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Diffstat (limited to 'Documentation/i386/boot.txt')
| -rw-r--r-- | Documentation/i386/boot.txt | 101 |
1 files changed, 78 insertions, 23 deletions
diff --git a/Documentation/i386/boot.txt b/Documentation/i386/boot.txt index 6498666ea330..d01b7a2a0f2e 100644 --- a/Documentation/i386/boot.txt +++ b/Documentation/i386/boot.txt @@ -2,7 +2,7 @@ ---------------------------- 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 @@ -11,7 +11,7 @@ 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. +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. @@ -183,9 +183,9 @@ filled out, however: 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 @@ -210,6 +210,9 @@ filled out, however: 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 @@ -278,14 +281,54 @@ command line is entered using the following protocol: 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: @@ -301,22 +344,33 @@ 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 */ @@ -324,12 +378,11 @@ Such a boot loader should enter the following fields in the header: 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); @@ -375,10 +428,11 @@ conflict with actual kernel options now or in the future. 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! @@ -428,7 +482,7 @@ In our example from above, we would do: /* 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 */ @@ -460,8 +514,9 @@ IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and 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. |
