boot/head.S

/*
* linux/boot/head.S
*
* Copyright (C) 1991, 1992 Linus Torvalds
*/

/*
* head.S contains the 32-bit startup code.
*/

.text
.globl _idt,_gdt,
.globl _swapper_pg_dir,_pg0
.globl _empty_bad_page
.globl _empty_bad_page_table
.globl _empty_zero_page
.globl _tmp_floppy_area,_floppy_track_buffer

#include <linux/tasks.h>
#include <linux/segment.h>

#define CL_MAGIC_ADDR 0x90020
#define CL_MAGIC 0xA33F
#define CL_BASE_ADDR 0x90000
#define CL_OFFSET 0x90022

/*
* swapper_pg_dir is the main page directory, address 0x00001000 (or at
* address 0x00101000 for a compressed boot).
*/
startup_32:
cld
movl $(KERNEL_DS),%eax #初始化段寄存器
mov %ax,%ds
mov %ax,%es
mov %ax,%fs
mov %ax,%gs
lss _stack_start,%esp #加载堆栈
/*
* Clear BSS first so that there are no surprises... 首先清空BSS段
*/
xorl %eax,%eax
movl $__edata,%edi
movl $__end,%ecx
subl %edi,%ecx
cld
rep
stosb
/*
* start system 32-bit setup. We need to re-do some of the things done
* in 16-bit mode for the "real" operations.
* 首先设置idt，然后检测A20地址线
*/
call setup_idt
xorl %eax,%eax
1: incl %eax # check that A20 really IS enabled
movl %eax,0x000000 # loop forever if it isn't
cmpl %eax,0x100000
je 1b
/*
* Initialize eflags. Some BIOS's leave bits like NT set. This would
* confuse the debugger if this code is traced.
* XXX - best to initialize before switching to protected mode.
* 初始化eflag寄存器
*/
pushl $0
popfl
/*
* Copy bootup parameters out of the way. First 2kB of
* _empty_zero_page is for boot parameters, second 2kB
* is for the command line.
* 拷贝启动参数在2k空间内，然后预留2k空间加载命令行参数
*/
movl $0x90000,%esi //esi指向0x90000，从ds:esi处存放着系统启动参数
movl $_empty_zero_page,%edi //edi指向_empty_zero_page
movl $512,%ecx //ecx计数器初始化为512
cld
rep
movsl //将启动参数从es:esi拷贝到_empty_zero_page页中，共计2k
xorl %eax,%eax //eax清零
movl $512,%ecx //ecx计数器初始化为512
rep
stosl //后2k清零
cmpw $(CL_MAGIC),CL_MAGIC_ADDR //比较地址CL_MAGIC_ADDR处内容是否为0xA33F
jne 1f
movl $_empty_zero_page+2048,%edi //edi指向后2k开始地址
movzwl CL_OFFSET,%esi //esi指向CL_OFFSET
addl $(CL_BASE_ADDR),%esi //开始处的内容放入esi
movl $2048,%ecx //初始化计数器
rep
movsb //循环拷贝命令行参数
1:
/* check if it is 486 or 386. */
/*
* XXX - this does a lot of unnecessary setup. Alignment checks don't
* apply at our cpl of 0 and the stack ought to be aligned already, and
* we don't need to preserve eflags.
* 检测是486还是386
*/
movl %esp,%edi # save stack pointer
andl $0xfffffffc,%esp # align stack to avoid AC fault
movl $3,_x86
pushfl # push EFLAGS
popl %eax # get EFLAGS
movl %eax,%ecx # save original EFLAGS
xorl $0x40000,%eax # flip AC bit in EFLAGS
pushl %eax # copy to EFLAGS
popfl # set EFLAGS
pushfl # get new EFLAGS
popl %eax # put it in eax
xorl %ecx,%eax # change in flags
andl $0x40000,%eax # check if AC bit changed
je is386
movl $4,_x86
movl %ecx,%eax
xorl $0x200000,%eax # check ID flag
pushl %eax
popfl # if we are on a straight 486DX, SX, or
pushfl # 487SX we can't change it
popl %eax
xorl %ecx,%eax
andl $0x200000,%eax
je is486
isnew: pushl %ecx # restore original EFLAGS
popfl
movl $1, %eax # Use the CPUID instruction to
.byte 0x0f, 0xa2 # check the processor type
andl $0xf00, %eax # Set _x86 with the family
shrl $8, %eax # returned.
movl %eax, _x86
movl %edi,%esp # restore esp
movl %cr0,%eax # 486+
andl $0x80000011,%eax # Save PG,PE,ET
orl $0x50022,%eax # set AM, WP, NE and MP
jmp 2f
is486: pushl %ecx # restore original EFLAGS
popfl
movl %edi,%esp # restore esp
movl %cr0,%eax # 486
andl $0x80000011,%eax # Save PG,PE,ET
orl $0x50022,%eax # set AM, WP, NE and MP
jmp 2f
is386: pushl %ecx # restore original EFLAGS
popfl
movl %edi,%esp # restore esp
movl %cr0,%eax # 386
andl $0x80000011,%eax # Save PG,PE,ET
orl $2,%eax # set MP
2: movl %eax,%cr0
call check_x87
call setup_paging
lgdt gdt_descr
lidt idt_descr
ljmp $(KERNEL_CS),$1f
1: movl $(KERNEL_DS),%eax # reload all the segment registers
mov %ax,%ds # after changing gdt.
mov %ax,%es
mov %ax,%fs
mov %ax,%gs
lss _stack_start,%esp
xorl %eax,%eax
lldt %ax
pushl %eax # These are the parameters to main :-)
pushl %eax
pushl %eax
cld # gcc2 wants the direction flag cleared at all times
call _start_kernel #跳转到init文件夹下的main文件中的start_kernel执行
L6:
jmp L6 # main should never return here, but
# just in case, we know what happens.

/*
* We depend on ET to be correct. This checks for 287/387.
*/
check_x87:
movl $0,_hard_math
clts
fninit
fstsw %ax
cmpb $0,%al
je 1f
movl %cr0,%eax /* no coprocessor: have to set bits */
xorl $4,%eax /* set EM */
movl %eax,%cr0
ret
.align 2
1: movl $1,_hard_math
.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 doesn't actually load
* idt - that can be done only after paging has been enabled
* and the kernel moved to 0xC0000000. Interrupts
* are enabled elsewhere, when we can be relatively
* sure everything is ok.
*/
setup_idt:
lea ignore_int,%edx
movl $(KERNEL_CS << 16),%eax
movw %dx,%ax /* selector = 0x0010 = 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
ret

/*
* Setup_paging
*
* This routine sets up paging by setting the page bit
* in cr0. The page tables are set up, identity-mapping
* the first 4MB. The rest are initialized later.
*
* (ref: added support for up to 32mb, 17Apr92) -- Rik Faith
* (ref: update, 25Sept92) -- croutons@crunchy.uucp
* (ref: 92.10.11 - Linus Torvalds. Corrected 16M limit - no upper memory limit)
*/
.align 2
setup_paging:
movl $1024*2,%ecx /* 2 pages - swapper_pg_dir+1 page table */ /* 将1024 * 2 赋值给 ecx 做为计数器 ，准备初始化2页*/
xorl %eax,%eax /* eax清零*/
movl $_swapper_pg_dir,%edi /* swapper_pg_dir is at 0x1000 */ /* edi指向页目录表*/
cld;rep;stosl /* 将eax中的内存，放入到es:edi位置处，每次放四个字节，共放置两页*/
/* Identity-map the kernel in low 4MB memory for ease of transition */ /* */
movl $_pg0+7,_swapper_pg_dir /* set present bit/user r/w */ /* 设置页目录表中的项*/
/* But the real place is at 0xC0000000 */ /* 但是真正的位置在0xC0000000处*/
movl $_pg0+7,_swapper_pg_dir+3072 /* set present bit/user r/w */ /* 设置页目录表中第3072项*/
movl $_pg0+4092,%edi /* 填写页表内容，pg0页中的最后一项*/
movl $0x03ff007,%eax /* 4Mb - 4096 + 7 (r/w user,p) */ /* 最后一项对应的内存地址+属性标志*/
std /* 方向位置位，edi每次减4*/
1: stosl /* fill the page backwards - more efficient :-) */ /* 填充*/
subl $0x1000,%eax /* 每填好一项，eax减0x1000*/
jge 1b /* 没填好，则跳转到标号1，否则执行下一步*/
cld /* */
movl $_swapper_pg_dir,%eax /* eax指向页目录地址*/
movl %eax,%cr3 /* cr3 - page directory start */ /* cr3寄存器指向页目录地址*/
movl %cr0,%eax /* eax指向cr0寄存器*/
orl $0x80000000,%eax /* 准备开启分页机制*/
movl %eax,%cr0 /* set paging (PG) bit */ /* 设置pg位，开启分页*/
ret /* this also flushes the prefetch-queue */ /* 返回*/

/*
* page 0 is made non-existent, so that kernel NULL pointer references get
* caught. Thus the swapper page directory has been moved to 0x1000
*
* XXX Actually, the swapper page directory is at 0x1000 plus 1 megabyte,
* with the introduction of the compressed boot code. Theoretically,
* the original design of overlaying the startup code with the swapper
* page directory is still possible --- it would reduce the size of the kernel
* by 2-3k. This would be a good thing to do at some point.....
*/
.org 0x1000 /*在物理地址0x1000处放置页目录表 _swapper_pg_dir*/
_swapper_pg_dir:
/*
* The page tables are initialized to only 4MB here - the final page
* 页表只初始化开始的4M空间，最终页表在后面根据内存大小进行设置
* tables are set up later depending on memory size.
*/
.org 0x2000 /*在0x2000处放置页表0*/
_pg0:

.org 0x3000 /*在0x3000处放置页_empty_bad_page*/
_empty_bad_page:

.org 0x4000 /*在0x4000处放置页表_empty_bad_page_table*/
_empty_bad_page_table:

.org 0x5000 /*在0x5000处放置页_empty_zero_page*/
_empty_zero_page:

.org 0x6000
/*
* 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
/*
* floppy_track_buffer is used to buffer one track of floppy data: it
* has to be separate from the tmp_floppy area, as otherwise a single-
* sector read/write can mess it up. It can contain one full track of
* data (18*2*512 bytes).
*/
_floppy_track_buffer:
.fill 512*2*18,1,0

/* This is the default interrupt "handler" :-) */
int_msg:
.asciz "Unknown interrupt\n"
.align 2
ignore_int:
cld
pushl %eax
pushl %ecx
pushl %edx
push %ds
push %es
push %fs
movl $(KERNEL_DS),%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

/*
* The interrupt descriptor table has room for 256 idt's
*/
.align 4
.word 0
idt_descr:
.word 256*8-1 # idt contains 256 entries
.long 0xc0000000+_idt

.align 4
_idt:
.fill 256,8,0 # idt is uninitialized

.align 4
.word 0
gdt_descr:
.word (8+2*NR_TASKS)*8-1
.long 0xc0000000+_gdt

/*
* This gdt setup gives the kernel a 1GB address space at virtual
* address 0xC0000000 - space enough for expansion, I hope.
*/
.align 4
_gdt:
.quad 0x0000000000000000 /* NULL descriptor */
.quad 0x0000000000000000 /* not used */
.quad 0xc0c39a000000ffff /* 0x10 kernel 1GB code at 0xC0000000 */
.quad 0xc0c392000000ffff /* 0x18 kernel 1GB data at 0xC0000000 */
.quad 0x00cbfa000000ffff /* 0x23 user 3GB code at 0x00000000 */
.quad 0x00cbf2000000ffff /* 0x2b user 3GB data at 0x00000000 */
.quad 0x0000000000000000 /* not used */
.quad 0x0000000000000000 /* not used */
.fill 2*NR_TASKS,8,0 /* space for LDT's and TSS's etc */