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
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
0x226 - 0x7ff		setup.S code.

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 )