简单的引导示例在示例中，我们假设有实模式内存段布局：
当加载内核实模式部分到0x90000后，该内存段中的内容为：
0x0000-0x7fff 内核实模式部分 0x8000-0xdfff 堆栈 0xe000-0xffff 内核命令行
当加载内核实模式部分到0x90000或者引导协议为2.01及之前的版本：
0x0000-0x7fff 内核实模式部分 0x8000-0x97ff 堆栈 0x9800-0x9fff 内核命令行
内核加载程序应该对数据头做以下检查：
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 >= 0x0202 && loadflags & 0x01 ) heap_end = 0xe000; else heap_end = 0x9800;
if ( protocol >= 0x0201 ) { heap_end_ptr = heap_end - 0x200; loadflags |= 0x80; /* CAN_USE_HEAP */ }
if ( protocol >= 0x0202 ) { cmd_line_ptr = base_ptr + heap_end; strcpy(cmd_line_ptr, cmdline); } else { cmd_line_magic = 0xA33F; 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 = heap_end;
/* 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); 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); } 加载内核的其余部分内核的32位（非实模式）部分处于内核文件的(setup_sects+1)*512偏移处（再次提醒一下，如果setup_sects == 0，其真实值是4）。如果是Image/zImage类型的内核，它需要被加载到0x10000处。如果是bzImage类型的内核则需要加载到0x100000。 如果引导一些大于等于2.00并且loadflags的LOAD_HIGH位置位的话，那么内核肯定是bzImage类型的。也就是说，代码可以写成： 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. 需要特别留意的几个命令行选项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 (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!
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. |
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