boot.txt

ARM 嵌入式 系统 设计与实例开发 实验教材 二源码

**** THE KERNEL COMMAND LINE

The kernel command line has become an important way for the boot
loader to communicate with the kernel.  Some of its options are also
relevant to the boot loader itself, see "special command line options"
below.

The kernel command line is a null-terminated string up to 255
characters long, plus the final null.

If the boot protocol version is 2.02 or later, the address of the
kernel command line is given by the header field cmd_line_ptr (see
above.)

If the protocol version is *not* 2.02 or higher, the kernel
command line is entered using the following protocol:

	At offset 0x0020 (word), "cmd_line_magic", enter the magic
	number 0xA33F.

	At offset 0x0022 (word), "cmd_line_offset", enter the offset
	of the kernel command line (relative to the start of the
	real-mode kernel).
	
	The kernel command line *must* be within the memory region
	covered by setup_move_size, so you may need to adjust this
	field.


**** SAMPLE BOOT CONFIGURATION

As a sample configuration, assume the following layout of the real
mode segment:

	0x0000-0x7FFF	Real mode kernel
	0x8000-0x8FFF	Stack and heap
	0x9000-0x90FF	Kernel command line

Such a boot loader should enter the following fields in the header:

	unsigned long base_ptr;	/* base address for real-mode segment */

	if ( setup_sects == 0 ) {
		setup_sects = 4;
	}

	if ( protocol >= 0x0200 ) {
		type_of_loader = <type code>;
		if ( loading_initrd ) {
			ramdisk_image = <initrd_address>;
			ramdisk_size = <initrd_size>;
		}
		if ( protocol >= 0x0201 ) {
			heap_end_ptr = 0x9000 - 0x200;
			loadflags |= 0x80; /* CAN_USE_HEAP */
		}
		if ( protocol >= 0x0202 ) {
			cmd_line_ptr = base_ptr + 0x9000;
		} else {
			cmd_line_magic	= 0xA33F;
			cmd_line_offset = 0x9000;
			setup_move_size = 0x9100;
		}
	} else {
		/* Very old kernel */

		cmd_line_magic	= 0xA33F;
		cmd_line_offset = 0x9000;

		/* 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 */
		}

		/* It is recommended to clear memory up to the 32K mark */
		memset(0x90000 + (setup_sects+1)*512, 0,
		       (64-(setup_sects+1))*512);
	}


**** LOADING THE REST OF THE KERNEL

The non-real-mode kernel starts at offset (setup_sects+1)*512 in the
kernel file (again, if setup_sects == 0 the real value is 4.)  It
should be loaded at address 0x10000 for Image/zImage kernels and
0x100000 for bzImage kernels.

The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
bit (LOAD_HIGH) in the loadflags field is set:

	is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
	load_address = is_bzImage ? 0x100000 : 0x10000;

Note that Image/zImage kernels can be up to 512K in size, and thus use
the entire 0x10000-0x90000 range of memory.  This means it is pretty
much a requirement for these kernels to load the real-mode part at
0x90000.  bzImage kernels allow much more flexibility.


**** SPECIAL COMMAND LINE OPTIONS

If the command line provided by the boot loader is entered by the
user, the user may expect the following command line options to work.
They should normally not be deleted from the kernel command line even
though not all of them are actually meaningful to the kernel.  Boot
loader authors who need additional command line options for the boot
loader itself should get them registered in
linux/Documentation/kernel-parameters.txt to make sure they will not
conflict with actual kernel options now or in the future.

  vga=<mode>
	<mode> here is either an integer (in C notation, either
	decimal, octal, or hexadecimal) or one of the strings
	"normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
	(meaning 0xFFFD).  This value should be entered into the
	vid_mode field, as it is used by the kernel before the command
	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
	memory.  Note that this is an option to *both* the kernel and
	the bootloader!

  initrd=<file>
	An initrd should be loaded.  The meaning of <file> is
	obviously bootloader-dependent, and some boot loaders
	(e.g. LILO) do not have such a command.

In addition, some boot loaders add the following options to the
user-specified command line:

  BOOT_IMAGE=<file>
	The boot image which was loaded.  Again, the meaning of <file>
	is obviously bootloader-dependent.

  auto
	The kernel was booted without explicit user intervention.

If these options are added by the boot loader, it is highly
recommended that they are located *first*, before the user-specified
or configuration-specified command line.  Otherwise, "init=/bin/sh"
gets confused by the "auto" option.


**** RUNNING THE KERNEL

The kernel is started by jumping to the kernel entry point, which is
located at *segment* offset 0x20 from the start of the real mode
kernel.  This means that if you loaded your real-mode kernel code at
0x90000, the kernel entry point is 9020:0000.

At entry, ds = es = ss should point to the start of the real-mode
kernel code (0x9000 if the code is loaded at 0x90000), sp should be
set up properly, normally pointing to the top of the heap, and
interrupts should be disabled.  Furthermore, to guard against bugs in
the kernel, it is recommended that the boot loader sets fs = gs = ds =
es = ss.

In our example from above, we would do:

	/* Note: in the case of the "old" kernel protocol, base_ptr must
	   be == 0x90000 at this point; see the previous sample code */

	seg = base_ptr >> 4;

	cli();	/* Enter with interrupts disabled! */

	/* Set up the real-mode kernel stack */
	_SS = seg;
	_SP = 0x9000;	/* Load SP immediately after loading SS! */

	_DS = _ES = _FS = _GS = seg;
	jmp_far(seg+0x20, 0);	/* Run the kernel */

If your boot sector accesses a floppy drive, it is recommended to
switch off the floppy motor before running the kernel, since the
kernel boot leaves interrupts off and thus the motor will not be
switched off, especially if the loaded kernel has the floppy driver as
a demand-loaded module!


**** ADVANCED BOOT TIME HOOKS

If the boot loader runs in a particularly hostile environment (such as
LOADLIN, which runs under DOS) it may be impossible to follow the
standard memory location requirements.  Such a boot loader may use the
following hooks that, if set, are invoked by the kernel at the
appropriate time.  The use of these hooks should probably be
considered an absolutely last resort!

IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
%edi across invocation.

  realmode_swtch:
	A 16-bit real mode far subroutine invoked immediately before
	entering protected mode.  The default routine disables NMI, so
	your routine should probably do so, too.

  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.

	After completing your hook, you should jump to the address
	that was in this field before your boot loader overwrote it.