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diff --git a/doc/linux-i386-boot.txt b/doc/linux-i386-boot.txt new file mode 100644 index 0000000..afa6d12 --- /dev/null +++ b/doc/linux-i386-boot.txt @@ -0,0 +1,438 @@ + THE LINUX/I386 BOOT PROTOCOL + ---------------------------- + + H. Peter Anvin <hpa@zytor.com> + Last update 2002-01-01 + +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. + + +**** 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 setup_sects The size of the setup in sectors +01F2/2 ALL root_flags If set, the root is mounted readonly +01F4/2 ALL syssize DO NOT USE - for bootsect.S use only +01F6/2 ALL swap_dev DO NOT USE - obsolete +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 + +For backwards compatibility, if the setup_sects field contains 0, the +real value is 4. + +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 + + 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 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 +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. |