लिनक्स/linux-0.01.tar/boot/boot.s
< लिनक्स | linux-0.01.tar
| | boot.s | | boot.s is loaded at 0x7c00 by the bios-startup routines, and moves itself | out of the way to address 0x90000, and jumps there. | | It then loads the system at 0x10000, using BIOS interrupts. Thereafter | it disables all interrupts, moves the system down to 0x0000, changes | to protected mode, and calls the start of system. System then must | RE-initialize the protected mode in it's own tables, and enable | interrupts as needed. | | 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 - in fact more would mean we'd have to move | not just these start-up routines, but also do something about the cache- | memory (block IO devices). The area left over in the lower 640 kB is meant | for these. No other memory is assumed to be "physical", ie all memory | over 1Mb is demand-paging. All addresses under 1Mb are guaranteed to match | their physical addresses. | | NOTE1 abouve is no longer valid in it's entirety. cache-memory is allocated | above the 1Mb mark as well as below. Otherwise it is mainly correct. | | NOTE 2! The boot disk type must be set at compile-time, by setting | the following equ. Having the boot-up procedure hunt for the right | disk type is severe brain-damage. | The loader has been made as simple as possible (had to, to get it | in 512 bytes with the code to move to protected mode), 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. | 1.44Mb disks: sectors = 18 | 1.2Mb disks: | sectors = 15 | 720kB disks: | sectors = 9 .globl begtext, begdata, begbss, endtext, enddata, endbss .text begtext: .data begdata: .bss begbss: .text BOOTSEG = 0x07c0 INITSEG = 0x9000 SYSSEG = 0x1000 | system loaded at 0x10000 (65536). ENDSEG = SYSSEG + SYSSIZE entry start 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 go: mov ax,cs mov ds,ax mov es,ax mov ss,ax mov sp,#0x400 | arbitrary value >>512 mov ah,#0x03 | read cursor pos xor bh,bh int 0x10 mov cx,#24 mov bx,#0x0007 | page 0, attribute 7 (normal) mov bp,#msg1 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 | if the read went well we get current cursor position ans save it for | posterity. mov ah,#0x03 | read cursor pos xor bh,bh int 0x10 | save it in known place, con_init fetches mov [510],dx | it from 0x90510. | 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 j do_move | then we load the segment descriptors end_move: mov ax,cs | right, forgot this at first. didn't work :-) mov ds,ax lidt idt_48 | load idt with 0,0 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 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 | 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) | | This routine has to be recompiled to fit another drive type, | just change the "sectors" variable at the start of the file | (originally 18, for a 1.44Mb drive) | sread: .word 1 | 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 rp_read: mov ax,es cmp ax,#ENDSEG | have we loaded all yet? jb ok1_read ret ok1_read: 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 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 ax,#0x1000 mov es,ax xor bx,bx jmp rp_read read_track: push ax push bx push cx push dx mov dx,track mov cx,sread inc cx mov ch,dl mov dx,head mov dh,dl mov dl,#0 and dx,#0x0100 mov ah,#2 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 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 gdt,0x9 | gdt base = 0X9xxxx msg1: .byte 13,10 .ascii "Loading system ..." .byte 13,10,13,10 .text endtext: .data enddata: .bss endbss: