THE LINUX/I386 BOOT PROTOCOL ---------------------------- H. Peter Anvin <hpa@zytor.com> Last update 2005-09-02 On the i386 platform, the Linux kernel uses a rather complicated boot convention. This has evolved partially due to historical aspects, as well as the desire in the early days to have the kernel itself be a bootable image, the complicated PC memory model and due to changed 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. Old kernels: zImage/Image support only. Some very early kernels may not even support a command line. Protocol 2.00: (Kernel 1.3.73) Added bzImage and initrd support, as well as a formalized way to communicate between the boot loader and the kernel. setup.S made relocatable, although the traditional setup area still assumed writable. Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning. Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol. Lower the conventional memory ceiling. No overwrite of the traditional setup area, thus making booting safe for systems which use the EBDA from SMM or 32-bit BIOS entry points. zImage deprecated but still supported. Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible initrd address available to the bootloader. Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes. **** MEMORY LAYOUT The traditional memory map for the kernel loader, used for Image or zImage kernels, typically looks like: | | 0A0000 +------------------------+ | Reserved for BIOS | Do not use. Reserved for BIOS EBDA. 09A000 +------------------------+ | Stack/heap/cmdline | For use by the kernel real-mode code. 098000 +------------------------+ | Kernel setup | The kernel real-mode code. 090200 +------------------------+ | Kernel boot sector | The kernel legacy boot sector. 090000 +------------------------+ | Protected-mode kernel | The bulk of the kernel image. 010000 +------------------------+ | Boot loader | <- Boot sector entry point 0000:7C00 001000 +------------------------+ | Reserved for MBR/BIOS | 000800 +------------------------+ | Typically used by MBR | 000600 +------------------------+ | BIOS use only | 000000 +------------------------+ When using bzImage, the protected-mode kernel was relocated to 0x100000 ("high memory"), and the kernel real-mode block (boot sector, setup, and stack/heap) was made relocatable to any address between 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and 2.01 the command line is still required to live in the 0x9XXXX memory range, and that memory range is still overwritten by the early kernel. The 2.02 protocol resolves that problem. It is desirable to keep the "memory ceiling" -- the highest point in low memory touched by the boot loader -- as low as possible, since some newer BIOSes have begun to allocate some rather large amounts of memory, called the Extended BIOS Data Area, near the top of low memory. The boot loader should use the "INT 12h" BIOS call to verify how much low memory is available. Unfortunately, if INT 12h reports that the amount of memory is too low, there is usually nothing the boot loader can do but to report an error to the user. The boot loader should therefore be designed to take up as little space in low memory as it reasonably can. For zImage or old bzImage kernels, which need data written into the 0x90000 segment, the boot loader should make sure not to use memory above the 0x9A000 point; too many BIOSes will break above that point. **** THE REAL-MODE KERNEL HEADER In the following text, and anywhere in the kernel boot sequence, "a sector" refers to 512 bytes. It is independent of the actual sector size of the underlying medium. The first step in loading a Linux kernel should be to load the real-mode code (boot sector and setup code) and then examine the following header at offset 0x01f1. The real-mode code can total up to 32K, although the boot loader may choose to load only the first two sectors (1K) and then examine the bootup sector size. The header looks like: Offset Proto Name Meaning /Size 01F1/1 ALL(1 setup_sects The size of the setup in sectors 01F2/2 ALL root_flags If set, the root is mounted readonly 01F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras 01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only 01FA/2 ALL vid_mode Video mode control 01FC/2 ALL root_dev Default root device number 01FE/2 ALL boot_flag 0xAA55 magic number 0200/2 2.00+ jump Jump instruction 0202/4 2.00+ header Magic signature "HdrS" 0206/2 2.00+ version Boot protocol version supported 0208/4 2.00+ realmode_swtch Boot loader hook (see below) 020C/2 2.00+ start_sys The load-low segment (0x1000) (obsolete) 020E/2 2.00+ kernel_version Pointer to kernel version string 0210/1 2.00+ type_of_loader Boot loader identifier 0211/1 2.00+ loadflags Boot protocol option flags 0212/2 2.00+ setup_move_size Move to high memory size (used with hooks) 0214/4 2.00+ code32_start Boot loader hook (see below) 0218/4 2.00+ ramdisk_image initrd load address (set by boot loader) 021C/4 2.00+ ramdisk_size initrd size (set by boot loader) 0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only 0224/2 2.01+ heap_end_ptr Free memory after setup end 0226/2 N/A pad1 Unused 0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line 022C/4 2.03+ initrd_addr_max Highest legal initrd address (1) For backwards compatibility, if the setup_sects field contains 0, the real value is 4. (2) For boot protocol prior to 2.04, the upper two bytes of the syssize field are unusable, which means the size of a bzImage kernel cannot be determined. If the "HdrS" (0x53726448) magic number is not found at offset 0x202, the boot protocol version is "old". Loading an old kernel, the following parameters should be assumed: Image type = zImage initrd not supported Real-mode kernel must be located at 0x90000. Otherwise, the "version" field contains the protocol version, e.g. protocol version 2.01 will contain 0x0201 in this field. When setting fields in the header, you must make sure only to set fields supported by the protocol version in use. The "kernel_version" field, if set to a nonzero value, contains a pointer to a null-terminated human-readable kernel version number string, less 0x200. This can be used to display the kernel version to the user. This value should be less than (0x200*setup_sects). For example, if this value is set to 0x1c00, the kernel version number string can be found at offset 0x1e00 in the kernel file. This is a valid value if and only if the "setup_sects" field contains the value 14 or higher. Most boot loaders will simply load the kernel at its target address directly. Such boot loaders do not need to worry about filling in most of the fields in the header. The following fields should be filled out, however: vid_mode: Please see the section on SPECIAL COMMAND LINE OPTIONS. type_of_loader: If your boot loader has an assigned id (see table below), enter 0xTV here, where T is an identifier for the boot loader and V is a version number. Otherwise, enter 0xFF here. Assigned boot loader ids: 0 LILO 1 Loadlin 2 bootsect-loader 3 SYSLINUX 4 EtherBoot 5 ELILO 7 GRuB 8 U-BOOT Please contact <hpa@zytor.com> if you need a bootloader ID value assigned. loadflags, heap_end_ptr: If the protocol version is 2.01 or higher, enter the offset limit of the setup heap into heap_end_ptr and set the 0x80 bit (CAN_USE_HEAP) of loadflags. heap_end_ptr appears to be relative to the start of setup (offset 0x0200). setup_move_size: When using protocol 2.00 or 2.01, if the real mode kernel is not loaded at 0x90000, it gets moved there later in the loading sequence. Fill in this field if you want additional data (such as the kernel command line) moved in addition to the real-mode kernel itself. 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 and the ramdisk_size to the size of the ramdisk data. The initrd should typically be located as high in memory as possible, as it may otherwise get overwritten by the early kernel initialization sequence. However, it must never be located above the address specified in the initrd_addr_max field. The initrd should be at least 4K page aligned. cmd_line_ptr: If the protocol version is 2.02 or higher, this is a 32-bit pointer to the kernel command line. The kernel command line can be located anywhere between the end of setup and 0xA0000. Fill in this field even if your boot loader does not support a command line, in which case you can point this to an empty string (or better yet, to the string "auto".) If this field is left at zero, the kernel will assume that your boot loader does not support the 2.02+ protocol. ramdisk_max: The maximum address that may be occupied by the initrd contents. For boot protocols 2.02 or earlier, this field is not present, and the maximum address is 0x37FFFFFF. (This address is defined as the address of the highest safe byte, so if your ramdisk is exactly 131072 bytes long and this field is 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.) **** 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 currently up to 255 characters long, plus the final null. A string that is too long will be automatically truncated by the kernel, a boot loader may allow a longer command line to be passed to permit future kernels to extend this limit. 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.) This address can be anywhere between the end of the setup heap and 0xA0000. 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 (this is a typical, and recommended layout): 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 32-bit (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 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.