linux 0.11 trace boot/bootsect.s

 .code16
# rewrite with AT&T syntax by falcon  at 081012
#
# SYS_SIZE is the number of clicks (16 bytes) to be loaded.
# 0x3000 is 0x30000 bytes = 196kB, more than enough for current
# versions of linux
#
 .equ SYSSIZE, 0x3000
#
# bootsect.s  (C) 1991 Linus Torvalds
#
# bootsect.s is loaded at 0x7c00 by the bios-startup routines, and moves
# iself out of the way to address 0x90000, and jumps there.
#
# It then loads 'setup' directly after itself (0x90200), and the system
# at 0x10000, using BIOS interrupts. 
#
# NOTE! currently system is at most 8*65536 bytes long. This should be no
# problem, even in the future. I want to keep it simple. This 512 kB
# kernel size should be enough, especially as this doesn't contain the
# buffer cache as in minix
#
# 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 sectors at a time whenever possible.

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

 .equ SETUPLEN, 4  # nr of setup-sectors
 .equ BOOTSEG, 0x07c0  # original address of boot-sector
 .equ INITSEG, 0x9000  # we move boot here - out of the way
 .equ SETUPSEG, 0x9020  # setup starts here
 .equ SYSSEG, 0x1000  # system loaded at 0x10000 (65536).
 .equ ENDSEG, SYSSEG + SYSSIZE # where to stop loading

# ROOT_DEV: 0x000 - same type of floppy as boot.
#  0x301 - first partition on first drive etc
 .equ ROOT_DEV, 0x301
 ljmp    $BOOTSEG, $_start #跳至cs:eip，這樣一行可以不用加，因為現在的cs即為BOOTSEG(0x07c0)
_start:      #將BOOTSEG上的資料依序往後複製至INITSEG，共複製256回合，每回合複製兩個byte(即一個word)，即從BOOTSEG複製512bytes到INITSEG上
 mov $BOOTSEG, %ax
 mov %ax, %ds
 mov $INITSEG, %ax
 mov %ax, %es        #es設定為INITSEG，之後ljmp會用到
 mov $256, %cx
 sub %si, %si
 sub %di, %di
 rep 
 movsw
 ljmp $INITSEG, $go    #跳至INITSEG seg(跳過去後，會執行複製後的bootsect code)
go: mov %cs, %ax       #ax=cs=INITSEG 
 mov %ax, %ds          #ds=ax=INITSEG 
 mov %ax, %es          #es=ax=INITSEG
# put stack at 0x9ff00.
 mov %ax, %ss          #ss=ax=INITSEG=0x9000
 mov $0xFF00, %sp  # arbitrary value >>512   

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

load_setup:
 mov $0x0000, %dx  # drive 0, head 0
 mov $0x0002, %cx  # sector 2, track 0    #從sector2開始讀(sector2,3放得是setup部分的code)
 mov $0x0200, %bx  # address = 512, in INITSEG
 .equ    AX, 0x0200+SETUPLEN
 mov     $AX, %ax  # service 2, nr of sectors    #ax=0x0204(AH=02h: Read Sectors From Drive AL=Sectors To Read Count)
 int $0x13   # read it
 jnc ok_load_setup  # ok - continue    #會把讀到的兩個sector放在es(INITSEG):bx(0x0200)上，即為SETUPSEG
 mov $0x0000, %dx
 mov $0x0000, %ax  # reset the diskette
 int $0x13
 jmp load_setup

ok_load_setup:

# Get disk drive parameters, specifically nr of sectors/track

 mov $0x00, %dl
 mov $0x0800, %ax  # AH=8 is get drive parameters
 int $0x13
 mov $0x00, %ch
 #seg cs
 mov %cx, %cs:sectors+0 # %cs means sectors is in %cs
 mov $INITSEG, %ax
 mov %ax, %es

# Print some inane message

 mov $0x03, %ah  # read cursor pos
 xor %bh, %bh
 int $0x10
 
 mov $24, %cx
 mov $0x0007, %bx  # page 0, attribute 7 (normal)
 #lea msg1, %bp
 mov     $msg1, %bp
 mov $0x1301, %ax  # write string, move cursor
 int $0x10

# ok, we've written the message, now
# we want to load the system (at 0x10000)

 mov $SYSSEG, %ax
 mov %ax, %es  # segment of 0x010000
 call read_it
 call kill_motor

# 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.

 #seg cs
 mov %cs:root_dev+0, %ax
 cmp $0, %ax
 jne root_defined
 #seg cs
 mov %cs:sectors+0, %bx
 mov $0x0208, %ax  # /dev/ps0 - 1.2Mb
 cmp $15, %bx
 je root_defined
 mov $0x021c, %ax  # /dev/PS0 - 1.44Mb
 cmp $18, %bx
 je root_defined
undef_root:
 jmp undef_root
root_defined:
 #seg cs
 mov %ax, %cs:root_dev+0

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

 ljmp $SETUPSEG, $0    #跳至SETUPSEG，執行setup code ，需看boot/setup.s

# 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+ SETUPLEN # sectors read of current track
head: .word 0   # current head
track: .word 0   # current track

read_it:
 mov %es, %ax
 test $0x0fff, %ax
die: jne  die   # es must be at 64kB boundary
 xor  %bx, %bx  # bx is starting address within segment
rp_read:
 mov  %es, %ax
  cmp  $ENDSEG, %ax  # have we loaded all yet?
 jb ok1_read
 ret
ok1_read:
 #seg cs
 mov %cs:sectors+0, %ax
 sub sread, %ax
 mov %ax, %cx
 shl $9, %cx
 add %bx, %cx
 jnc  ok2_read
 je  ok2_read
 xor  %ax, %ax
 sub  %bx, %ax
 shr  $9, %ax
ok2_read:
 call  read_track
 mov  %ax, %cx
 add  sread, %ax
 #seg cs
 cmp  %cs:sectors+0, %ax
 jne  ok3_read
 mov  $1, %ax
 sub  head, %ax
 jne  ok4_read
 incw    track 
ok4_read:
 mov %ax, head
 xor %ax, %ax
ok3_read:
 mov %ax, sread
 shl $9, %cx
 add %cx, %bx
 jnc rp_read
 mov %es, %ax
 add $0x1000, %ax
 mov %ax, %es
 xor %bx, %bx
 jmp rp_read

read_track:
 push %ax
 push %bx
 push %cx
 push %dx
 mov track, %dx
 mov sread, %cx
 inc %cx
 mov %dl, %ch
 mov head, %dx
 mov %dl, %dh
 mov $0, %dl
 and $0x0100, %dx
 mov $2, %ah
 int $0x13
 jc bad_rt
 pop %dx
 pop %cx
 pop %bx
 pop %ax
 ret
bad_rt: mov $0, %ax
 mov $0, %dx
 int $0x13
 pop %dx
 pop %cx
 pop %bx
 pop %ax
 jmp read_track

#/*
# * 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 $0x3f2, %dx
 mov $0, %al
 outsb
 pop %dx
 ret

sectors:
 .word 0

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

 .org 508
root_dev:
 .word ROOT_DEV
boot_flag:
 .word 0xAA55
 
 .text
 endtext:
 .data
 enddata:
 .bss
 endbss: