Re: empty_zero_page+setup on intel?

Hans Lermen (
Sun, 31 May 1998 23:37:58 +0200 (MEST)

On Sat, 30 May 1998, Martin Mares wrote:

> Hi,


> > Ok, in order to not do the work twice, I'll append a summary, that should
> > perhaps go some where under linux/Documents.
> That's exactly I was going to do... ;-) [well, at least for one year...]

Good, so we agree;-)
Could you please manage to pass it to the right places while submitting
the related patches (else the below summary won't match)

Here the re-worked version (assuming your patches will be applied):

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:


Offset Type Description
------ ---- -----------
0 18 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
V = version
0x211 char loadflags:
bit0 = 1: kernel is loaded high (bzImage)
bit7 = 1: Heap and pointer (see below) set by boot
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 )


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