != equipment_word ) ||
             ( check_num_drives != num_drives ) ) {
                int13_sync_num_drives();
        }
}

/**
 * INT 13, 00 - Reset disk system
 *
 * @v sandev            SAN device
 * @ret status          Status code
 */
static int int13_reset ( struct san_device *sandev,
                         struct i386_all_regs *ix86 __unused ) {
        int rc;

        DBGC2 ( sandev->drive, "Reset drive\n" );

        /* Reset SAN device */
        if ( ( rc = sandev_reset ( sandev ) ) != 0 )
                return -INT13_STATUS_RESET_FAILED;

        return 0;
}

/**
 * INT 13, 01 - Get status of last operation
 *
 * @v sandev            SAN device
 * @ret status          Status code
 */
static int int13_get_last_status ( struct san_device *sandev,
                                   struct i386_all_regs *ix86 __unused ) {
        struct int13_data *int13 = sandev->priv;

        DBGC2 ( sandev->drive, "Get status of last operation\n" );
        return int13->last_status;
}

/**
 * Read / write sectors
 *
 * @v sandev            SAN device
 * @v al                Number of sectors to read or write (must be nonzero)
 * @v ch                Low bits of cylinder number
 * @v cl (bits 7:6)     High bits of cylinder number
 * @v cl (bits 5:0)     Sector number
 * @v dh                Head number
 * @v es:bx             Data buffer
 * @v sandev_rw         SAN device read/write method
 * @ret status          Status code
 * @ret al              Number of sectors read or written
 */
static int int13_rw_sectors ( struct san_device *sandev,
                              struct i386_all_regs *ix86,
                              int ( * sandev_rw ) ( struct san_device *sandev,
                                                    uint64_t lba,
                                                    unsigned int count,
                                                    userptr_t buffer ) ) {
        struct int13_data *int13 = sandev->priv;
        unsigned int cylinder, head, sector;
        unsigned long lba;
        unsigned int count;
        userptr_t buffer;
        int rc;

        /* Validate blocksize */
        if ( sandev_blksize ( sandev ) != INT13_BLKSIZE ) {
                DBGC ( sandev->drive, "\nINT 13 drive %02x invalid blocksize "
                       "(%zd) for non-extended read/write\n",
                       sandev->drive, sandev_blksize ( sandev ) );
                return -INT13_STATUS_INVALID;
        }

        /* Calculate parameters */
        cylinder = ( ( ( ix86->regs.cl & 0xc0 ) << 2 ) | ix86->regs.ch );
        head = ix86->regs.dh;
        sector = ( ix86->regs.cl & 0x3f );
        if ( ( cylinder >= int13->cylinders ) ||
             ( head >= int13->heads ) ||
             ( sector < 1 ) || ( sector > int13->sectors_per_track ) ) {
                DBGC ( sandev->drive, "C/H/S %d/%d/%d out of range for "
                       "geometry %d/%d/%d\n", cylinder, head, sector,
                       int13->cylinders, int13->heads,
                       int13->sectors_per_track );
                return -INT13_STATUS_INVALID;
        }
        lba = ( ( ( ( cylinder * int13->heads ) + head )
                  * int13->sectors_per_track ) + sector - 1 );
        count = ix86->regs.al;
        buffer = real_to_user ( ix86->segs.es, ix86->regs.bx );

        DBGC2 ( sandev->drive, "C/H/S %d/%d/%d = LBA %08lx <-> %04x:%04x "
                "(count %d)\n", cylinder, head, sector, lba, ix86->segs.es,
                ix86->regs.bx, count );

        /* Read from / write to block device */
        if ( ( rc = sandev_rw ( sandev, lba, count, buffer ) ) != 0 ){
                DBGC ( sandev->drive, "INT13 drive %02x I/O failed: %s\n",
                       sandev->drive, strerror ( rc ) );
                return -INT13_STATUS_READ_ERROR;
        }

        return 0;
}

/**
 * INT 13, 02 - Read sectors
 *
 * @v sandev            SAN device
 * @v al                Number of sectors to read (must be nonzero)
 * @v ch                Low bits of cylinder number
 * @v cl (bits 7:6)     High bits of cylinder number
 * @v cl (bits 5:0)     Sector number
 * @v dh                Head number
 * @v es:bx             Data buffer
 * @ret status          Status code
 * @ret al              Number of sectors read
 */
static int int13_read_sectors ( struct san_device *sandev,
                                struct i386_all_regs *ix86 ) {

        DBGC2 ( sandev->drive, "Read: " );
        return int13_rw_sectors ( sandev, ix86, sandev_read );
}

/**
 * INT 13, 03 - Write sectors
 *
 * @v sandev            SAN device
 * @v al                Number of sectors to write (must be nonzero)
 * @v ch                Low bits of cylinder number
 * @v cl (bits 7:6)     High bits of cylinder number
 * @v cl (bits 5:0)     Sector number
 * @v dh                Head number
 * @v es:bx             Data buffer
 * @ret status          Status code
 * @ret al              Number of sectors written
 */
static int int13_write_sectors ( struct san_device *sandev,
                                 struct i386_all_regs *ix86 ) {

        DBGC2 ( sandev->drive, "Write: " );
        return int13_rw_sectors ( sandev, ix86, sandev_write );
}

/**
 * INT 13, 08 - Get drive parameters
 *
 * @v sandev            SAN device
 * @ret status          Status code
 * @ret ch              Low bits of maximum cylinder number
 * @ret cl (bits 7:6)   High bits of maximum cylinder number
 * @ret cl (bits 5:0)   Maximum sector number
 * @ret dh              Maximum head number
 * @ret dl              Number of drives
 */
static int int13_get_parameters ( struct san_device *sandev,
                                  struct i386_all_regs *ix86 ) {
        struct int13_data *int13 = sandev->priv;
        unsigned int max_cylinder = int13->cylinders - 1;
        unsigned int max_head = int13->heads - 1;
        unsigned int max_sector = int13->sectors_per_track; /* sic */

        DBGC2 ( sandev->drive, "Get drive parameters\n" );

        /* Validate blocksize */
        if ( sandev_blksize ( sandev ) != INT13_BLKSIZE ) {
                DBGC ( sandev->drive, "\nINT 13 drive %02x invalid blocksize "
                       "(%zd) for non-extended parameters\n",
                       sandev->drive, sandev_blksize ( sandev ) );
                return -INT13_STATUS_INVALID;
        }

        /* Common parameters */
        ix86->regs.ch = ( max_cylinder & 0xff );
        ix86->regs.cl = ( ( ( max_cylinder >> 8 ) << 6 ) | max_sector );
        ix86->regs.dh = max_head;
        ix86->regs.dl = ( int13_is_fdd ( sandev ) ? num_fdds : num_drives );

        /* Floppy-specific parameters */
        if ( int13_is_fdd ( sandev ) ) {
                ix86->regs.bl = INT13_FDD_TYPE_1M44;
                ix86->segs.es = rm_ds;
                ix86->regs.di = __from_data16 ( &int13_fdd_params );
        }

        return 0;
}

/**
 * INT 13, 15 - Get disk type
 *
 * @v sandev            SAN device
 * @ret ah              Type code
 * @ret cx:dx           Sector count
 * @ret status          Status code / disk type
 */
static int int13_get_disk_type ( struct san_device *sandev,
                                 struct i386_all_regs *ix86 ) {
        uint32_t blocks;

        DBGC2 ( sandev->drive, "Get disk type\n" );

        if ( int13_is_fdd ( sandev ) ) {
                return INT13_DISK_TYPE_FDD;
        } else {
                blocks = int13_capacity32 ( sandev );
                ix86->regs.cx = ( blocks >> 16 );
                ix86->regs.dx = ( blocks & 0xffff );
                return INT13_DISK_TYPE_HDD;
        }
}

/**
 * INT 13, 41 - Extensions installation check
 *
 * @v sandev            SAN device
 * @v bx                0x55aa
 * @ret bx              0xaa55
 * @ret cx              Extensions API support bitmap
 * @ret status          Status code / API version
 */
static int int13_extension_check ( struct san_device *sandev,
                                   struct i386_all_regs *ix86 ) {

        if ( ( ix86->regs.bx == 0x55aa ) && ! int13_is_fdd ( sandev ) ) {
                DBGC2 ( sandev->drive, "INT13 extensions check\n" );
                ix86->regs.bx = 0xaa55;
                ix86->regs.cx = ( INT13_EXTENSION_LINEAR |
                                  INT13_EXTENSION_EDD |
                                  INT13_EXTENSION_64BIT );
                return INT13_EXTENSION_VER_3_0;
        } else {
                return -INT13_STATUS_INVALID;
        }
}

/**
 * Extended read / write
 *
 * @v sandev            SAN device
 * @v ds:si             Disk address packet
 * @v sandev_rw         SAN device read/write method
 * @ret status          Status code
 */
static int int13_extended_rw ( struct san_device *sandev,
                               struct i386_all_regs *ix86,
                               int ( * sandev_rw ) ( struct san_device *sandev,
                                                     uint64_t lba,
                                                     unsigned int count,
                                                     userptr_t buffer ) ) {
        struct int13_disk_address addr;
        uint8_t bufsize;
        uint64_t lba;
        unsigned long count;
        userptr_t buffer;
        int rc;

        /* Extended reads are not allowed on floppy drives.
         * ELTORITO.SYS seems to assume that we are really a CD-ROM if
         * we support extended reads for a floppy drive.
         */
        if ( int13_is_fdd ( sandev ) )
                return -INT13_STATUS_INVALID;

        /* Get buffer size */
        get_real ( bufsize, ix86->segs.ds,
                   ( ix86->regs.si + offsetof ( typeof ( addr ), bufsize ) ) );
        if ( bufsize < offsetof ( typeof ( addr ), buffer_phys ) ) {
                DBGC2 ( sandev->drive, "<invalid buffer size %#02x\n>\n",
                        bufsize );
                return -INT13_STATUS_INVALID;
        }

        /* Read parameters from disk address structure */
        memset ( &addr, 0, sizeof ( addr ) );
        copy_from_real ( &addr, ix86->segs.ds, ix86->regs.si, bufsize );
        lba = addr.lba;
        DBGC2 ( sandev->drive, "LBA %08llx <-> ",
                ( ( unsigned long long ) lba ) );
        if ( ( addr.count == 0xff ) ||
             ( ( addr.buffer.segment == 0xffff ) &&
               ( addr.buffer.offset == 0xffff ) ) ) {
                buffer = phys_to_user ( addr.buffer_phys );
                DBGC2 ( sandev->drive, "%08llx",
                        ( ( unsigned long long ) addr.buffer_phys ) );
        } else {
                buffer = real_to_user ( addr.buffer.segment,
                                        addr.buffer.offset );
                DBGC2 ( sandev->drive, "%04x:%04x", addr.buffer.segment,
                        addr.buffer.offset );
        }
        if ( addr.count <= 0x7f ) {
                count = addr.count;
        } else if ( addr.count == 0xff ) {
                count = addr.long_count;
        } else {
                DBGC2 ( sandev->drive, " <invalid count %#02x>\n", addr.count );
                return -INT13_STATUS_INVALID;
        }
        DBGC2 ( sandev->drive, " (count %ld)\n", count );

        /* Read from / write to block device */
        if ( ( rc = sandev_rw ( sandev, lba, count, buffer ) ) != 0 ) {
                DBGC ( sandev->drive, "INT13 drive %02x extended I/O failed: "
                       "%s\n", sandev->drive, strerror ( rc ) );
                /* Record that no blocks were transferred successfully */
                addr.count = 0;
                put_real ( addr.count, ix86->segs.ds,
                           ( ix86->regs.si +
                             offsetof ( typeof ( addr ), count ) ) );
                return -INT13_STATUS_READ_ERROR;
        }

        return 0;
}

/**
 * INT 13, 42 - Extended read
 *
 * @v sandev            SAN device
 * @v ds:si             Disk address packet
 * @ret status          Status code
 */
static int int13_extended_read ( struct san_device *sandev,
                                 struct i386_all_regs *ix86 ) {

        DBGC2 ( sandev->drive, "Extended read: " );
        return int13_extended_rw ( sandev, ix86, sandev_read );
}

/**
 * INT 13, 43 - Extended write
 *
 * @v sandev            SAN device
 * @v ds:si             Disk address packet
 * @ret status          Status code
 */
static int int13_extended_write ( struct san_device *sandev,
                                  struct i386_all_regs *ix86 ) {

        DBGC2 ( sandev->drive, "Extended write: " );
        return int13_extended_rw ( sandev, ix86, sandev_write );
}

/**
 * INT 13, 44 - Verify sectors
 *
 * @v sandev            SAN device
 * @v ds:si             Disk address packet
 * @ret status          Status code
 */
static int int13_extended_verify ( struct san_device *sandev,
                                   struct i386_all_regs *ix86 ) {
        struct int13_disk_address addr;
        uint64_t lba;
        unsigned long count;

        /* Read parameters from disk address structure */
        if ( DBG_EXTRA ) {
                copy_from_real ( &addr, ix86->segs.ds, ix86->regs.si,
                                 sizeof ( addr ));
                lba = addr.lba;
                count = addr.count;
                DBGC2 ( sandev->drive, "Verify: LBA %08llx (count %ld)\n",
                        ( ( unsigned long long ) lba ), count );
        }

        /* We have no mechanism for verifying sectors */
        return -INT13_STATUS_INVALID;
}

/**
 * INT 13, 44 - Extended seek
 *
 * @v sandev            SAN device
 * @v ds:si             Disk address packet
 * @ret status          Status code
 */
static int int13_extended_seek ( struct san_device *sandev,
                                 struct i386_all_regs *ix86 ) {
        struct int13_disk_address addr;
        uint64_t lba;
        unsigned long count;

        /* Read parameters from disk address structure */
        if ( DBG_EXTRA ) {
                copy_from_real ( &addr, ix86->segs.ds, ix86->regs.si,
                                 sizeof ( addr ));
                lba = addr.lba;
                count = addr.count;
                DBGC2 ( sandev->drive, "Seek: LBA %08llx (count %ld)\n",
                        ( ( unsigned long long ) lba ), count );
        }

        /* Ignore and return success */
        return 0;
}

/**
 * Build device path information
 *
 * @v sandev            SAN device
 * @v dpi               Device path information
 * @ret rc              Return status code
 */
static int int13_device_path_info ( struct san_device *sandev,
                                    struct edd_device_path_information *dpi ) {
        struct san_path *sanpath;
        struct device *device;
        struct device_description *desc;
        unsigned int i;
        uint8_t sum = 0;
        int rc;

        /* Reopen block device if necessary */
        if ( sandev_needs_reopen ( sandev ) &&
             ( ( rc = sandev_reopen ( sandev ) ) != 0 ) )
                return rc;
        sanpath = sandev->active;
        assert ( sanpath != NULL );

        /* Get underlying hardware device */
        device = identify_device ( &sanpath->block );
        if ( ! device ) {
                DBGC ( sandev->drive, "INT13 drive %02x cannot identify "
                       "hardware device\n", sandev->drive );
                return -ENODEV;
        }

        /* Fill in bus type and interface path */
        desc = &device->desc;
        switch ( desc->bus_type ) {
        case BUS_TYPE_PCI:
                dpi->host_bus_type.type = EDD_BUS_TYPE_PCI;
                dpi->interface_path.pci.bus = PCI_BUS ( desc->location );
                dpi->interface_path.pci.slot = PCI_SLOT ( desc->location );
                dpi->interface_path.pci.function = PCI_FUNC ( desc->location );
                dpi->interface_path.pci.channel = 0xff; /* unused */
                break;
        default:
                DBGC ( sandev->drive, "INT13 drive %02x unrecognised bus "
                       "type %d\n", sandev->drive, desc->bus_type );
                return -ENOTSUP;
        }

        /* Get EDD block device description */
        if ( ( rc = edd_describe ( &sanpath->block, &dpi->interface_type,
                                   &dpi->device_path ) ) != 0 ) {
                DBGC ( sandev->drive, "INT13 drive %02x cannot identify "
                       "block device: %s\n", sandev->drive, strerror ( rc ) );
                return rc;
        }

        /* Fill in common fields and fix checksum */
        dpi->key = EDD_DEVICE_PATH_INFO_KEY;
        dpi->len = sizeof ( *dpi );
        for ( i = 0 ; i < sizeof ( *dpi ) ; i++ )
                sum += *( ( ( uint8_t * ) dpi ) + i );
        dpi->checksum -= sum;

        return 0;
}

/**
 * INT 13, 48 - Get extended parameters
 *
 * @v sandev            SAN device
 * @v ds:si             Drive parameter table
 * @ret status          Status code
 */
static int int13_get_extended_parameters ( struct san_device *sandev,
                                           struct i386_all_regs *ix86 ) {
        struct int13_data *int13 = sandev->priv;
        struct int13_disk_parameters params;
        struct segoff address;
        size_t len = sizeof ( params );
        uint16_t bufsize;
        int rc;

        /* Get buffer size */
        get_real ( bufsize, ix86->segs.ds,
                   ( ix86->regs.si + offsetof ( typeof ( params ), bufsize )));

        DBGC2 ( sandev->drive, "Get extended drive parameters to "
                "%04x:%04x+%02x\n", ix86->segs.ds, ix86->regs.si, bufsize );

        /* Build drive parameters */
        memset ( &params, 0, sizeof ( params ) );
        params.flags = INT13_FL_DMA_TRANSPARENT;
        if ( ( int13->cylinders < 1024 ) &&
             ( sandev_capacity ( sandev ) <= INT13_MAX_CHS_SECTORS ) ) {
                params.flags |= INT13_FL_CHS_VALID;
        }
        params.cylinders = int13->cylinders;
        params.heads = int13->heads;
        params.sectors_per_track = int13->sectors_per_track;
        params.sectors = sandev_capacity ( sandev );
        params.sector_size = sandev_blksize ( sandev );
        memset ( &params.dpte, 0xff, sizeof ( params.dpte ) );
        if ( ( rc = int13_device_path_info ( sandev, &params.dpi ) ) != 0 ) {
                DBGC ( sandev->drive, "INT13 drive %02x could not provide "
                       "device path information: %s\n",
                       sandev->drive, strerror ( rc ) );
                len = offsetof ( typeof ( params ), dpi );
        }

        /* Calculate returned "buffer size" (which will be less than
         * the length actually copied if device path information is
         * present).
         */
        if ( bufsize < offsetof ( typeof ( params ), dpte ) )
                return -INT13_STATUS_INVALID;
        if ( bufsize < offsetof ( typeof ( params ), dpi ) ) {
                params.bufsize = offsetof ( typeof ( params ), dpte );
        } else {
                params.bufsize = offsetof ( typeof ( params ), dpi );
        }

        DBGC ( sandev->drive, "INT 13 drive %02x described using extended "
               "parameters:\n", sandev->drive );
        address.segment = ix86->segs.ds;
        address.offset = ix86->regs.si;
        DBGC_HDA ( sandev->drive, address, &params, len );

        /* Return drive parameters */
        if ( len > bufsize )
                len = bufsize;
        copy_to_real ( ix86->segs.ds, ix86->regs.si, &params, len );

        return 0;
}

/**
 * INT 13, 4b - Get status or terminate CD-ROM emulation
 *
 * @v sandev            SAN device
 * @v ds:si             Specification packet
 * @ret status          Status code
 */
static int int13_cdrom_status_terminate ( struct san_device *sandev,
                                          struct i386_all_regs *ix86 ) {
        struct int13_cdrom_specification specification;

        DBGC2 ( sandev->drive, "Get CD-ROM emulation status to %04x:%04x%s\n",
                ix86->segs.ds, ix86->regs.si,
                ( ix86->regs.al ? "" : " and terminate" ) );

        /* Fail if we are not a CD-ROM */
        if ( ! sandev->is_cdrom ) {
                DBGC ( sandev->drive, "INT13 drive %02x is not a CD-ROM\n",
                       sandev->drive );
                return -INT13_STATUS_INVALID;
        }

        /* Build specification packet */
        memset ( &specification, 0, sizeof ( specification ) );
        specification.size = sizeof ( specification );
        specification.drive = sandev->drive;

        /* Return specification packet */
        copy_to_real ( ix86->segs.ds, ix86->regs.si, &specification,
                       sizeof ( specification ) );

        return 0;
}


/**
 * INT 13, 4d - Read CD-ROM boot catalog
 *
 * @v sandev            SAN device
 * @v ds:si             Command packet
 * @ret status          Status code
 */
static int int13_cdrom_read_boot_catalog ( struct san_device *sandev,
                                           struct i386_all_regs *ix86 ) {
        struct int13_data *int13 = sandev->priv;
        struct int13_cdrom_boot_catalog_command command;
        unsigned int start;
        int rc;

        /* Read parameters from command packet */
        copy_from_real ( &command, ix86->segs.ds, ix86->regs.si,
                         sizeof ( command ) );
        DBGC2 ( sandev->drive, "Read CD-ROM boot catalog to %08x\n",
                command.buffer );

        /* Fail if we have no boot catalog */
        if ( ! int13->boot_catalog ) {
                DBGC ( sandev->drive, "INT13 drive %02x has no boot catalog\n",
                       sandev->drive );
                return -INT13_STATUS_INVALID;
        }
        start = ( int13->boot_catalog + command.start );

        /* Read from boot catalog */
        if ( ( rc = sandev_read ( sandev, start, command.count,
                                  phys_to_user ( command.buffer ) ) ) != 0 ) {
                DBGC ( sandev->drive, "INT13 drive %02x could not read boot "
                       "catalog: %s\n", sandev->drive, strerror ( rc ) );
                return -INT13_STATUS_READ_ERROR;
        }

        return 0;
}

/**
 * INT 13 handler
 *
 */
static __asmcall __used void int13 ( struct i386_all_regs *ix86 ) {
        int command = ix86->regs.ah;
        unsigned int bios_drive = ix86->regs.dl;
        struct san_device *sandev;
        struct int13_data *int13;
        int status;

        /* Check BIOS hasn't killed off our drive */
        int13_check_num_drives();

        for_each_sandev ( sandev ) {

                int13 = sandev->priv;
                if ( bios_drive != sandev->drive ) {
                        /* Remap any accesses to this drive's natural number */
                        if ( bios_drive == int13->natural_drive ) {
                                DBGC2 ( sandev->drive, "INT13,%02x (%02x) "
                                        "remapped to (%02x)\n", ix86->regs.ah,
                                        bios_drive, sandev->drive );
                                ix86->regs.dl = sandev->drive;
                                return;
                        } else if ( ( ( bios_drive & 0x7f ) == 0x7f ) &&
                                    ( command == INT13_CDROM_STATUS_TERMINATE )
                                    && sandev->is_cdrom ) {
                                /* Catch non-drive-specific CD-ROM calls */
                        } else {
                                continue;
                        }
                }
                
                DBGC2 ( sandev->drive, "INT13,%02x (%02x): ",
                        ix86->regs.ah, bios_drive );

                switch ( command ) {
                case INT13_RESET:
                        status = int13_reset ( sandev, ix86 );
                        break;
                case INT13_GET_LAST_STATUS:
                        status = int13_get_last_status ( sandev, ix86 );
                        break;
                case INT13_READ_SECTORS:
                        status = int13_read_sectors ( sandev, ix86 );
                        break;
                case INT13_WRITE_SECTORS:
                        status = int13_write_sectors ( sandev, ix86 );
                        break;
                case INT13_GET_PARAMETERS:
                        status = int13_get_parameters ( sandev, ix86 );
                        break;
                case INT13_GET_DISK_TYPE:
                        status = int13_get_disk_type ( sandev, ix86 );
                        break;
                case INT13_EXTENSION_CHECK:
                        status = int13_extension_check ( sandev, ix86 );
                        break;
                case INT13_EXTENDED_READ:
                        status = int13_extended_read ( sandev, ix86 );
                        break;
                case INT13_EXTENDED_WRITE:
                        status = int13_extended_write ( sandev, ix86 );
                        break;
                case INT13_EXTENDED_VERIFY:
                        status = int13_extended_verify ( sandev, ix86 );
                        break;
                case INT13_EXTENDED_SEEK:
                        status = int13_extended_seek ( sandev, ix86 );
                        break;
                case INT13_GET_EXTENDED_PARAMETERS:
                        status = int13_get_extended_parameters ( sandev, ix86 );
                        break;
                case INT13_CDROM_STATUS_TERMINATE:
                        status = int13_cdrom_status_terminate ( sandev, ix86 );
                        break;
                case INT13_CDROM_READ_BOOT_CATALOG:
                        status = int13_cdrom_read_boot_catalog ( sandev, ix86 );
                        break;
                default:
                        DBGC2 ( sandev->drive, "*** Unrecognised INT13 ***\n" );
                        status = -INT13_STATUS_INVALID;
                        break;
                }

                /* Store status for INT 13,01 */
                int13->last_status = status;

                /* Negative status indicates an error */
                if ( status < 0 ) {
                        status = -status;
                        DBGC ( sandev->drive, "INT13,%02x (%02x) failed with "
                               "status %02x\n", ix86->regs.ah, sandev->drive,
                               status );
                } else {
                        ix86->flags &= ~CF;
                }
                ix86->regs.ah = status;

                /* Set OF to indicate to wrapper not to chain this call */
                ix86->flags |= OF;

                return;
        }
}

/**
 * Hook INT 13 handler
 *
 */
static void int13_hook_vector ( void ) {
        /* Assembly wrapper to call int13().  int13() sets OF if we
         * should not chain to the previous handler.  (The wrapper
         * clears CF and OF before calling int13()).
         */
        __asm__  __volatile__ (
               TEXT16_CODE ( "\nint13_wrapper:\n\t"
                             /* Preserve %ax and %dx for future reference */
                             "pushw %%bp\n\t"
                             "movw %%sp, %%bp\n\t"                           
                             "pushw %%ax\n\t"
                             "pushw %%dx\n\t"
                             /* Clear OF, set CF, call int13() */
                             "orb $0, %%al\n\t" 
                             "stc\n\t"
                             VIRT_CALL ( int13 )
                             /* Chain if OF not set */
                             "jo 1f\n\t"
                             "pushfw\n\t"
                             "lcall *%%cs:int13_vector\n\t"
                             "\n1:\n\t"
                             /* Overwrite flags for iret */
                             "pushfw\n\t"
                             "popw 6(%%bp)\n\t"
                             /* Fix up %dl:
                              *
                              * INT 13,15 : do nothing if hard disk
                              * INT 13,08 : load with number of drives
                              * all others: restore original value
                              */
                             "cmpb $0x15, -1(%%bp)\n\t"
                             "jne 2f\n\t"
                             "testb $0x80, -4(%%bp)\n\t"
                             "jnz 3f\n\t"
                             "\n2:\n\t"
                             "movb -4(%%bp), %%dl\n\t"
                             "cmpb $0x08, -1(%%bp)\n\t"
                             "jne 3f\n\t"
                             "testb $0x80, %%dl\n\t"
                             "movb %%cs:num_drives, %%dl\n\t"
                             "jnz 3f\n\t"
                             "movb %%cs:num_fdds, %%dl\n\t"
                             /* Return */
                             "\n3:\n\t"
                             "movw %%bp, %%sp\n\t"
                             "popw %%bp\n\t"
                             "iret\n\t" ) : : );

        hook_bios_interrupt ( 0x13, ( intptr_t ) int13_wrapper, &int13_vector );
}

/**
 * Unhook INT 13 handler
 */
static void int13_unhook_vector ( void ) {
        unhook_bios_interrupt ( 0x13, ( intptr_t ) int13_wrapper,
                                &int13_vector );
}

/**
 * Hook INT 13 SAN device
 *
 * @v drive             Drive number
 * @v uris              List of URIs
 * @v count             Number of URIs
 * @v flags             Flags
 * @ret drive           Drive number, or negative error
 *
 * Registers the drive with the INT 13 emulation subsystem, and hooks
 * the INT 13 interrupt vector (if not already hooked).
 */
static int int13_hook ( unsigned int drive, struct uri **uris,
                        unsigned int count, unsigned int flags ) {
        struct san_device *sandev;
        struct int13_data *int13;
        unsigned int natural_drive;
        void *scratch;
        int need_hook = ( ! have_sandevs() );
        int rc;

        /* Calculate natural drive number */
        int13_sync_num_drives();
        natural_drive = ( ( drive & 0x80 ) ? ( num_drives | 0x80 ) : num_fdds );

        /* Use natural drive number if directed to do so */
        if ( ( drive & 0x7f ) == 0x7f )
                drive = natural_drive;

        /* Allocate SAN device */
        sandev = alloc_sandev ( uris, count, sizeof ( *int13 ) );
        if ( ! sandev ) {
                rc = -ENOMEM;
                goto err_alloc;
        }
        int13 = sandev->priv;
        int13->natural_drive = natural_drive;

        /* Register SAN device */
        if ( ( rc = register_sandev ( sandev, drive, flags ) ) != 0 ) {
                DBGC ( drive, "INT13 drive %02x could not register: %s\n",
                       drive, strerror ( rc ) );
                goto err_register;
        }

        /* Allocate scratch area */
        scratch = malloc ( sandev_blksize ( sandev ) );
        if ( ! scratch )
                goto err_alloc_scratch;

        /* Parse parameters, if present */
        if ( sandev->is_cdrom &&
             ( ( rc = int13_parse_eltorito ( sandev, scratch ) ) != 0 ) )
                goto err_parse_eltorito;

        /* Give drive a default geometry, if applicable */
        if ( ( sandev_blksize ( sandev ) == INT13_BLKSIZE ) &&
             ( ( rc = int13_guess_geometry ( sandev, scratch ) ) != 0 ) )
                goto err_guess_geometry;

        DBGC ( drive, "INT13 drive %02x (naturally %02x) registered with "
               "C/H/S geometry %d/%d/%d\n", drive, int13->natural_drive,
               int13->cylinders, int13->heads, int13->sectors_per_track );

        /* Hook INT 13 vector if not already hooked */
        if ( need_hook ) {
                int13_hook_vector();
                devices_get();
        }

        /* Update BIOS drive count */
        int13_sync_num_drives();

        free ( scratch );
        return drive;

 err_guess_geometry:
 err_parse_eltorito:
        free ( scratch );
 err_alloc_scratch:
        unregister_sandev ( sandev );
 err_register:
        sandev_put ( sandev );
 err_alloc:
        return rc;
}

/**
 * Unhook INT 13 SAN device
 *
 * @v drive             Drive number
 *
 * Unregisters the drive from the INT 13 emulation subsystem.  If this
 * is the last SAN device, the INT 13 vector is unhooked (if
 * possible).
 */
static void int13_unhook ( unsigned int drive ) {
        struct san_device *sandev;

        /* Find drive */
        sandev = sandev_find ( drive );
        if ( ! sandev ) {
                DBGC ( drive, "INT13 drive %02x is not a SAN drive\n", drive );
                return;
        }

        /* Unregister SAN device */
        unregister_sandev ( sandev );

        /* Should adjust BIOS drive count, but it's difficult
         * to do so reliably.
         */

        DBGC ( drive, "INT13 drive %02x unregistered\n", drive );

        /* Unhook INT 13 vector if no more drives */
        if ( ! have_sandevs() ) {
                devices_put();
                int13_unhook_vector();
        }

        /* Drop reference to drive */
        sandev_put ( sandev );
}

/**
 * Load and verify master boot record from INT 13 drive
 *
 * @v drive             Drive number
 * @v address           Boot code address to fill in
 * @ret rc              Return status code
 */
static int int13_load_mbr ( unsigned int drive, struct segoff *address ) {
        uint16_t status;
        int discard_b, discard_c, discard_d;
        uint16_t magic;

        /* Use INT 13, 02 to read the MBR */
        address->segment = 0;
        address->offset = 0x7c00;
        __asm__ __volatile__ ( REAL_CODE ( "pushw %%es\n\t"
                                           "pushl %%ebx\n\t"
                                           "popw %%bx\n\t"
                                           "popw %%es\n\t"
                                           "stc\n\t"
                                           "sti\n\t"
                                           "int $0x13\n\t"
                                           "sti\n\t" /* BIOS bugs */
                                           "jc 1f\n\t"
                                           "xorw %%ax, %%ax\n\t"
                                           "\n1:\n\t"
                                           "popw %%es\n\t" )
                               : "=a" ( status ), "=b" ( discard_b ),
                                 "=c" ( discard_c ), "=d" ( discard_d )
                               : "a" ( 0x0201 ), "b" ( *address ),
                                 "c" ( 1 ), "d" ( drive ) );
        if ( status ) {
                DBGC ( drive, "INT13 drive %02x could not read MBR (status "
                       "%04x)\n", drive, status );
                return -EIO;
        }

        /* Check magic signature */
        get_real ( magic, address->segment,
                   ( address->offset +
                     offsetof ( struct master_boot_record, magic ) ) );
        if ( magic != INT13_MBR_MAGIC ) {
                DBGC ( drive, "INT13 drive %02x does not contain a valid MBR\n",
                       drive );
                return -ENOEXEC;
        }

        return 0;
}

/** El Torito boot catalog command packet */
static struct int13_cdrom_boot_catalog_command __data16 ( eltorito_cmd ) = {
        .size = sizeof ( struct int13_cdrom_boot_catalog_command ),
        .count = 1,
        .buffer = 0x7c00,
        .start = 0,
};
#define eltorito_cmd __use_data16 ( eltorito_cmd )

/** El Torito disk address packet */
static struct int13_disk_address __bss16 ( eltorito_address );
#define eltorito_address __use_data16 ( eltorito_address )

/**
 * Load and verify El Torito boot record from INT 13 drive
 *
 * @v drive             Drive number
 * @v address           Boot code address to fill in
 * @ret rc              Return status code
 */
static int int13_load_eltorito ( unsigned int drive, struct segoff *address ) {
        struct {
                struct eltorito_validation_entry valid;
                struct eltorito_boot_entry boot;
        } __attribute__ (( packed )) catalog;
        uint16_t status;

        /* Use INT 13, 4d to read the boot catalog */
        __asm__ __volatile__ ( REAL_CODE ( "stc\n\t"
                                           "sti\n\t"
                                           "int $0x13\n\t"
                                           "sti\n\t" /* BIOS bugs */
                                           "jc 1f\n\t"
                                           "xorw %%ax, %%ax\n\t"
                                           "\n1:\n\t" )
                               : "=a" ( status )
                               : "a" ( 0x4d00 ), "d" ( drive ),
                                 "S" ( __from_data16 ( &eltorito_cmd ) ) );
        if ( status ) {
                DBGC ( drive, "INT13 drive %02x could not read El Torito boot "
                       "catalog (status %04x)\n", drive, status );
                return -EIO;
        }
        copy_from_user ( &catalog, phys_to_user ( eltorito_cmd.buffer ), 0,
                         sizeof ( catalog ) );

        /* Sanity checks */
        if ( catalog.valid.platform_id != ELTORITO_PLATFORM_X86 ) {
                DBGC ( drive, "INT13 drive %02x El Torito specifies unknown "
                       "platform %02x\n", drive, catalog.valid.platform_id );
                return -ENOEXEC;
        }
        if ( catalog.boot.indicator != ELTORITO_BOOTABLE ) {
                DBGC ( drive, "INT13 drive %02x El Torito is not bootable\n",
                       drive );
                return -ENOEXEC;
        }
        if ( catalog.boot.media_type != ELTORITO_NO_EMULATION ) {
                DBGC ( drive, "INT13 drive %02x El Torito requires emulation "
                       "type %02x\n", drive, catalog.boot.media_type );
                return -ENOTSUP;
        }
        DBGC ( drive, "INT13 drive %02x El Torito boot image at LBA %08x "
               "(count %d)\n", drive, catalog.boot.start, catalog.boot.length );
        address->segment = ( catalog.boot.load_segment ?
                             catalog.boot.load_segment : 0x7c0 );
        address->offset = 0;
        DBGC ( drive, "INT13 drive %02x El Torito boot image loads at "
               "%04x:%04x\n", drive, address->segment, address->offset );

        /* Use INT 13, 42 to read the boot image */
        eltorito_address.bufsize =
                offsetof ( typeof ( eltorito_address ), buffer_phys );
        eltorito_address.count = catalog.boot.length;
        eltorito_address.buffer = *address;
        eltorito_address.lba = catalog.boot.start;
        __asm__ __volatile__ ( REAL_CODE ( "stc\n\t"
                                           "sti\n\t"
                                           "int $0x13\n\t"
                                           "sti\n\t" /* BIOS bugs */
                                           "jc 1f\n\t"
                                           "xorw %%ax, %%ax\n\t"
                                           "\n1:\n\t" )
                               : "=a" ( status )
                               : "a" ( 0x4200 ), "d" ( drive ),
                                 "S" ( __from_data16 ( &eltorito_address ) ) );
        if ( status ) {
                DBGC ( drive, "INT13 drive %02x could not read El Torito boot "
                       "image (status %04x)\n", drive, status );
                return -EIO;
        }

        return 0;
}

/**
 * Attempt to boot from an INT 13 drive
 *
 * @v drive             Drive number
 * @v config            Boot configuration parameters
 * @ret rc              Return status code
 *
 * This boots from the specified INT 13 drive by loading the Master
 * Boot Record to 0000:7c00 and jumping to it.  INT 18 is hooked to
 * capture an attempt by the MBR to boot the next device.  (This is
 * the closest thing to a return path from an MBR).
 *
 * Note that this function can never return success, by definition.
 */
static int int13_boot ( unsigned int drive,
                        struct san_boot_config *config __unused ) {
        struct memory_map memmap;
        struct segoff address;
        int rc;

        /* Look for a usable boot sector */
        if ( ( ( rc = int13_load_mbr ( drive, &address ) ) != 0 ) &&
             ( ( rc = int13_load_eltorito ( drive, &address ) ) != 0 ) )
                return rc;

        /* Dump out memory map prior to boot, if memmap debugging is
         * enabled.  Not required for program flow, but we have so
         * many problems that turn out to be memory-map related that
         * it's worth doing.
         */
        get_memmap ( &memmap );

        /* Jump to boot sector */
        if ( ( rc = call_bootsector ( address.segment, address.offset,
                                      drive ) ) != 0 ) {
                DBGC ( drive, "INT13 drive %02x boot returned: %s\n",
                       drive, strerror ( rc ) );
                return rc;
        }

        return -ECANCELED; /* -EIMPOSSIBLE */
}

/** Maximum size of boot firmware table(s) */
#define XBFTAB_SIZE 768

/** Alignment of boot firmware table entries */
#define XBFTAB_ALIGN 16

/** The boot firmware table(s) generated by iPXE */
static uint8_t __bss16_array ( xbftab, [XBFTAB_SIZE] )
        __attribute__ (( aligned ( XBFTAB_ALIGN ) ));
#define xbftab __use_data16 ( xbftab )

/** Total used length of boot firmware tables */
static size_t xbftab_used;

/**
 * Install ACPI table
 *
 * @v acpi              ACPI description header
 * @ret rc              Return status code
 */
static int int13_install ( struct acpi_header *acpi ) {
        struct segoff xbft_address;
        struct acpi_header *installed;
        size_t len;

        /* Check length */
        len = acpi->length;
        if ( len > ( sizeof ( xbftab ) - xbftab_used ) ) {
                DBGC ( acpi, "INT13 out of space for %s table\n",
                       acpi_name ( acpi->signature ) );
                return -ENOSPC;
        }

        /* Install table */
        installed = ( ( ( void * ) xbftab ) + xbftab_used );
        memcpy ( installed, acpi, len );
        xbft_address.segment = rm_ds;
        xbft_address.offset = __from_data16 ( installed );

        /* Fill in common parameters */
        strncpy ( installed->oem_id, "FENSYS",
                  sizeof ( installed->oem_id ) );
        strncpy ( installed->oem_table_id, "iPXE",
                  sizeof ( installed->oem_table_id ) );

        /* Fix checksum */
        acpi_fix_checksum ( installed );

        /* Update used length */
        xbftab_used = ( ( xbftab_used + len + XBFTAB_ALIGN - 1 ) &
                        ~( XBFTAB_ALIGN - 1 ) );

        DBGC ( acpi, "INT13 installed %s:\n",
               acpi_name ( installed->signature ) );
        DBGC_HDA ( acpi, xbft_address, installed, len );
        return 0;
}

/**
 * Describe SAN devices for SAN-booted operating system
 *
 * @ret rc              Return status code
 */
static int int13_describe ( void ) {
        int rc;

        /* Clear tables */
        memset ( &xbftab, 0, sizeof ( xbftab ) );
        xbftab_used = 0;

        /* Install ACPI tables */
        if ( ( rc = acpi_install ( int13_install ) ) != 0 ) {
                DBG ( "INT13 could not install ACPI tables: %s\n",
                      strerror ( rc ) );
                return rc;
        }

        return 0;
}

PROVIDE_SANBOOT ( pcbios, san_hook, int13_hook );
PROVIDE_SANBOOT ( pcbios, san_unhook, int13_unhook );
PROVIDE_SANBOOT ( pcbios, san_boot, int13_boot );
PROVIDE_SANBOOT ( pcbios, san_describe, int13_describe );