diff options
Diffstat (limited to 'linux/boot/boot.s')
| -rw-r--r-- | linux/boot/boot.s | 329 |
1 files changed, 329 insertions, 0 deletions
diff --git a/linux/boot/boot.s b/linux/boot/boot.s new file mode 100644 index 0000000..e19bbd2 --- /dev/null +++ b/linux/boot/boot.s @@ -0,0 +1,329 @@ +| +| 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 +| +| NOTE1 abouve is no longer valid in it's entirety.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: |