linux kernel 0.11 setup

 

setup作用

①读取参数放在0x90000处。

②将原本在0x10000处的system模块移至0x00000处

③加载中断描述符表，全局描述符表，进入32位保护模式。

概念

关于实模式和保护模式区别及寻址方式，该博客已经很详尽：http://blog.csdn.net/rosetta/article/details/8933200，只是有个别信息没有。

IDT：Interrupt Descriptor Table--中断描述表

GDT：Global Descriptor Table   --全局描述表

LDT：Local Descriptor Table    --局部描述表

    在Intel架构中，更准确的说是保护模式下，大部分内存管理和中断服务例程都通过描述符表来控制。每个描述符存储了CPU随时可能需要获取的一个单个对象（例如服务例程、任务、一段代码或数据等）的信息。如果试图装载一个数据到一个段寄存器中，CPU需要进行安全性和访问控制检查，来确认是否获得了访问该内存区域的许可。一旦检查结束，一些有用的信息（例如最低和最高地址）被缓存在CPU中的几个不可见的寄存器中。

    Intel定义了3种类型的描述符表：中断描述符表IDT（用以替换中断向量表IVT）、全局描述符表GDT和局部描述符表LDT。每个表分别通过LIDT、LGDT、LLDT指令以(size, linear address)的形式定义（注：是载入描述符表的指令，这里就是说表包含了大小和基址组成。）。大多数情况下，操作系统中启动期指定这些表的位置，然后通过一个指针直接读写这些表。

　　1. 全局描述符表GDT：

 

全局描述符表在系统中只能有一个,且可以被每一个任务所共享.任何描述符都可以放在GDT中,但中断门和陷阱门放在GDT中是不会起作用的.能被多个任务共享的内存区就是通过GDT完成的,

 

　　2. 局部描述符表LDT:

 

局部描述符表在系统中可以有多个,通常情况下是与任务的数量保持对等,但任务可以没有局部描述符表.任务间不相干的部分也是通过LDT实现的.这里涉及到地址映射的问题.和GDT一样,中断门和陷阱门放在LDT中是不会起作用的.

 

　　3. 中断描述符表IDT:

 

和GDT一样,中断描述符表在系统最多只能有一个,中断描述符表内可以存放256个描述符,分别对应256个中断.因为每个描述符占用8个字节,所以IDT的长度可达2K.中断描述符表中可以有任务门、中断门、陷阱门三个门描述符，其它的描述符在中断描述符表中无意义。

 

　　4. 段选择子

在保护模式下,段寄存器的内容已不是段值,而称其为选择子.该选择子指示描述符在上面这三个表中的位置,所以说选择子即是索引值。

当我们把段选择子装入寄存器时不仅使该寄存器值，同时CPU将该选择子所对应的GDT或LDT中的描述符装入了不可见部分。这样只要我们不进行代码切换（不重新装入新的选择子）CPU就会不会对不可见部分存储的描述符进行更新，可以直接进行访问，加快了访问速度。一旦寄存器被重新赋值，不可见部分也将被重新赋值。

　　setup源代码注释

 

!
!    setup.s        (C) 1991 Linus Torvalds
!
! setup.s is responsible for getting the system data from the BIOS,
! and putting them into the appropriate places in system memory.
! both setup.s and system has been loaded by the bootblock.
!
! This code asks the bios for memory/disk/other parameters, and
! puts them in a "safe" place: 0x90000-0x901FF, ie where the
! boot-block used to be. It is then up to the protected mode
! system to read them from there before the area is overwritten
! for buffer-blocks.
!

! NOTE! These had better be the same as in bootsect.s!

INITSEG  = 0x9000    ! we move boot here - out of the way
SYSSEG   = 0x1000    ! system loaded at 0x10000 (65536).
SETUPSEG = 0x9020    ! this is the current segment

.globl begtext, begdata, begbss, endtext, enddata, endbss
.text
begtext:
.data
begdata:
.bss
begbss:
.text

entry start
start:

! ok, the read went well so we get current cursor position and save it for
! posterity.
                            !    input:
                            !    BH = page number.
                            !    return:
                            !    DH = row.
                            !    DL = column.
                            !    CH = cursor start line.
                            !    CL = cursor bottom line.
    mov    ax,#INITSEG    ! this is done in bootsect already, but...
    mov    ds,ax
    mov    ah,#0x03    ! read cursor pos
    xor    bh,bh
    int    0x10        ! save it in known place, con_init fetches
    mov    [0],dx        ! it from 0x90000.光标的行 列的值

! Get memory size (extended mem, kB)    内存大小

    mov    ah,#0x88
    int    0x15
    mov    [2],ax

! Get video-card data:                                声卡数据

    mov    ah,#0x0f
    int    0x10
    mov    [4],bx        ! bh = display page
    mov    [6],ax        ! al = video mode, ah = window width

! check for EGA/VGA and some config parameters    VGA和配置参数

    mov    ah,#0x12
    mov    bl,#0x10
    int    0x10
    mov    [8],ax
    mov    [10],bx
    mov    [12],cx

! Get hd0 data                                磁盘第一分区数据

    mov    ax,#0x0000
    mov    ds,ax
    lds    si,[4*0x41]
    mov    ax,#INITSEG
    mov    es,ax
    mov    di,#0x0080
    mov    cx,#0x10
    rep
    movsb

! Get hd1 data                                磁盘第二分区数据

    mov    ax,#0x0000
    mov    ds,ax
    lds    si,[4*0x46]
    mov    ax,#INITSEG
    mov    es,ax
    mov    di,#0x0090
    mov    cx,#0x10
    rep
    movsb

! Check that there IS a hd1 :-)    检查是否存在第二个硬盘，不存在则清空磁盘二的参数表

    mov    ax,#0x01500
    mov    dl,#0x81
    int    0x13
    jc    no_disk1
    cmp    ah,#3
    je    is_disk1
no_disk1:        !清空磁盘二的参数表
    mov    ax,#INITSEG
    mov    es,ax
    mov    di,#0x0090
    mov    cx,#0x10
    mov    ax,#0x00
    rep
    stosb
is_disk1:

! now we want to move to protected mode ...开始我们要保护模式方面的工作了。

    cli            ! no interrupts allowed !cli是关中断，防止有些硬件中断对程序的干扰 sti是开中断，允许硬件中断

! first we move the system to it's rightful place

    mov    ax,#0x0000
    cld            ! 'direction'=0, movs moves forward  cld即告诉程序si，di向前移动，std指令为设置方向，告诉程序si，di向后移动
do_move:    !    0x1000=64k 循环移动ds:si->es:di数据，总共移动cx=(0x9000-0x1000)=0x8000的数据
    mov    es,ax        ! destination segment
    add    ax,#0x1000
    cmp    ax,#0x9000
    jz    end_move
    mov    ds,ax        ! source segment
    sub    di,di
    sub    si,si
    mov     cx,#0x8000
    rep
    movsw
    jmp    do_move

! then we load the segment descriptors

end_move:                        !    从实模式进入保护模式
    mov    ax,#SETUPSEG    ! right, forgot this at first. didn't work :-)
    mov    ds,ax
    lidt    idt_48        ! load idt with 0,0 指令以(size=0, linear address=0)的形式加载中断描述符表
    lgdt    gdt_48        ! load gdt with whatever appropriate
                                    !    指令以(size=0x800, linear address=512+gdt,0x9)的形式加载全局描述符表

! that was painless, now we enable A20

    call    empty_8042!等待为空
    mov    al,#0xD1        ! command write
    out    #0x64,al
    call    empty_8042
    mov    al,#0xDF        ! A20 on
    out    #0x60,al
    call    empty_8042

! well, that went ok, I hope. Now we have to reprogram the interrupts :-(
! we put them right after the intel-reserved hardware interrupts, at
! int 0x20-0x2F. There they won't mess up anything. Sadly IBM really
! messed this up with the original PC, and they haven't been able to
! rectify it afterwards. Thus the bios puts interrupts at 0x08-0x0f,
! which is used for the internal hardware interrupts as well. We just
! have to reprogram the 8259's, and it isn't fun.

    mov    al,#0x11        ! initialization sequence
    out    #0x20,al        ! send it to 8259A-1
    .word    0x00eb,0x00eb        ! jmp $+2, jmp $+2
    out    #0xA0,al        ! and to 8259A-2
    .word    0x00eb,0x00eb
    mov    al,#0x20        ! start of hardware int's (0x20)
    out    #0x21,al
    .word    0x00eb,0x00eb
    mov    al,#0x28        ! start of hardware int's 2 (0x28)
    out    #0xA1,al
    .word    0x00eb,0x00eb
    mov    al,#0x04        ! 8259-1 is master
    out    #0x21,al
    .word    0x00eb,0x00eb
    mov    al,#0x02        ! 8259-2 is slave
    out    #0xA1,al
    .word    0x00eb,0x00eb
    mov    al,#0x01        ! 8086 mode for both
    out    #0x21,al
    .word    0x00eb,0x00eb
    out    #0xA1,al
    .word    0x00eb,0x00eb
    mov    al,#0xFF        ! mask off all interrupts for now
    out    #0x21,al
    .word    0x00eb,0x00eb
    out    #0xA1,al

! well, that certainly wasn't fun :-(. Hopefully it works, and we don't
! need no steenking BIOS anyway (except for the initial loading :-).
! The BIOS-routine wants lots of unnecessary data, and it's less
! "interesting" anyway. This is how REAL programmers do it.
!
! Well, now's the time to actually move into protected mode. To make
! things as simple as possible, we do no register set-up or anything,
! we let the gnu-compiled 32-bit programs do that. We just jump to
! absolute address 0x00000, in 32-bit protected mode.

    mov    ax,#0x0001    ! protected mode (PE) bit
    lmsw    ax        ! This is it!
    jmpi    0,8        ! jmp offset 0 of segment 8 (cs)

! This routine checks that the keyboard command queue is empty
! No timeout is used - if this hangs there is something wrong with
! the machine, and we probably couldn't proceed anyway.
empty_8042:
    .word    0x00eb,0x00eb
    in    al,#0x64    ! 8042 status port
    test    al,#2        ! is input buffer full?
    jnz    empty_8042    ! yes - loop
    ret

gdt:
    .word    0,0,0,0        ! dummy

    .word    0x07FF        ! 8Mb - limit=2047 (2048*4096=8Mb)
    .word    0x0000        ! base address=0
    .word    0x9A00        ! code read/exec
    .word    0x00C0        ! granularity=4096, 386

    .word    0x07FF        ! 8Mb - limit=2047 (2048*4096=8Mb)
    .word    0x0000        ! base address=0
    .word    0x9200        ! data read/write
    .word    0x00C0        ! granularity=4096, 386

idt_48:
    .word    0            ! idt limit=0 
    .word    0,0            ! idt base=0L 

gdt_48:
    .word    0x800        ! gdt limit=2048, 256 GDT entries
    .word    512+gdt,0x9    ! gdt base = 0X9xxxx
    
.text
endtext:
.data
enddata:
.bss
endbss: