Linux 内核解读之Boot----Head

 Linux 内核解读之Boot----Head

Linux跑到Head时，已近开始有Task了，Head.s主要做的动作，先是加载数据段寄存器，并且设置256项中断描述符表，也就是说中断项最多有256个

然后设置一遍GDT全局描述符表，接着会做A20地址的检测看是否开启了A20，没有开启系统就没有办法跑下去了，得认为重启机器。之后开启内存管理的

分页机制，注意这里是作为一项内存线性地址和逻辑地址映射的不一样的关键，分页OK后，就要开始进入init/main.c了，真正的跑起Linux的task.

程序描述流程图如下:

/* *  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. */ .text .globl _idt,_gdt,_pg_dir,_tmp_floppy_area _pg_dir: startup_32: movl $0x10,%eaxmov %ax,%dsmov %ax,%esmov %ax,%fsmov %ax,%gslss _stack_start,%espcall setup_idtcall setup_gdtmovl $0x10,%eax# reload all the segment registersmov %ax,%ds# after changing gdt. CS was alreadymov %ax,%es# reloaded in 'setup_gdt'mov %ax,%fsmov %ax,%gslss _stack_start,%espxorl %eax,%eax 1:incl %eax# check that A20 really IS enabled movl %eax,0x000000# loop forever if it isn'tcmpl %eax,0x100000je 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 chipandl $0x80000011,%eax# Save PG,PE,ET /* "orl $0x10020,%eax" here for 486 might be good */ orl $2,%eax# set MPmovl %eax,%cr0call check_x87jmp after_page_tables/* * We depend on ET to be correct. This checks for 287/387. */ check_x87: fninitfstsw %axcmpb $0,%alje 1f/* no coprocessor: have to set bits */movl %cr0,%eaxxorl $6,%eax/* reset MP, set EM */movl %eax,%cr0ret .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: lea ignore_int,%edxmovl $0x00080000,%eaxmovw %dx,%ax/* selector = 0x0008 = cs */movw $0x8E00,%dx/* interrupt gate - dpl=0, present */lea _idt,%edimov $256,%ecx rp_sidt: movl %eax,(%edi)movl %edx,4(%edi)addl $8,%edidec %ecxjne rp_sidtlidt idt_descrret/* *  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: lgdt gdt_descrret/* * I put the kernel page tables right after the page directory, * using 4 of them to span 16 Mb of physical memory. People with * more than 16MB will have to expand this. */ .org 0x1000 pg0:.org 0x2000 pg1:.org 0x3000 pg2:.org 0x4000 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 1024,1,0after_page_tables: pushl $0# These are the parameters to main :-)pushl $0pushl $0pushl $L6# return address for main, if it decides to.pushl $_mainjmp setup_paging L6: jmp L6# main should never return here, but # just in case, we know what happens./* This is the default interrupt "handler" :-) */ int_msg: .asciz "Unknown interrupt\n\r" .align 2 ignore_int: pushl %eaxpushl %ecxpushl %edxpush %dspush %espush %fsmovl $0x10,%eaxmov %ax,%dsmov %ax,%esmov %ax,%fspushl $int_msgcall _printkpopl %eaxpop %fspop %espop %dspopl %edxpopl %ecxpopl %eaxiret/* * 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: movl $1024*5,%ecx/* 5 pages - pg_dir+4 page tables */xorl %eax,%eaxxorl %edi,%edi/* pg_dir is at 0x000 */cld;rep;stoslmovl $pg0+7,_pg_dir/* set present bit/user r/w */movl $pg1+7,_pg_dir+4/*  --------- " " --------- */movl $pg2+7,_pg_dir+8/*  --------- " " --------- */movl $pg3+7,_pg_dir+12/*  --------- " " --------- */movl $pg3+4092,%edimovl $0xfff007,%eax/*  16Mb - 4096 + 7 (r/w user,p) */std 1:stosl/* fill pages backwards - more efficient :-) */ subl $0x1000,%eaxjge 1bxorl %eax,%eax/* pg_dir is at 0x0000 */movl %eax,%cr3/* cr3 - page directory start */movl %cr0,%eaxorl $0x80000000,%eaxmovl %eax,%cr0/* set paging (PG) bit */ret/* this also flushes prefetch-queue */.align 2 .word 0 idt_descr: .word 256*8-1# idt contains 256 entries.long _idt .align 2 .word 0 gdt_descr: .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_gdt:.quad 0x0000000000000000/* NULL descriptor */ .quad 0x00c09a0000000fff/* 16Mb */.quad 0x00c0920000000fff/* 16Mb */.quad 0x0000000000000000/* TEMPORARY - don't use */.fill 252,8,0/* space for LDT's and TSS's etc */