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 )
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