----------------------------
H. Peter Anvin <hpa@zytor.com>
- Last update 2007-03-06
+ Last update 2007-05-07
On the i386 platform, the Linux kernel uses a rather complicated boot
convention. This has evolved partially due to historical aspects, as
expectations in the PC industry caused by the effective demise of
real-mode DOS as a mainstream operating system.
-Currently, four versions of the Linux/i386 boot protocol exist.
+Currently, the following versions of the Linux/i386 boot protocol exist.
Old kernels: zImage/Image support only. Some very early kernels
may not even support a command line.
a version number. Otherwise, enter 0xFF here.
Assigned boot loader ids:
- 0 LILO
+ 0 LILO (0x00 reserved for pre-2.00 bootloader)
1 Loadlin
- 2 bootsect-loader
+ 2 bootsect-loader (0x20, all other values reserved)
3 SYSLINUX
4 EtherBoot
5 ELILO
additional data (such as the kernel command line) moved in
addition to the real-mode kernel itself.
+ The unit is bytes starting with the beginning of the boot
+ sector.
+
ramdisk_image, ramdisk_size:
If your boot loader has loaded an initial ramdisk (initrd),
set ramdisk_image to the 32-bit pointer to the ramdisk data
field.
+**** MEMORY LAYOUT OF THE REAL-MODE CODE
+
+The real-mode code requires a stack/heap to be set up, as well as
+memory allocated for the kernel command line. This needs to be done
+in the real-mode accessible memory in bottom megabyte.
+
+It should be noted that modern machines often have a sizable Extended
+BIOS Data Area (EBDA). As a result, it is advisable to use as little
+of the low megabyte as possible.
+
+Unfortunately, under the following circumstances the 0x90000 memory
+segment has to be used:
+
+ - When loading a zImage kernel ((loadflags & 0x01) == 0).
+ - When loading a 2.01 or earlier boot protocol kernel.
+
+ -> For the 2.00 and 2.01 boot protocols, the real-mode code
+ can be loaded at another address, but it is internally
+ relocated to 0x90000. For the "old" protocol, the
+ real-mode code must be loaded at 0x90000.
+
+When loading at 0x90000, avoid using memory above 0x9a000.
+
+For boot protocol 2.02 or higher, the command line does not have to be
+located in the same 64K segment as the real-mode setup code; it is
+thus permitted to give the stack/heap the full 64K segment and locate
+the command line above it.
+
+The kernel command line should not be located below the real-mode
+code, nor should it be located in high memory.
+
+
**** SAMPLE BOOT CONFIGURATION
As a sample configuration, assume the following layout of the real
-mode segment (this is a typical, and recommended layout):
+mode segment:
+
+ When loading below 0x90000, use the entire segment:
+
+ 0x0000-0x7fff Real mode kernel
+ 0x8000-0xdfff Stack and heap
+ 0xe000-0xffff Kernel command line
- 0x0000-0x7FFF Real mode kernel
- 0x8000-0x8FFF Stack and heap
- 0x9000-0x90FF Kernel command line
+ When loading at 0x90000 OR the protocol version is 2.01 or earlier:
+
+ 0x0000-0x7fff Real mode kernel
+ 0x8000-0x97ff Stack and heap
+ 0x9800-0x9fff Kernel command line
Such a boot loader should enter the following fields in the header:
ramdisk_image = <initrd_address>;
ramdisk_size = <initrd_size>;
}
+
+ if ( protocol >= 0x0202 && loadflags & 0x01 )
+ heap_end = 0xe000;
+ else
+ heap_end = 0x9800;
+
if ( protocol >= 0x0201 ) {
- heap_end_ptr = 0x9000 - 0x200;
+ heap_end_ptr = heap_end - 0x200;
loadflags |= 0x80; /* CAN_USE_HEAP */
}
+
if ( protocol >= 0x0202 ) {
- cmd_line_ptr = base_ptr + 0x9000;
+ cmd_line_ptr = base_ptr + heap_end;
+ strcpy(cmd_line_ptr, cmdline);
} else {
cmd_line_magic = 0xA33F;
- cmd_line_offset = 0x9000;
- setup_move_size = 0x9100;
+ cmd_line_offset = heap_end;
+ setup_move_size = heap_end + strlen(cmdline)+1;
+ strcpy(base_ptr+cmd_line_offset, cmdline);
}
} else {
/* Very old kernel */
+ heap_end = 0x9800;
+
cmd_line_magic = 0xA33F;
- cmd_line_offset = 0x9000;
+ cmd_line_offset = heap_end;
/* A very old kernel MUST have its real-mode code
loaded at 0x90000 */
if ( base_ptr != 0x90000 ) {
/* Copy the real-mode kernel */
memcpy(0x90000, base_ptr, (setup_sects+1)*512);
- /* Copy the command line */
- memcpy(0x99000, base_ptr+0x9000, 256);
-
base_ptr = 0x90000; /* Relocated */
}
+ strcpy(0x90000+cmd_line_offset, cmdline);
+
/* It is recommended to clear memory up to the 32K mark */
memset(0x90000 + (setup_sects+1)*512, 0,
(64-(setup_sects+1))*512);
line is parsed.
mem=<size>
- <size> is an integer in C notation optionally followed by K, M
- or G (meaning << 10, << 20 or << 30). This specifies the end
- of memory to the kernel. This affects the possible placement
- of an initrd, since an initrd should be placed near end of
+ <size> is an integer in C notation optionally followed by
+ (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
+ << 30, << 40, << 50 or << 60). This specifies the end of
+ memory to the kernel. This affects the possible placement of
+ an initrd, since an initrd should be placed near end of
memory. Note that this is an option to *both* the kernel and
the bootloader!
/* Set up the real-mode kernel stack */
_SS = seg;
- _SP = 0x9000; /* Load SP immediately after loading SS! */
+ _SP = heap_end;
_DS = _ES = _FS = _GS = seg;
jmp_far(seg+0x20, 0); /* Run the kernel */
code32_start:
A 32-bit flat-mode routine *jumped* to immediately after the
transition to protected mode, but before the kernel is
- uncompressed. No segments, except CS, are set up; you should
- set them up to KERNEL_DS (0x18) yourself.
+ uncompressed. No segments, except CS, are guaranteed to be
+ set up (current kernels do, but older ones do not); you should
+ set them up to BOOT_DS (0x18) yourself.
After completing your hook, you should jump to the address
that was in this field before your boot loader overwrote it.
projects / linux-2.6 / commitdiff