linux內(nèi)核原代碼head.s部分的注釋
 

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第八軍團(tuán) 時(shí)間：2004-1-23 15:47:40 
   
/*
　　* 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
　　_pg_dir:
　　startup_32:
　　movl $0x10,%eax @@ds,es,fs,gs指向內(nèi)核數(shù)據(jù)段
　　mov %ax,%ds
　　mov %ax,%es
　　mov %ax,%fs
　　mov %ax,%gs
　　lss _stack_start,%esp @@ds送ss esp 指向stack_start (在sched.c定義）
　　 @@進(jìn)入保護(hù)模式的堆棧段的第一次變化，很奇怪
　　 @@為什么堆棧段也可正向增漲？
　　call setup_idt
　　call setup_gdt
　　movl $0x10,%eax # reload all the segment registers
　　mov %ax,%ds # after changing gdt. CS was already
　　mov %ax,%es # reloaded in 'setup_gdt' @@ 有reload??
　　mov %ax,%fs
　　mov %ax,%gs
　　lss _stack_start,%esp
　　xorl %eax,%eax
　　1: incl %eax # check that A20 really IS enabled
　　movl %eax,0x000000
　　cmpl %eax,0x100000 @@這是怎么測(cè)的 0x100000為什么值，
　　 @@明白，a20 not
　　enable,0x000000就是0x100000
　　je 1b
　　movl %cr0,%eax # check math chip
　　andl $0x80000011,%eax # Save PG,ET,PE
　　testl $0x10,%eax
　　jne 1f # ET is set - 387 is present
　　orl $4,%eax # else set emulate bit
　　1: movl %eax,%cr0
　　jmp after_page_tables @@注意，用jmp 不call，不返回
　　/*
　　* 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,%edx
　　movl $0x00080000,%eax
　　movw %dx,%ax /* selector = 0x0008 = cs */
　　movw $0x8E00,%dx /* interrupt gate - dpl=0, present */
　　 @@ ignore_int 低16-->ax 高16-->edx高字 
@@ 8e00-->dx 8-->eax高字
　　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:
　　lgdt gdt_descr
　　ret @@跳到main函數(shù) 

.org 0x1000
　　pg0: 

.org 0x2000
　　pg1: 

.org 0x3000
　　pg2: # This is not used yet, but if you
　　 # want to expand past 8 Mb, you'll have
　　 # to use it. 

.org 0x4000
　　after_page_tables:
　　pushl $0 # These are the parameters to main :-)
　　pushl $0
　　pushl $0
　　pushl $L6 # return address for main, if it decides to.
　　pushl $_main
　　jmp setup_paging @@再jmp
　　L6:
　　jmp L6 # main should never return here, but
　　 # just in case, we know what happens. 

/* This is the default interrupt "handler" :-) */
　　.align 2
　　ignore_int:
　　incb 0xb8000+160 # put something on the screen
　　movb $2,0xb8000+161 # so that we know something
　　iret # happened
　　 

/*
　　* Setup_paging
　　*
　　* This routine sets up paging by setting the page bit
　　* in cr0. The page tables are set up, identity-mapping
　　* the first 8MB. 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 8 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 8Mb, 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 "8Mb"), but I
　　* won't guarantee that's all :-( )
　　*/
　　.align 2
　　setup_paging:
　　movl $1024*3,%ecx @@pg_dir pg_table 清零
　　xorl %eax,%eax
　　xorl %edi,%edi /* pg_dir is at 0x000 */
　　cld;rep;stosl
　　movl $pg0+7,_pg_dir /* set present bit/user r/w */
　　movl $pg1+7,_pg_dir+4 /* --------- " " --------- */
　　movl $pg1+4092,%edi
　　movl $0x7ff007,%eax /* 8Mb - 4096 + 7 (r/w user,p) */
　　std
　　1: stosl /* fill pages backwards - more efficient :-) */
　　subl $0x1000,%eax
　　jge 1b @@ greater or equal
　　xorl %eax,%eax /* pg_dir is at 0x0000 */
　　movl %eax,%cr3 /* cr3 - page directory start */
　　movl %cr0,%eax
　　orl $0x80000000,%eax
　　movl %eax,%cr0 /* set paging (PG) bit */
　　ret /* this also flushes prefetch-queue */ 

@@頁(yè)目錄pg_dir=0 僅兩項(xiàng)(8M),
　　@@pg1,pg0頁(yè)表逆向填充，pg0的首頁(yè)仍為零
　　@@這樣映射，線性地址=物理地址 

.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 @@和前面head.S差一??
　　.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 0x00c09a00000007ff /* 8Mb */
　　.quad 0x00c09200000007ff /* 8Mb */
　　.quad 0x0000000000000000 /* TEMPORARY - don't use */
　　.fill 252,8,0 /* space for LDT's and TSS's etc */
　　@@內(nèi)核 邏輯地址（不含段寄存器）=線性地址