通讯了一次文档，并作了部分理解注释。
本注释力图简洁，在阅读时，参考了网上的代码以增加理解。
! ! 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 ! 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. .globl begtext, begdata, begbss, endtext, enddata, endbss .text begtext: .data begdata: .bss begbss: .text SETUPLEN = 4 ! nr of setup-sectors BOOTSEG = 0x07c0 ! original address of boot-sector INITSEG = 0x9000 ! we move boot here - out of the way SETUPSEG = 0x9020 ! setup starts here SYSSEG = 0x1000 ! system loaded at 0x10000 (65536). ENDSEG = SYSSEG + SYSSIZE ! where to stop loading ! ROOT_DEV: 0x000 - same type of floppy as boot. ! 0x301 - first partition on first drive etc ROOT_DEV = 0x306 entry start ! 下面的代码将自身从0x07c0移到0x9000. ! X86系列的CPU在起启时，BIOS自动将磁盘上的起动代码调入 ! 到0x07c00执行。 ! 至于为何要移动，也许是这个空间还有其它用途 start: mov ax,#BOOTSEG mov ds,ax mov ax,#INITSEG mov es,ax mov cx,#256 sub si,si sub di,di rep movw jmpi go,INITSEG !INITSEG 指出跳转目的段地址 go: mov ax,cs !此时CS==0x9000。 mov ds,ax mov es,ax ! put stack at 0x9ff00. mov ss,ax mov sp,#0xFF00 ! arbitrary value >>512 ! load the setup-sectors directly after the bootblock. ! Note that 'es' is already set up. load_setup: ! BIOS的0x13号中断为磁盘中断，有关内容可参见X86CPU的相关资料。 ! 下面的代码为读扇区： mov dx,#0x0000 ! drive 0, head 0 mov cx,#0x0002 ! sector 2, track 0 mov bx,#0x0200 ! address = 512, in INITSEG mov ax,#0x0200+SETUPLEN ! service 2, nr of sectors ! 服务号为2（AH=2）， ! 扇区数量为SETUPLEN(AL=SETUPLEN) int 0x13 ! read it jnc ok_load_setup ! ok - continue mov dx,#0x0000 ! err - reset mov ax,#0x0000 ! reset the diskette int 0x13 j load_setup ok_load_setup: ! Get disk drive parameters, specifically nr of sectors/track ! 服务号AH=8，取磁盘参数 ! 有关磁盘中断0x13可参考相关资料 mov dl,#0x00 mov ax,#0x0800 ! AH=8 is get drive parameters int 0x13 mov ch,#0x00 seg cs ! 指定操作数所在段 mov sectors,cx ! 保存每磁道扇区数 mov ax,#INITSEG mov es,ax ! 修正ES的值 ! Print some inane message ! 显示字符串 "Loading system ..." ! BIOS的0x10号中断为显示中断，可参考X86CPU的相关资料 ! mov ah,#0x03 ! read cursor pos xor bh,bh int 0x10 mov cx,#24 ! 显示的字符数量为24 mov bx,#0x0007 ! page 0, attribute 7 (normal) mov bp,#msg1 ! "Loading system ..." mov ax,#0x1301 ! write string, move cursor int 0x10 ! 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 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 ax,root_dev cmp ax,#0 ! 根设备号是否为0 jne root_defined ! 如果不为0，则直接使用给定的根设备 seg cs ! 根据参数作判断处理 mov bx,sectors ! 如果sectors=15 =>>1.2Mb软盘 ! sectors=18 =>>1.44Mb软盘 mov ax,#0x0208 ! /dev/ps0 - 1.2Mb 存入AX备存 cmp bx,#15 ! ==1.2Mb? je root_defined ! 相行则用其作为根设备 mov ax,#0x021c ! /dev/PS0 - 1.44Mb cmp bx,#18 ! ==1.44Mb? je root_defined undef_root: ! 都不是，死循环 jmp undef_root root_defined: seg cs mov root_dev,ax ! 设备号保存起来。 ! after that (everyting loaded), we jump to ! the setup-routine loaded directly after ! the bootblock: 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+SETUPLEN ! sectors read of current track head: .word 0 ! current head track: .word 0 ! current track 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 ! bx=0x00;即BX清零 rp_read: mov ax,es cmp ax,#ENDSEG ! have we loaded all yet? jb ok1_read ret ok1_read: seg cs mov ax,sectors sub ax,sread mov cx,ax shl cx,#9 ! cx<<=9，即*0x200，转为字节数 add cx,bx ! cx为16位，最大计数为64K,相加而无进位， jnc ok2_read ! 表明未超过64K。 je ok2_read xor ax,ax ! ax=0x00 sub ax,bx ! 0x10000-bx即64K-BX,得到尚要读取的字节数 shr ax,#9 ! ax>>=9;即ax/=0x200，即扇区数量 ok2_read: call read_track ! 通过磁盘中断INT3读出扇区内容 mov cx,ax ! 本次读取的扇区数。 add ax,sread ! 当前磁道上已经读取的扇区数 seg cs cmp ax,sectors ! 未读完转到ok3_read jne ok3_read mov ax,#1 sub ax,head ! head=1? jne ok4_read ! 转去读1磁头的扇区 inc track ! 否则去读下一磁道 ok4_read: mov head,ax ! 保存当前的磁头号。 xor ax,ax ! 已经读扇区数量在新的磁头号下清零。 ok3_read: mov sread,ax ! 当前磁道已读扇区数。 shl cx,#9 ! 已读扇区转换为已读字节数。 add bx,cx ! 累加已读字节数 jnc rp_read ! 累加结果没有进位，表示小64KB据。 mov ax,es add ax,#0x1000 ! 调整段地址，指向下一个段（64K大小）。 mov es,ax xor bx,bx ! 基址清零 jmp rp_read ! 以下为通过BIOS的0x13号中断读取数据，可参才 ! X86CPU的有关资料。 read_track: push ax push bx push cx push dx mov dx,track ! 当前磁道 mov cx,sread ! 磁道上已读扇区。 inc cx mov ch,dl ! 磁道号。 mov dx,head ! 磁头号。 mov dh,dl ! dx<<=8,即将head移入到dh内。 mov dl,#0 ! 驱动器号=0 and dx,#0x0100 mov ah,#2 ! server = 2，磁盘中断服务号。 int 0x13 jc bad_rt pop dx pop cx pop bx pop ax ret bad_rt: mov ax,#0 mov dx,#0 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 dx,#0x3f2 mov al,#0 outb pop dx ret sectors: .word 0 ! 每磁道的扇区数。 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:
