extlinux.asm

linux内核

; -*- fundamental -*- (asm-mode sucks)
; $Id$
; ****************************************************************************
;
;  extlinux.asm
;
;  A program to boot Linux kernels off an ext2/ext3 filesystem.
;
;   Copyright (C) 1994-2005  H. Peter Anvin
;
;  This program is free software; you can redistribute it and/or modify
;  it under the terms of the GNU General Public License as published by
;  the Free Software Foundation, Inc., 53 Temple Place Ste 330,
;  Boston MA 02111-1307, USA; either version 2 of the License, or
;  (at your option) any later version; incorporated herein by reference.
; 
; ****************************************************************************

%define IS_EXTLINUX 1
%include "macros.inc"
%include "config.inc"
%include "kernel.inc"
%include "bios.inc"
%include "tracers.inc"
%include "layout.inc"

%include "ext2_fs.inc"

;
; Some semi-configurable constants... change on your own risk.
;
my_id		equ extlinux_id
; NASM 0.98.38 croaks if these are equ's rather than macros...
FILENAME_MAX_LG2 equ 8			; log2(Max filename size Including final null)
FILENAME_MAX	equ (1 << FILENAME_MAX_LG2)	; Max mangled filename size
NULLFILE	equ 0			; Null character == empty filename
NULLOFFSET	equ 0			; Position in which to look
retry_count	equ 16			; How patient are we with the disk?
%assign HIGHMEM_SLOP 0			; Avoid this much memory near the top
LDLINUX_MAGIC	equ 0x3eb202fe		; A random number to identify ourselves with

MAX_OPEN_LG2	equ 6			; log2(Max number of open files)
MAX_OPEN	equ (1 << MAX_OPEN_LG2)

SECTOR_SHIFT	equ 9
SECTOR_SIZE	equ (1 << SECTOR_SHIFT)

MAX_SYMLINKS	equ 64			; Maximum number of symlinks per lookup
SYMLINK_SECTORS	equ 2			; Max number of sectors in a symlink
					; (should be >= FILENAME_MAX)

;
; This is what we need to do when idle
;
%macro	RESET_IDLE 0
	; Nothing
%endmacro
%macro	DO_IDLE 0
	; Nothing
%endmacro

;
; The following structure is used for "virtual kernels"; i.e. LILO-style
; option labels.  The options we permit here are `kernel' and `append
; Since there is no room in the bottom 64K for all of these, we
; stick them at vk_seg:0000 and copy them down before we need them.
;
		struc vkernel
vk_vname:	resb FILENAME_MAX	; Virtual name **MUST BE FIRST!**
vk_rname:	resb FILENAME_MAX	; Real name
vk_appendlen:	resw 1
		alignb 4
vk_append:	resb max_cmd_len+1	; Command line
		alignb 4
vk_end:		equ $			; Should be <= vk_size
		endstruc

;
; Segment assignments in the bottom 640K
; Stick to the low 512K in case we're using something like M-systems flash
; which load a driver into low RAM (evil!!)
;
; 0000h - main code/data segment (and BIOS segment)
;
real_mode_seg	equ 4000h
cache_seg	equ 3000h		; 64K area for metadata cache
vk_seg          equ 2000h		; Virtual kernels
xfer_buf_seg	equ 1000h		; Bounce buffer for I/O to high mem
comboot_seg	equ real_mode_seg	; COMBOOT image loading zone

;
; File structure.  This holds the information for each currently open file.
;
		struc open_file_t
file_left	resd 1			; Number of sectors left (0 = free)
file_sector	resd 1			; Next linear sector to read
file_in_sec	resd 1			; Sector where inode lives
file_in_off	resw 1
file_mode	resw 1
		endstruc

%ifndef DEPEND
%if (open_file_t_size & (open_file_t_size-1))
%error "open_file_t is not a power of 2"
%endif
%endif

; ---------------------------------------------------------------------------
;   BEGIN CODE
; ---------------------------------------------------------------------------

;
; Memory below this point is reserved for the BIOS and the MBR
;
		section .earlybss
trackbufsize	equ 8192
trackbuf	resb trackbufsize	; Track buffer goes here
getcbuf		resb trackbufsize
		; ends at 4800h

		section .latebss
SuperBlock	resb 1024		; ext2 superblock
SuperInfo	resq 16			; DOS superblock expanded
ClustSize	resd 1			; Bytes/cluster ("block")
SecPerClust	resd 1			; Sectors/cluster
ClustMask	resd 1			; Sectors/cluster - 1
PtrsPerBlock1	resd 1			; Pointers/cluster
PtrsPerBlock2	resd 1			; (Pointers/cluster)^2
DriveNumber	resb 1			; BIOS drive number
ClustShift	resb 1			; Shift count for sectors/cluster
ClustByteShift	resb 1			; Shift count for bytes/cluster

		alignb open_file_t_size
Files		resb MAX_OPEN*open_file_t_size

;
; Constants for the xfer_buf_seg
;
; The xfer_buf_seg is also used to store message file buffers.  We
; need two trackbuffers (text and graphics), plus a work buffer
; for the graphics decompressor.
;
xbs_textbuf	equ 0			; Also hard-coded, do not change
xbs_vgabuf	equ trackbufsize
xbs_vgatmpbuf	equ 2*trackbufsize


		section .text
;
; Some of the things that have to be saved very early are saved
; "close" to the initial stack pointer offset, in order to
; reduce the code size...
;
StackBuf	equ $-44-32		; Start the stack here (grow down - 4K)
PartInfo	equ StackBuf		; Saved partition table entry
FloppyTable	equ PartInfo+16		; Floppy info table (must follow PartInfo)
OrigFDCTabPtr	equ StackBuf-4		; The high dword on the stack

;
; Primary entry point.  Tempting as though it may be, we can't put the
; initial "cli" here; the jmp opcode in the first byte is part of the
; "magic number" (using the term very loosely) for the DOS superblock.
;
bootsec		equ $
		jmp short start		; 2 bytes
		nop			; 1 byte
;
; "Superblock" follows -- it's in the boot sector, so it's already
; loaded and ready for us
;
bsOemName	db 'EXTLINUX'		; The SYS command sets this, so...
;
; These are the fields we actually care about.  We end up expanding them
; all to dword size early in the code, so generate labels for both
; the expanded and unexpanded versions.
;
%macro		superb 1
bx %+ %1	equ SuperInfo+($-superblock)*8+4
bs %+ %1	equ $
		zb 1
%endmacro
%macro		superw 1
bx %+ %1	equ SuperInfo+($-superblock)*8
bs %+ %1	equ $
		zw 1
%endmacro
%macro		superd 1
bx %+ %1	equ $			; no expansion for dwords
bs %+ %1	equ $
		zd 1
%endmacro
superblock	equ $
		superw BytesPerSec
		superb SecPerClust
		superw ResSectors
		superb FATs
		superw RootDirEnts
		superw Sectors
		superb Media
		superw FATsecs
		superw SecPerTrack
		superw Heads
superinfo_size	equ ($-superblock)-1	; How much to expand
		superd Hidden
		superd HugeSectors
		;
		; This is as far as FAT12/16 and FAT32 are consistent
		;
		zb 54			; FAT12/16 need 26 more bytes,
					; FAT32 need 54 more bytes
superblock_len	equ $-superblock

;
; Note we don't check the constraints above now; we did that at install
; time (we hope!)
;
start:
		cli			; No interrupts yet, please
		cld			; Copy upwards
;
; Set up the stack
;
		xor ax,ax
		mov ss,ax
		mov sp,StackBuf		; Just below BSS
		mov es,ax
;
; DS:SI may contain a partition table entry.  Preserve it for us.
;
		mov cx,8		; Save partition info
		mov di,sp
		rep movsw

		mov ds,ax		; Now we can initialize DS...

;
; Now sautee the BIOS floppy info block to that it will support decent-
; size transfers; the floppy block is 11 bytes and is stored in the
; INT 1Eh vector (brilliant waste of resources, eh?)
;
; Of course, if BIOSes had been properly programmed, we wouldn't have
; had to waste precious space with this code.
;
		mov bx,fdctab
		lfs si,[bx]		; FS:SI -> original fdctab
		push fs			; Save on stack in case we need to bail
		push si

		; Save the old fdctab even if hard disk so the stack layout
		; is the same.  The instructions above do not change the flags
		mov [DriveNumber],dl	; Save drive number in DL
		and dl,dl		; If floppy disk (00-7F), assume no
					; partition table
		js harddisk

floppy:
		mov cl,6		; 12 bytes (CX == 0)
		; es:di -> FloppyTable already
		; This should be safe to do now, interrupts are off...
		mov [bx],di		; FloppyTable
		mov [bx+2],ax		; Segment 0
		fs rep movsw		; Faster to move words
		mov cl,[bsSecPerTrack]  ; Patch the sector count
		mov [di-8],cl
		; AX == 0 here
		int 13h			; Some BIOSes need this

		jmp short not_harddisk
;
; The drive number and possibly partition information was passed to us
; by the BIOS or previous boot loader (MBR).  Current "best practice" is to
; trust that rather than what the superblock contains.
;
; Would it be better to zero out bsHidden if we don't have a partition table?
;
; Note: di points to beyond the end of PartInfo
;
harddisk:
		test byte [di-16],7Fh	; Sanity check: "active flag" should
		jnz no_partition	; be 00 or 80
		mov eax,[di-8]		; Partition offset (dword)
		mov [bsHidden],eax
no_partition:
;
; Get disk drive parameters (don't trust the superblock.)  Don't do this for
; floppy drives -- INT 13:08 on floppy drives will (may?) return info about
; what the *drive* supports, not about the *media*.  Fortunately floppy disks
; tend to have a fixed, well-defined geometry which is stored in the superblock.
;
		; DL == drive # still
		mov ah,08h
		int 13h
		jc no_driveparm
		and ah,ah
		jnz no_driveparm
		shr dx,8
		inc dx			; Contains # of heads - 1
		mov [bsHeads],dx
		and cx,3fh
		mov [bsSecPerTrack],cx
no_driveparm:
not_harddisk:
;
; Ready to enable interrupts, captain
;
		sti

;
; Do we have EBIOS (EDD)?
;
eddcheck:
		mov bx,55AAh
		mov ah,41h		; EDD existence query
		mov dl,[DriveNumber]
		int 13h
		jc .noedd
		cmp bx,0AA55h
		jne .noedd
		test cl,1		; Extended disk access functionality set
		jz .noedd
		;
		; We have EDD support...
		;
		mov byte [getlinsec.jmp+1],(getlinsec_ebios-(getlinsec.jmp+2))
.noedd:

;
; Load the first sector of LDLINUX.SYS; this used to be all proper
; with parsing the superblock and root directory; it doesn't fit
; together with EBIOS support, unfortunately.
;
		mov eax,[FirstSector]	; Sector start
		mov bx,ldlinux_sys	; Where to load it
		call getonesec
		
		; Some modicum of integrity checking
		cmp dword [ldlinux_magic+4],LDLINUX_MAGIC^HEXDATE
		jne kaboom

		; Go for it...
		jmp ldlinux_ent

;
; getonesec: get one disk sector
;
getonesec:
		mov bp,1		; One sector
		; Fall through

;
; getlinsec: load a sequence of BP floppy sector given by the linear sector
;	     number in EAX into the buffer at ES:BX.  We try to optimize
;	     by loading up to a whole track at a time, but the user
;	     is responsible for not crossing a 64K boundary.
;	     (Yes, BP is weird for a count, but it was available...)
;
;	     On return, BX points to the first byte after the transferred
;	     block.
;
;            This routine assumes CS == DS, and trashes most registers.
;
; Stylistic note: use "xchg" instead of "mov" when the source is a register
; that is dead from that point; this saves space.  However, please keep
; the order to dst,src to keep things sane.
;
getlinsec:
		add eax,[bsHidden]		; Add partition offset
		xor edx,edx			; Zero-extend LBA (eventually allow 64 bits)

.jmp:		jmp strict short getlinsec_cbios

;
; getlinsec_ebios:
;
; getlinsec implementation for EBIOS (EDD)
;
getlinsec_ebios:
.loop:
                push bp                         ; Sectors left
.retry2:
		call maxtrans			; Enforce maximum transfer size
		movzx edi,bp			; Sectors we are about to read
		mov cx,retry_count
.retry:

		; Form DAPA on stack
		push edx
		push eax
		push es
		push bx
		push di
		push word 16
		mov si,sp
		pushad
                mov dl,[DriveNumber]
		push ds
		push ss
		pop ds				; DS <- SS
                mov ah,42h                      ; Extended Read
		int 13h
		pop ds
		popad
		lea sp,[si+16]			; Remove DAPA
		jc .error
        	pop bp
		add eax,edi			; Advance sector pointer
		sub bp,di			; Sectors left
                shl di,SECTOR_SHIFT		; 512-byte sectors
                add bx,di                   	; Advance buffer pointer
                and bp,bp
                jnz .loop

                ret

.error:
		; Some systems seem to get "stuck" in an error state when
		; using EBIOS.  Doesn't happen when using CBIOS, which is
		; good, since some other systems get timeout failures
		; waiting for the floppy disk to spin up.

		pushad				; Try resetting the device
		xor ax,ax
		mov dl,[DriveNumber]
		int 13h
		popad
		loop .retry			; CX-- and jump if not zero

		;shr word [MaxTransfer],1	; Reduce the transfer size
		;jnz .retry2

		; Total failure.  Try falling back to CBIOS.
		mov byte [getlinsec.jmp+1],(getlinsec_cbios-(getlinsec.jmp+2))
		;mov byte [MaxTransfer],63	; Max possibe CBIOS transfer

		pop bp
		; ... fall through ...

;
; getlinsec_cbios:
;
; getlinsec implementation for legacy CBIOS
;
getlinsec_cbios:
.loop:
		push edx
		push eax
		push bp
		push bx

		movzx esi,word [bsSecPerTrack]
		movzx edi,word [bsHeads]
		;
		; Dividing by sectors to get (track,sector): we may have
		; up to 2^18 tracks, so we need to use 32-bit arithmetric.
		;
		div esi
		xor cx,cx
		xchg cx,dx		; CX <- sector index (0-based)
					; EDX <- 0
		; eax = track #
		div edi			; Convert track to head/cyl

		; We should test this, but it doesn't fit...
		; cmp eax,1023
		; ja .error

		;
		; Now we have AX = cyl, DX = head, CX = sector (0-based),
		; BP = sectors to transfer, SI = bsSecPerTrack,
		; ES:BX = data target
		;

		call maxtrans			; Enforce maximum transfer size

		; Must not cross track boundaries, so BP <= SI-CX
		sub si,cx
		cmp bp,si
		jna .bp_ok
		mov bp,si
.bp_ok:	

		shl ah,6		; Because IBM was STOOPID
					; and thought 8 bits were enough
					; then thought 10 bits were enough...
		inc cx			; Sector numbers are 1-based, sigh
		or cl,ah
		mov ch,al
		mov dh,dl
		mov dl,[DriveNumber]
		xchg ax,bp		; Sector to transfer count
		mov ah,02h		; Read sectors
		mov bp,retry_count
.retry:
		pushad
		int 13h
		popad
		jc .error
.resume:
		movzx ecx,al		; ECX <- sectors transferred
		shl ax,SECTOR_SHIFT	; Convert sectors in AL to bytes in AX
		pop bx
		add bx,ax
		pop bp
		pop eax
		pop edx
		add eax,ecx
		sub bp,cx
		jnz .loop
		ret

.error:
		dec bp
		jnz .retry

		xchg ax,bp		; Sectors transferred <- 0
		shr word [MaxTransfer],1
		jnz .resume
		; Fall through to disk_error
	
;
; kaboom: write a message and bail out.
;
disk_error:
kaboom:
		xor si,si
		mov ss,si		
		mov sp,StackBuf-4 	; Reset stack
		mov ds,si		; Reset data segment
		pop dword [fdctab]	; Restore FDC table
.patch:					; When we have full code, intercept here
		mov si,bailmsg

		; Write error message, this assumes screen page 0
.loop:		lodsb
		and al,al
                jz .done
		mov ah,0Eh		; Write to screen as TTY
		mov bx,0007h		; Attribute
		int 10h
		jmp short .loop
.done:
		cbw			; AH <- 0
		int 16h			; Wait for keypress
		int 19h			; And try once more to boot...
.norge:		jmp short .norge	; If int 19h returned; this is the end

;