/*
* 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,陎 @@ds,es,fs,gs指向内核数据段
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定义）
@@进入保护模式的堆栈段的第一次变化，很奇怪
@@为什么堆栈段也可正向增涨？
call setup_idt
call setup_gdt
movl $0x10,陎 # 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 陎,陎
1: incl 陎 # check that A20 really IS enabled
movl 陎,0x000000
cmpl 陎,0x100000 @@这是怎么测的 0x100000为什么值，
@@明白，a20 not
enable,0x000000就是0x100000
je 1b
movl %cr0,陎 # check math chip
andl $0x80000011,陎 # Save PG,ET,PE
testl $0x10,陎
jne 1f # ET is set - 387 is present
orl $4,陎 # else set emulate bit
1: movl 陎,%cr0
jmp after_page_s @@注意，用jmp 不call，不返回
/*
* setup_idt
*
* sets up a idt with 256 enies pointing to
* ignore_int, interrupt gates. It then loads
* idt. Everything that wants to install itself
* in the idt- 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 s.
*/
setup_idt:
lea ignore_int,韝
movl $0x00080000,陎
movw %dx,%ax /* selector = 0x0008 = cs */
movw $0x8E00,%dx /* interrupt gate - dpl=0, present */
@@ ignore_int 低16ax 高16edx高字
@@ 8e00dx 8eax高字
lea _idt,韎
mov $256,靫
rp_sidt:
movl 陎,(韎)
movl 韝,4(韎)
addl $8,韎
dec 靫
jne rp_sidt
lidt idt_descr
ret
/*
* setup_gdt
*
* This routines sets up a new gdt and loads it.
* Only two enies 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 s.
*/
setup_gdt:
lgdt gdt_descr
ret @@跳到main函数
.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_s:
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 s 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 ack 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 ied to show which constants to change by having
* some kind of marker at them (search for "8Mb"), but I
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