Linux0.11版源代码分析——boot/head.s
Linux0.11版源代码分析第三篇，head.s源码注释分析。
/* * linux/boot/head.s * * (C) 1991 Linus Torvalds */ /* * head.s contains the 32-bit startup code. * * NOTE!!! Startup happens at absolute address 0x00000000, which is also where * the page directory will exist. The startup code will be overwritten by * the page directory. */ /* * head.s 包含32位启动代码. * * 注意!!! 启动代码位于绝对地址0x00000000处,将来也是页目录所在 * 启动代码将被页目录所覆盖. */ !定义全局标识符 .text .globl _idt,_gdt,_pg_dir,_tmp_floppy_area _pg_dir: !页目录所在处 startup_32: !启动（32位模式） movl $0x10,%eax !段选择符 mov %ax,%ds !设置段寄存器 mov %ax,%es mov %ax,%fs mov %ax,%gs lss _stack_start,%esp !加载系统堆栈 call setup_idt !加载中断描述符表 call setup_gdt !加载全局描述符表 movl $0x10,%eax # reload all the segment registers !重新设置段寄存器 mov %ax,%ds # after changing gdt. CS was already !在改变gdt后，cs已经 mov %ax,%es # reloaded in 'setup_gdt' !重新加载在setup_gdt mov %ax,%fs ! mov %ax,%gs ! lss _stack_start,%esp !加载系统堆栈 xorl %eax,%eax !清空eax 1: incl %eax # check that A20 really IS enabled !检测A20地址线 movl %eax,0x000000 # loop forever if it isn't cmpl %eax,0x100000 je 1b /* * NOTE! 486 should set bit 16, to check for write-protect in supervisor * mode. Then it would be unnecessary with the "verify_area()"-calls. * 486 users probably want to set the NE (#5) bit also, so as to use * int 16 for math errors. */ movl %cr0,%eax # check math chip !检测数学芯片 andl $0x80000011,%eax # Save PG,PE,ET /* "orl $0x10020,%eax" here for 486 might be good */ orl $2,%eax # set MP !设置mp movl %eax,%cr0 ! call check_x87 !调用check_x87 jmp after_page_tables !跳转到after_page_tables，设置目录页表 /* * We depend on ET to be correct. This checks for 287/387. !通过ET检测数学协处理器 */ check_x87: !检测x87芯片 fninit !初始化协处理器 fstsw %ax !将该处理器状态放置到ax中 cmpb $0,%al !比较，如果是0，则不存在 je 1f /* no coprocessor: have to set bits */ !不存在，则跳转到标号1处 movl %cr0,%eax !将cr0寄存器内容复制到eax xorl $6,%eax /* reset MP, set EM */ !和6进行异或 movl %eax,%cr0 !复制到cr0 ret .align 2 1: .byte 0xDB,0xE4 /* fsetpm for 287, ignored by 387 */ ret /* * setup_idt * * sets up a idt with 256 entries pointing to * ignore_int, interrupt gates. It then loads * idt. Everything that wants to install itself * in the idt-table may do so themselves. Interrupts * are enabled elsewhere, when we can be relatively * sure everything is ok. This routine will be over- * written by the page tables. */ setup_idt: !设置idt lea ignore_int,%edx movl $0x00080000,%eax movw %dx,%ax /* selector = 0x0008 = cs */ movw $0x8E00,%dx /* interrupt gate - dpl=0, present */ lea _idt,%edi mov $256,%ecx rp_sidt: ! movl %eax,(%edi) movl %edx,4(%edi) addl $8,%edi dec %ecx jne rp_sidt lidt idt_descr ret /* * setup_gdt * * This routines sets up a new gdt and loads it. * Only two entries are currently built, the same * ones that were built in init.s. The routine * is VERY complicated at two whole lines, so this * rather long comment is certainly needed :-). * This routine will beoverwritten by the page tables. */ setup_gdt: !设置gdt lgdt gdt_descr ret /* * I put the kernel page tables right after the page directory, !我放置内核页表在页目录之后 * using 4 of them to span 16 Mb of physical memory. People with !使用16M物理内存中的4M。如果 * more than 16MB will have to expand this. !你有超过16M内存可以自己扩展 */ .org 0x1000 !偏移为0x1000为表1 pg0: .org 0x2000 !偏移为0x2000出放置表2 pg1: .org 0x3000 !放置表3 pg2: .org 0x4000 !放置表4 pg3: .org 0x5000 /* * tmp_floppy_area is used by the floppy-driver when DMA cannot * reach to a buffer-block. It needs to be aligned, so that it isn't * on a 64kB border. */ _tmp_floppy_area: .fill 1024,1,0 after_page_tables: !设置页目录表 pushl $0 # These are the parameters to main :-) !main函数的参数，但是实际上main函数不要参数，所以设置为0 pushl $0 pushl $0 pushl $L6 # return address for main, if it decides to. !main函数返回地址 pushl $_main !setup_paging返回地址，即setup_paging返回后执行main函数 jmp setup_paging !跳转到setup_paging L6: jmp L6 # main should never return here, but !main函数将永远不会返回到这里，但是一旦进入到这里，我们 # just in case, we know what happens. !将知道发生什么了 /* This is the default interrupt "handler" :-) */ !这是默认的中断处理 int_msg: !中断消息 .asciz "Unknown interrupt\n\r" .align 2 ignore_int: pushl %eax pushl %ecx pushl %edx push %ds push %es push %fs movl $0x10,%eax mov %ax,%ds mov %ax,%es mov %ax,%fs pushl $int_msg call _printk popl %eax pop %fs pop %es pop %ds popl %edx popl %ecx popl %eax iret //中断返回 /* * Setup_paging * * This routine sets up paging by setting the page bit * in cr0. The page tables are set up, identity-mapping * the first 16MB. The pager assumes that no illegal * addresses are produced (ie >4Mb on a 4Mb machine). * * NOTE! Although all physical memory should be identity * mapped by this routine, only the kernel page functions * use the >1Mb addresses directly. All "normal" functions * use just the lower 1Mb, or the local data space, which * will be mapped to some other place - mm keeps track of * that. * * For those with more memory than 16 Mb - tough luck. I've * not got it, why should you :-) The source is here. Change * it. (Seriously - it shouldn't be too difficult. Mostly * change some constants etc. I left it at 16Mb, as my machine * even cannot be extended past that (ok, but it was cheap :-) * I've tried to show which constants to change by having * some kind of marker at them (search for "16Mb"), but I * won't guarantee that's all :-( ) */ .align 2 setup_paging: !设置页目录表 movl $1024*5,%ecx /* 5 pages - pg_dir+4 page tables */ !五页，一个页目录表+四个页表，实际上作为计数器为下面循环计数 xorl %eax,%eax !清空eax xorl %edi,%edi /* pg_dir is at 0x000 */ !清空edi cld;rep;stosl !清楚方向位标识，以四字节为单位循环将eax中的数据拷贝到es:edi中，即将前五页内存清零 movl $pg0+7,_pg_dir /* set present bit/user r/w */ !设置页目录表 movl $pg1+7,_pg_dir+4 /* --------- " " --------- */ movl $pg2+7,_pg_dir+8 /* --------- " " --------- */ movl $pg3+7,_pg_dir+12 /* --------- " " --------- */ movl $pg3+4092,%edi !也表中最后一项 movl $0xfff007,%eax /* 16Mb - 4096 + 7 (r/w user,p) */ !页表填充 std 1: stosl /* fill pages backwards - more efficient :-) */ subl $0x1000,%eax jge 1b xorl %eax,%eax /* pg_dir is at 0x0000 */ !设置页目录表起始地址 movl %eax,%cr3 /* cr3 - page directory start */ movl %cr0,%eax !启动分页处理机制 orl $0x80000000,%eax movl %eax,%cr0 /* set paging (PG) bit */ ret /* this also flushes prefetch-queue */ !将main地址弹出栈，从而进行main函数执行 .align 2 .word 0 idt_descr: !中断描述符表 .word 256*8-1 # idt contains 256 entries .long _idt .align 2 .word 0 gdt_descr: !全局段描述符表 .word 256*8-1 # so does gdt (not that that's any ! .long _gdt # magic number, but it works for me :^) .align 3 _idt: .fill 256,8,0 # idt is uninitialized !中断描述符256个未初始化 _gdt: .quad 0x0000000000000000 /* NULL descriptor */ !空选择符 .quad 0x00c09a0000000fff /* 16Mb */ !代码段 .quad 0x00c0920000000fff /* 16Mb */ !数据段 .quad 0x0000000000000000 /* TEMPORARY - don't use */ !临时的，不用 .fill 252,8,0 /* space for LDT's and TSS's etc */ !填充252个8字节的段选择符为0
至此，终于将boot文件夹中的三个文件注视，完毕，基本理解了Linux操作系统的boot机制。下一步，分析init文件夹中的代码。
