I386中bootloader与kernel之间的信息交换

有些信息需要从bootloader传递到内核中，empty_zero_page是i386交换信息的场所。

下面这段话来自于Documentation/i386/zero-page.txt

Summary of empty_zero_page layout (kernel point of view)
     ( collected by Hans Lermen and Martin Mares )
 
The contents of empty_zero_page are used to pass parameters from the
16-bit realmode code of the kernel to the 32-bit part. References/settings
to it mainly are in:

  arch/i386/boot/setup.S
  arch/i386/boot/video.S
  arch/i386/kernel/head.S
  arch/i386/kernel/setup.c
 

Offset    Type        Description
------  ----        -----------
    0    32 bytes    struct screen_info, SCREEN_INFO
            ATTENTION, overlaps the following !!!
    2    unsigned short    EXT_MEM_K, extended memory size in Kb (from int 0x15)
 0x20    unsigned short    CL_MAGIC, commandline magic number (=0xA33F)
 0x22    unsigned short    CL_OFFSET, commandline offset
            Address of commandline is calculated:
              0x90000 + contents of CL_OFFSET
            (only taken, when CL_MAGIC = 0xA33F)
 0x40    20 bytes    struct apm_bios_info, APM_BIOS_INFO
 0x80    16 bytes    hd0-disk-parameter from intvector 0x41
 0x90    16 bytes    hd1-disk-parameter from intvector 0x46

 0xa0    16 bytes    System description table truncated to 16 bytes.
            ( struct sys_desc_table_struct )
 0xb0 - 0x1df        Free. Add more parameters here if you really need them.

0x1e0    unsigned long    ALT_MEM_K, alternative mem check, in Kb
0x1e8    char        number of entries in E820MAP (below)
0x1e9    unsigned char    number of entries in EDDBUF (below)
0x1f1    char        size of setup.S, number of sectors
0x1f2    unsigned short    MOUNT_ROOT_RDONLY (if !=0)
0x1f4    unsigned short    size of compressed kernel-part in the
            (b)zImage-file (in 16 byte units, rounded up)
0x1f6    unsigned short    swap_dev (unused AFAIK)
0x1f8    unsigned short    RAMDISK_FLAGS
0x1fa    unsigned short    VGA-Mode (old one)
0x1fc    unsigned short    ORIG_ROOT_DEV (high=Major, low=minor)
0x1ff    char        AUX_DEVICE_INFO

0x200    short jump to start of setup code aka "reserved" field.
0x202    4 bytes        Signature for SETUP-header, ="HdrS"
0x206    unsigned short    Version number of header format
            Current version is 0x0201...
0x208    8 bytes        (used by setup.S for communication with boot loaders,
             look there)
0x210    char        LOADER_TYPE, = 0, old one
            else it is set by the loader:
            0xTV: T=0 for LILO
                1 for Loadlin
                2 for bootsect-loader
                3 for SYSLINUX
                4 for ETHERBOOT
                V = version
0x211    char        loadflags:
            bit0 = 1: kernel is loaded high (bzImage)
            bit7 = 1: Heap and pointer (see below) set by boot
                  loader.
0x212    unsigned short    (setup.S)
0x214    unsigned long    KERNEL_START, where the loader started the kernel
0x218    unsigned long    INITRD_START, address of loaded ramdisk image
0x21c    unsigned long    INITRD_SIZE, size in bytes of ramdisk image
0x220    4 bytes        (setup.S)
0x224    unsigned short    setup.S heap end pointer
0x228    4 bytes        unknown, but writing this in setup.S makes
            kernel fail to decompress on some systems
0x2cc    4 bytes        DISK80_SIG_BUFFER (setup.S)
0x2d0 - 0x600        E820MAP
0x600 - 0x7ff        EDDBUF (setup.S) for disk signature read sector
0x600 - 0x7de        EDDBUF (setup.S)

0x800    string, 2K max    COMMAND_LINE, the kernel commandline as
            copied using CL_OFFSET.
            Note: this will be copied once more by setup.c
            into a local buffer which is only 256 bytes long.
            ( #define COMMAND_LINE_SIZE 256 )