• CVS: CVSROOT=:pserver:anoncvs@subversions.gnu.org:/cvsroot/grub

  cvs -d:pserver:anoncvs@subversions.gnu.org:/cvsroot/grub
  Password:{Enter}
  cvs -d:pserver:anoncvs@subversions.gnu.org:/cvsroot/grub co grub
  

• Source code:ftp://alpha.gnu.org/gnu/grub/grub-0.92.tar.gz
• Binaries:ftp://alpha.gnu.org/gnu/grub/grub-0.92-i386-pc.tar.gz

DOS and Windows users will need PARTCOPY or RAWRITE:

• http://www.execpc.com/~geezer/johnfine/index.htm#zero
• http://uranus.it.swin.edu.au/~jn/linux/rawwrite.htm
• http://www.tux.org/pub/dos/rawrite/

• UNIX

  configure ; make ; make check ; make install
  

• DOS and Windows (ha! forget it)

The unmodified stage1 and stage2 files work only with 1.44 meg floppies.

1. Get the GRUB binaries (files "stage1" and "stage2")
2. Concatenate the files "stage1" and "stage2" into one file:

       (DOS)   copy /b stage1 + stage2 boot
   
       (UNIX)  cat stage1 stage2 >boot
   

3. Write the file "boot" directly to the floppy disk:

           (DOS)   rawrite boot a:
                       -OR-
           (DOS)   partcopy boot 0 168000 -f0
   
           (UNIX)  cat boot >/dev/fd0
   

   PARTCOPY will give an error message because the file "boot" is much shorter than 0x168000 bytes, but this is OK.

1. Make a bootable GRUB floppy with no filesystem, as described above.
2. Copy the files "stage1" and "stage2" to a second floppy disk, one formatted with a filesystem that GRUB recognizes. To use the GRUB "setup" command, these files must be stored in subdirectory "/boot/grub":

   (DOS)   mkdir a:\boot
           mkdir a:\boot\grub
           copy stage1 a:\boot\grub
           copy stage2 a:\boot\grub
   
   (UNIX)  mount /dev/fd0 /mnt
           mkdir /mnt/boot
           mkdir /mnt/boot/grub
           cp stage1 /mnt/boot/grub
           cp stage2 /mnt/boot/grub
   

3. After GRUB is installed on floppy disk #2, the file "stage2" must not be modified, deleted, defragged, or moved. If it is modified in any way, the disk will no longer be bootable. To prevent this, make the file read-only:

   (DOS)   attrib +r +s stage2
   
   (UNIX)  chmod a-w stage2
   

   The DOS command above makes "stage2" a System file as well as Read-only. This is needed to protect against DEFRAG.

   File "stage1" will be copied into the bootsector. If this file is moved or deleted after GRUB is installed, the disk will still be bootable.

4. Boot your computer from the floppy with GRUB but no filesystem. At the GRUB prompt, eject this floppy and insert the formatted floppy disk (with the filesystem and "stage1" and "stage2" files, possibly in directory "/boot/grub".
   • If files "stage1" and "stage2" are stored in "/boot/grub" on disk #2, you can install GRUB on disk #2 simply by typing:

         setup (fd0)
     

     This is apparently equivalent to this single command line:

         install /boot/grub/stage1 d (fd0) /boot/grub/stage2 p
                 /boot/grub/menu.lst
     

   • If files "stage1" and "stage2" are stored elsewhere, e.g. in subdirectory "/foo", install GRUB on the second floppy disk like this (this is also a single command line):

         install=(fd0)/foo/stage1 (fd0) (fd0)/foo/stage2 0x8000 p
                 (fd0)/foo/menu.lst
     

     Floppy disk #2 (the disk with the filesystem) is now bootable.

Install Syntax:

• 0x8000 -- This value gets embedded into the bootsector of the floppy to indicate the address that stage2 should be loaded into memory.
• p -- Modifies stage2, to report to the kernel the partition that stage 2 was found on (I think).
• (fd0)/boot/grub/menu.lst -- Modifies stage2, and tells it where to load the menu.lst (bootmenu) configuration file from.

Whatever its file format, your kernel MUST have a Multiboot header. This header

1. must be aligned on a dword (4-byte) boundary, and
2. must appear in the first 8K of the kernel file.

An address within the first 8K of the .text section is not necessarily within 8K of the start of the file.

GRUB understands the ELF file format directly. If your kernel is ELF, you can use the simple Multiboot header shown here:

MULTIBOOT_PAGE_ALIGN   equ 1<<0
MULTIBOOT_MEMORY_INFO  equ 1<<1

MULTIBOOT_HEADER_MAGIC equ 0x1BADB002
MULTIBOOT_HEADER_FLAGS equ MULTIBOOT_PAGE_ALIGN | MULTIBOOT_MEMORY_INFO
CHECKSUM               equ -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)

; The Multiboot header (in NASM syntax)
align 4
dd MULTIBOOT_HEADER_MAGIC
dd MULTIBOOT_HEADER_FLAGS
dd CHECKSUM

Put this near the beginning of your kernel startup code, then build your kernel. After the kernel is built, you can use the GRUB "mbchk" utility to test if the kernel complies with Multiboot.

mbchk in GRUB v0.90 will sometimes report that a kernel is Multiboot-compliant when it's not. This happens, for example, if you use the Multiboot header, no aout kludge (see below), and binary kernel file format.

GRUB reads the physical address (load address; LMA) of the kernel from the ELF file. This value must be

1. at or above 1 meg, and
2. below the end of physical RAM

If the load address is below 1 meg, you get error #7: Loading below 1MB is not supported This is a limitation of GRUB, not of Multiboot. If the load address is beyond the end of RAM, you get error #28: Selected item cannot fit into memory

And if you use a very high address like 0xC0000000, the math apparently overflows, and you get error #7 again. "mbchk" does not check for these errors. Normally, the physical address is the same as the VMA, and is set either in the linker script or on the linker command line ("ld -Ttext=0x100000 ..."). If your version of 'ld' supports it, the physical and virtual addresses can be specified separately in the linker script using 'AT':

&elf.ld;

DJGPP users can make ELF files using these tools: http://www.multimania.com/placr/binutils.html

This server is often difficult to reach. Someone should mirror these tools. I recommend building a regular COFF kernel, then doing this:

objcopy-elf -O elf32-i386 krnl.cof krnl.elf

Failing this, you can make GRUB load a COFF kernel by using the "aout kludge". This uses additional fields at the end of the Multiboot header, like this:

MULTIBOOT_PAGE_ALIGN   equ 1<<0
MULTIBOOT_MEMORY_INFO  equ 1<<1
MULTIBOOT_AOUT_KLUDGE  equ 1<<16

MULTIBOOT_HEADER_MAGIC equ 0x1BADB002
MULTIBOOT_HEADER_FLAGS equ MULTIBOOT_PAGE_ALIGN | MULTIBOOT_MEMORY_INFO | MULTIBOOT_AOUT_KLUDGE
CHECKSUM               equ -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)

; The Multiboot header
   align 4
mboot:
   dd MULTIBOOT_HEADER_MAGIC
   dd MULTIBOOT_HEADER_FLAGS
   dd CHECKSUM
; fields used if MULTIBOOT_AOUT_KLUDGE is set in MULTIBOOT_HEADER_FLAGS
   dd mboot ; these are PHYSICAL addresses
   dd code  ; start of kernel .text (code) section
   dd edata ; end of kernel .data section
   dd end   ; end of kernel BSS
   dd start ; kernel entry point (initial EIP)

The "aout kludge" works with binary and other file formats, too. (xxx - untested; should be correct)

GRUB v0.90 will ignore the aout kludge if it's present in an ELF file.

1. Make sure your kernel is some convenient place where GRUB can find it. It need not be on the floppy disk.
2. Boot from a GRUB floppy.
3. (Optional) Tell GRUB what device to use for its root directory:

           root (hd0,1)
   

   This "mounts" the 2nd primary partition on the 1st hard drive as the root directory.
4. Tell GRUB where your kernel is:

           kernel /krnl.elf
   

   If you did not specify the root device, you must give the device explicitly at the start of each path name:

           kernel (hd0,1)/krnl.elf
   

5. If GRUB has no complaints about the kernel file, boot it:

           boot
   

Upon entry to the 32-bit kernel

1. CS points to a code segment descriptor with base address 0 and limit 4 gig - 1
2. DS, SS, ES, FS, and GS point to a data segment descriptor with base address 0 and limit 4 gig - 1
3. A20 is enabled
4. Paging is disabled
5. Interrupts are disabled. No IDT is defined.
6. The size and location of the GDT and selector values are undefined. Your kernel should create it's own GDT as soon as possible.
7. EAX=0x2BADB002
8. EBX contains the linear address of (i.e. a pointer to) a block of system and bootstrap information:

   /* The Multiboot information.  */
   typedef struct multiboot_info
   {
      unsigned long flags;
      unsigned long mem_lower;
      unsigned long mem_upper;
      unsigned long boot_device;
      unsigned long cmdline;
      unsigned long mods_count;
      unsigned long mods_addr;
      union
      {
         aout_symbol_table_t aout_sym;
         elf_section_header_table_t elf_sec;
       } u;
       unsigned long mmap_length;
       unsigned long mmap_addr;
   } multiboot_info_t;
   

This information can be accessed from C code by pushing the pointer in EBX onto the stack before calling main():

...
push ebx
call _main 	; "call main" for Linux/ELF
...

#include <multiboot.h>
...
int main(multiboot_info_t *boot_info)
{       if(boot_info->flags & 2)
	{       kprintf("the command line is:\n'%s'\n",
			(char *)boot_info->cmdline); }
       	 	...

Example 1:

        # Entry 0:
        title   WildMagnolia
        root    (fd0)
        kernel  /boot/kernel.elf
        module  /boot/mod_a
        module  /boot/mod_b

Example 2:

&menu.lst;

gzip -c -9 krnl.elf > krnlz

kernel /boot/kernlz

• Origin of this tutorial
• GRUB project page
• Another GRUB tutorial
• http://www.gnu.org/software/grub/