《Linux内核设计的艺术》学习笔记（六）执行setup.s

　　参考资料

　　1. 8259A可编程中断控制器

jmpi , SETUPSEG // 0x90200

　　到这里，bootsect.s的执行就算结束了。控制权转移到了setup.s文件的手中。

　　setup程序的主要作用是利用ROM BIOS中断读取机器系统数据，并将这些数据保存到0x90000开始的位置。下表基本归纳了Line 109之前的所有代码逻辑。

　　表1 setup.s前半段代码的作用

内存地址	长度（字节）	名称	描述
0x90000	2	光标位置	行列都是从0开始，各占一个字节。由int 0x10中断（入口参数为AH=0x03）取得到DX中，并保存到内存。
0x90002	2	扩展内存数	系统从1MB开始的扩展内存数值（KB）。由0x15中断（入口参数为AH=0x88）取得到AX中，并保存到内存。
0x90004	2	当前活动页	当前显示的页面。由int 0x10中断（入口参数为AH=0x0F）取得到BH中，并保存到内存。
0x90006	1	显示模式	同上，由int 0x10中断（入口参数为AH=0x0F）取得到AL中，并保存到内存。
0x90007	1	字符的列数	同上，int 0x10中断（入口参数为AH=0x0F）取得到AH中，并保存到内存。
0x90008	2	未知	被破坏了。int 0x10中断（入口参数为AH=0x0F，BL=0x10）。
0x9000A	1	内存大小标识	同上，int 0x10中断（入口参数AH=0x0F，BL=0x10）取得到BL中，并保存到内存（0x00=64K，0x01=128K，0x02=192K，0x03=256K）。
0x9000B	1	视频状态	同上，int 0x10中断（入口参数AH=0x0F，BL=0x10）取得到BL中，并保存到内存（0x00为彩色模式，I/O端口为0x3DX；0x01为单色，I/O端口为0x3BX）。
0x9000C	2	显卡特性参数	同上，int 0x10中断（入口参数AH=0x0F，BL=0x10）取得到CX中，并保存到内存。CH和CL各有含义。
0x90080	16	硬盘hd0参数表	取中断向量0x41所指的内存值，放于0x9000：0x0080处。由movsb命令实现。
0x90090	16	硬盘hd1参数表	取中断向量0x46所指的内存值，放于0x9000:0x0090处。由movsb命令实现。
0x901FC	2	根设备号	根文件系统所在的设备号在bootsect.s中设置，这里没有代码参与。 见bootsect.s中第251行。

接下来使用cli命令屏蔽中断。

cld命令将DF设置为0，增址方向。std与之相反。

通过movsw命令，将system模块从0x10000到0x8FFFF的内存数据块（512KB）移动到0x00000开始的位置。每次移动64KB，共移动8次。

初始化IDT寄存器和GDT寄存器。

选通A20地址线。

重新对8259A中断控制器编程。

通过lmsw命令将0x0001写入到控制寄存器CR0中，打开保护模式。

最后，通过jmpi 0, 8命令跳转至CS段8，偏移地址为0处。即，0x00000处。

图1 setup.s程序结束后内存中程序示意图

setup.s代码如下：

 !
 !    setup.s        (C)  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 ().
 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.

     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    [],dx        ! it from 0x90000.

 ! Get memory size (extended mem, kB)

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

 ! Get video-card data:

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

 ! check for EGA/VGA and some config parameters

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

 ! Get hd0 data

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

 ! Get hd1 data

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

 ! Check that there IS a hd1 :-)

     mov    ax,#0x01500
     mov    dl,#0x81
     int    0x13
     jc    no_disk1
     cmp    ah,#
     je    is_disk1
 no_disk1:
     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 ...

     cli            ! no interrupts allowed !

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

     mov    ax,#0x0000
     cld            ! 'direction'=0, movs moves forward
 do_move:
     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:
     mov    ax,#SETUPSEG    ! right, forgot this at first. didn't work :-)
     mov    ds,ax
     lidt    idt_48        ! load idt with ,
     lgdt    gdt_48        ! load gdt with whatever appropriate

 ! that was painless, now we enable A20

     call    empty_8042
     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 's, and it isn't fun.

     mov    al,#0x11        ! initialization sequence
     out    #0x20,al        ! send it to 8259A-
     .word    0x00eb,0x00eb        ! jmp $+, jmp $+
     out    #0xA0,al        ! and to 8259A-
     .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  (0x28)
     out    #0xA1,al
     .word    0x00eb,0x00eb
     mov    al,#0x04        ! - is master
     out    #0x21,al
     .word    0x00eb,0x00eb
     mov    al,#0x02        ! - is slave
     out    #0xA1,al
     .word    0x00eb,0x00eb
     mov    al,#0x01        !  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 -bit programs do that. We just jump to
 ! absolute address 0x00000, in -bit protected mode.

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