boot/bootsect.S

!
! SYS_SIZE is the number of clicks (16 bytes) to be loaded.
! 0x7F00 is 0x7F000 bytes = 508kB, more than enough for current
! versions of linux which compress the kernel
!
#include <linux/config.h>
SYSSIZE = DEF_SYSSIZE
!
!    bootsect.s        Copyright (C) 1991, 1992 Linus Torvalds
!    modified by Drew Eckhardt
!    modified by Bruce Evans (bde)
!
! bootsect.s is loaded at 0x7c00 by the bios-startup routines, and moves
! itself out of the way to address 0x90000, and jumps there.
!
! bde - should not jump blindly, there may be systems with only 512K low
! memory.  Use int 0x12 to get the top of memory, etc.
!
! It then loads 'setup' directly after itself (0x90200), and the system
! at 0x10000, using BIOS interrupts.
!
! NOTE! currently system is at most (8*65536-4096) bytes long. This should
! be no problem, even in the future. I want to keep it simple. This 508 kB
! kernel size should be enough, especially as this doesn't contain the
! buffer cache as in minix (and especially now that the kernel is
! compressed :-)
!
! The loader has been made as simple as possible, and continuos
! read errors will result in a unbreakable loop. Reboot by hand. It
! loads pretty fast by getting whole tracks at a time whenever possible.

.text

SETUPSECS = 4                ! nr of setup-sectors                        setup程序所占的扇区数量
BOOTSEG   = 0x07C0            ! original address of boot-sector            bootsect程序最初位置
INITSEG   = DEF_INITSEG            ! we move boot here - out of the way     boot程序将要移动到的位置
SETUPSEG  = DEF_SETUPSEG        ! setup starts here                      setup程序的位置
SYSSEG    = DEF_SYSSEG            ! system loaded at 0x10000 (65536).      system程序所在位置

! ROOT_DEV & SWAP_DEV are now written by "build".
ROOT_DEV = 0                                                             ! 根设备号
SWAP_DEV = 0                                                             ! 交换设备号
#ifndef SVGA_MODE                                                        ! 定义宏SVGA_MODE为ASK_VGA
#define SVGA_MODE ASK_VGA
#endif
#ifndef RAMDISK                                                          ! 定义RAMDISK的大小
#define RAMDISK 0
#endif
#ifndef CONFIG_ROOT_RDONLY                                               ! 定义宏CONFIG_ROOT_RDONLY
#define CONFIG_ROOT_RDONLY 0
#endif

! ld86 requires an entry symbol. This may as well be the usual one.
.globl    _main                                                            ! 定义程序入口符号
_main:
#if 0 /* hook for debugger, harmless unless BIOS is fussy (old HP) */    ! 调试钩子
    int    3
#endif
    mov    ax,#BOOTSEG                                                      ! ds-->BOOTSEG
    mov    ds,ax
    mov    ax,#INITSEG                                                      ! es-->INITSEG
    mov    es,ax
    mov    cx,#256                                                          ! 初始化计数器
    sub    si,si                                                            ! si清零
    sub    di,di                                                            ! di清零
    cld                                                                  ! 置方向位
    rep                                                                  ! 循环拷贝
    movsw                                                                ! 每次拷贝两个字节，上面代码功能就是将bootsect移动到INITSEG处
    jmpi    go,INITSEG                                                   ! 跳转，这是段间跳转，跳转到INITSEG:go处执行

go:    mov    ax,cs                                                             ! 重新初始化段寄存器ds、es、ss
    mov    dx,#0x4000-12    ! 0x4000 is arbitrary value >= length of
                ! bootsect + length of setup + room for stack
                ! 12 is disk parm size

! bde - changed 0xff00 to 0x4000 to use debugger at 0x6400 up (bde).  We
! wouldn't have to worry about this if we checked the top of memory.  Also
! my BIOS can be configured to put the wini drive tables in high memory
! instead of in the vector table.  The old stack might have clobbered the
! drive table.

mov    ds,ax
    mov    es,ax
    mov    ss,ax        ! put stack at INITSEG:0x4000-12.                    ! 初始化堆栈ss：sp
    mov    sp,dx
/*
 *    Many BIOS's default disk parameter tables will not
 *    recognize multi-sector reads beyond the maximum sector number
 *    specified in the default diskette parameter tables - this may
 *    mean 7 sectors in some cases.
 *
 *    Since single sector reads are slow and out of the question,
 *    we must take care of this by creating new parameter tables
 *    (for the first disk) in RAM.  We will set the maximum sector
 *    count to 18 - the most we will encounter on an HD 1.44.  
 *
 *    High doesn't hurt.  Low does.
 *
 *    Segments are as follows: ds=es=ss=cs - INITSEG,
 *        fs = 0, gs = parameter table segment
 */

push    #0
    pop    fs
    mov    bx,#0x78        ! fs:bx is parameter table address
    seg fs
    lgs    si,(bx)            ! gs:si is source

mov    di,dx            ! es:di is destination
    mov    cx,#6            ! copy 12 bytes
    cld

rep
    seg gs
    movsw               ! 以上函数功能是将软盘参数表读取到指定位置

mov    di,dx
    movb    4(di),*18        ! patch sector count   指定参数为18

seg fs
    mov    (bx),di
    seg fs
    mov    2(bx),es

mov    ax,cs
    mov    fs,ax
    mov    gs,ax
    
    xor    ah,ah            ! reset FDC
    xor    dl,dl
    int     0x13        ! 重置软盘

! load the setup-sectors directly after the bootblock.
! Note that 'es' is already set up.

! 加载setup程序
load_setup:
    xor    dx, dx            ! drive 0, head 0
    mov    cx,#0x0002        ! sector 2, track 0
    mov    bx,#0x0200        ! address = 512, in INITSEG
    mov    ax,#0x0200+SETUPSECS    ! service 2, nr of sectors
                    ! (assume all on head 0, track 0)
    int    0x13            ! read it
    jnc    ok_load_setup        ! ok - continue

push    ax            ! dump error code
    call    print_nl
    mov    bp, sp
    call    print_hex
    pop    ax    
    
    xor    dl, dl            ! reset FDC
    xor    ah, ah
    int    0x13
    jmp    load_setup

! 加载setup程序成功
ok_load_setup:

! 下面开始获取磁盘驱动器参数
! Get disk drive parameters, specifically nr of sectors/track

#if 0

! bde - the Phoenix BIOS manual says function 0x08 only works for fixed
! disks.  It doesn't work for one of my BIOS's (1987 Award).  It was
! fatal not to check the error code.

xor    dl,dl
    mov    ah,#0x08        ! AH=8 is get drive parameters
    int    0x13
    xor    ch,ch
#else

! It seems that there is no BIOS call to get the number of sectors.  Guess
! 18 sectors if sector 18 can be read, 15 if sector 15 can be read.
! Otherwise guess 9.

! 确定软盘类型
    xor    dx, dx            ! drive 0, head 0
    mov    cx,#0x0012        ! sector 18, track 0
    mov    bx,#0x0200+SETUPSECS*0x200  ! address after setup (es = cs)
    mov    ax,#0x0201        ! service 2, 1 sector
    int    0x13
    jnc    got_sectors
    mov    cl,#0x0f        ! sector 15
    mov    ax,#0x0201        ! service 2, 1 sector
    int    0x13
    jnc    got_sectors
    mov    cl,#0x09

#endif

got_sectors:
    seg cs
    mov    sectors,cx
    mov    ax,#INITSEG
    mov    es,ax

! 打印一些消息
! Print some inane message

mov    ah,#0x03        ! read cursor pos
    xor    bh,bh
    int    0x10
    
    mov    cx,#9
    mov    bx,#0x0007        ! page 0, attribute 7 (normal)
    mov    bp,#msg1
    mov    ax,#0x1301        ! write string, move cursor
    int    0x10

! ok，我们已经输入一些消息，现在我们开始加载system部分
! ok, we've written the message, now
! we want to load the system (at 0x10000)

mov    ax,#SYSSEG
    mov    es,ax        ! segment of 0x010000
    call    read_it                                                ! 读取数据
    call    kill_motor                                             ! 关闭软驱马达
    call    print_nl                                               ! 输出加载消息

! After that we check which root-device to use. If the device is
! defined (!= 0), nothing is done and the given device is used.
! Otherwise, either /dev/PS0 (2,28) or /dev/at0 (2,8), depending
! on the number of sectors that the BIOS reports currently.
! 下面我们检测使用的根设备，如果设备被定义则什么都不做，给出使用的设备
! 否则，使用第一个软驱或者使用第一个硬盘，依赖于BIOS当前报告的扇区号
    seg cs
    mov    ax,root_dev
    or    ax,ax
    jne    root_defined
    seg cs
    mov    bx,sectors
    mov    ax,#0x0208        ! /dev/ps0 - 1.2Mb
    cmp    bx,#15
    je    root_defined
    mov    ax,#0x021c        ! /dev/PS0 - 1.44Mb
    cmp    bx,#18
    je    root_defined
    mov    ax,#0x0200        ! /dev/fd0 - autodetect
root_defined:
    seg cs
    mov    root_dev,ax

! after that (everyting loaded), we jump to
! the setup-routine loaded directly after
! the bootblock:

! 跳转到setup程序执行

jmpi    0,SETUPSEG

! This routine loads the system at address 0x10000, making sure
! no 64kB boundaries are crossed. We try to load it as fast as
! possible, loading whole tracks whenever we can.
!
! in:    es - starting address segment (normally 0x1000)
!
sread:    .word 1+SETUPSECS    ! sectors read of current track
head:    .word 0            ! current head
track:    .word 0            ! current track

! 以下是实际加载system的子程序
read_it:
    mov ax,es
    test ax,#0x0fff
die:    jne die            ! es must be at 64kB boundary
    xor bx,bx        ! bx is starting address within segment
rp_read:
    mov ax,es
    sub ax,#SYSSEG
    cmp ax,syssize        ! have we loaded all yet?
    jbe ok1_read
    ret
ok1_read:
    seg cs
    mov ax,sectors
    sub ax,sread
    mov cx,ax
    shl cx,#9
    add cx,bx
    jnc ok2_read
    je ok2_read
    xor ax,ax
    sub ax,bx
    shr ax,#9
ok2_read:
    call read_track
    mov cx,ax
    add ax,sread
    seg cs
    cmp ax,sectors
    jne ok3_read
    mov ax,#1
    sub ax,head
    jne ok4_read
    inc track
ok4_read:
    mov head,ax
    xor ax,ax
ok3_read:
    mov sread,ax
    shl cx,#9
    add bx,cx
    jnc rp_read
    mov ax,es
    add ah,#0x10
    mov es,ax
    xor bx,bx
    jmp rp_read

read_track:
    pusha
    pusha    
    mov    ax, #0xe2e     ! loading... message 2e = .
    mov    bx, #7
     int    0x10
    popa

mov    dx,track
    mov    cx,sread
    inc    cx
    mov    ch,dl
    mov    dx,head
    mov    dh,dl
    and    dx,#0x0100
    mov    ah,#2
    
    push    dx                ! save for error dump
    push    cx
    push    bx
    push    ax

int    0x13
    jc    bad_rt
    add    sp, #8
    popa
    ret

bad_rt:    push    ax                ! save error code
    call    print_all            ! ah = error, al = read
    
    
    xor ah,ah
    xor dl,dl
    int 0x13

add    sp, #10
    popa    
    jmp read_track

/*
 *    print_all is for debugging purposes.  
 *    It will print out all of the registers.  The assumption is that this is
 *    called from a routine, with a stack frame like
 *    dx
 *    cx
 *    bx
 *    ax
 *    error
 *    ret <- sp
 *
*/
 
print_all:
    mov    cx, #5        ! error code + 4 registers
    mov    bp, sp

print_loop:
    push    cx        ! save count left
    call    print_nl    ! nl for readability

cmp    cl, 5
    jae    no_reg        ! see if register name is needed
    
    mov    ax, #0xe05 + 'A - 1
    sub    al, cl
    int    0x10

mov    al, #'X
    int    0x10

mov    al, #':
    int    0x10

no_reg:
    add    bp, #2        ! next register
    call    print_hex    ! print it
    pop    cx
    loop    print_loop
    ret

print_nl:
    mov    ax, #0xe0d    ! CR
    int    0x10
    mov    al, #0xa    ! LF
    int     0x10
    ret

/*
 *    print_hex is for debugging purposes, and prints the word
 *    pointed to by ss:bp in hexadecmial.
*/

print_hex:
    mov    cx, #4        ! 4 hex digits
    mov    dx, (bp)    ! load word into dx
print_digit:
    rol    dx, #4        ! rotate so that lowest 4 bits are used
    mov    ah, #0xe    
    mov    al, dl        ! mask off so we have only next nibble
    and    al, #0xf
    add    al, #'0        ! convert to 0-based digit
    cmp    al, #'9        ! check for overflow
    jbe    good_digit
    add    al, #'A - '0 - 10

good_digit:
    int    0x10
    loop    print_digit
    ret

/*
 * This procedure turns off the floppy drive motor, so
 * that we enter the kernel in a known state, and
 * don't have to worry about it later.
 */
kill_motor:
    push dx
    mov dx,#0x3f2
    xor al, al
    outb
    pop dx
    ret

sectors:
    .word 0

msg1:
    .byte 13,10
    .ascii "Loading"

.org 498
root_flags:
    .word CONFIG_ROOT_RDONLY
syssize:
    .word SYSSIZE
swap_dev:
    .word SWAP_DEV
ram_size:
    .word RAMDISK
vid_mode:
    .word SVGA_MODE
root_dev:
    .word ROOT_DEV
boot_flag:
    .word 0xAA55