boot.asm

来自「GNU FreeDOS兼容MS DOS很好的东东.」· 汇编 代码 · 共 528 行 · 第 1/2 页

;
; File:
;                            boot.asm
; Description:
;                           DOS-C boot
;
;                       Copyright (c) 1997;
;                           Svante Frey
;                       All Rights Reserved
;
; This file is part of DOS-C.
;
; DOS-C 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; either version
; 2, or (at your option) any later version.
;
; DOS-C is distributed in the hope that it will be useful, but
; WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
; the GNU General Public License for more details.
;
; You should have received a copy of the GNU General Public
; License along with DOS-C; see the file COPYING.  If not,
; write to the Free Software Foundation, 675 Mass Ave,
; Cambridge, MA 02139, USA.
;
;
;	+--------+ 1FE0:7E00
;	|BOOT SEC|
;	|RELOCATE|
;	|--------| 1FE0:7C00
;	|        |
;	|--------| 1FE0:3000
;	| CLUSTER|
;	|  LIST  |
;	|--------| 1FE0:2000
;	|        |
;	|--------| 0000:7E00
;	|BOOT SEC| overwritten by max 128k FAT buffer
;	|ORIGIN  | and later by max 134k loaded kernel
;	|--------| 0000:7C00
;	|        |
;	|--------|
;	|KERNEL  | also used as max 128k FAT buffer
;	|LOADED  | before kernel loading starts
;	|--------| 0060:0000
;	|        |
;	+--------+


;%define ISFAT12         1
;%define ISFAT16         1


segment	.text

%define BASE            0x7c00

                org     BASE

Entry:          jmp     short real_start
		nop

;       bp is initialized to 7c00h
%define bsOemName       bp+0x03      ; OEM label
%define bsBytesPerSec   bp+0x0b      ; bytes/sector
%define bsSecPerClust   bp+0x0d      ; sectors/allocation unit
%define bsResSectors    bp+0x0e      ; # reserved sectors
%define bsFATs          bp+0x10      ; # of fats
%define bsRootDirEnts   bp+0x11      ; # of root dir entries
%define bsSectors       bp+0x13      ; # sectors total in image
%define bsMedia         bp+0x15      ; media descrip: fd=2side9sec, etc...
%define sectPerFat      bp+0x16      ; # sectors in a fat
%define sectPerTrack    bp+0x18      ; # sectors/track
%define nHeads          bp+0x1a      ; # heads
%define nHidden         bp+0x1c      ; # hidden sectors
%define nSectorHuge     bp+0x20      ; # sectors if > 65536
%define drive           bp+0x24      ; drive number
%define extBoot         bp+0x26      ; extended boot signature
%define volid           bp+0x27
%define vollabel        bp+0x2b
%define filesys         bp+0x36

%define LOADSEG         0x0060

%define FATBUF          0x2000          ; offset of temporary buffer for FAT
                                        ; chain

;       Some extra variables

;%define StoreSI         bp+3h          ;temp store

;       To save space, functions that are just called once are
;       implemented as macros instead. Four bytes are saved by
;       avoiding the call / ret instructions.


;       GETDRIVEPARMS:  Calculate start of some disk areas.
;

%macro		GETDRIVEPARMS	0
                mov     si, word [nHidden]
                mov     di, word [nHidden+2]
                add     si, word [bsResSectors]
                adc     di, byte 0              ; DI:SI = first FAT sector

                mov     word [fat_start], si
                mov     word [fat_start+2], di

                mov     al, [bsFATs]
                cbw
                mul     word [sectPerFat]       ; DX:AX = total number of FAT sectors

                add     si, ax
                adc     di, dx                  ; DI:SI = first root directory sector
                mov     word [root_dir_start], si
                mov     word [root_dir_start+2], di

                ; Calculate how many sectors the root directory occupies.
                mov     bx, [bsBytesPerSec]
                mov     cl, 5                   ; divide BX by 32
                shr     bx, cl                  ; BX = directory entries per sector

                mov     ax, [bsRootDirEnts]
                xor     dx, dx
                div     bx

                mov     word [RootDirSecs], ax  ; AX = sectors per root directory

                add     si, ax
                adc     di, byte 0              ; DI:SI = first data sector

                mov     [data_start], si
                mov     [data_start+2], di
%endmacro

;-----------------------------------------------------------------------

		times	0x3E-$+$$ db 0

%define loadsegoff_60	bp+loadseg_off-Entry
%define loadseg_60	bp+loadseg_seg-Entry

;%define LBA_PACKET      bp+0x42
;		db      10h  ; size of packet
;		db      0        ; const
;		dw      1        ; number of sectors to read
%define LBA_PACKET       bp-0x40
%define LBA_SIZE       word [LBA_PACKET]
%define LBA_SECNUM     word [LBA_PACKET+2]
%define LBA_OFF        LBA_PACKET+4
%define LBA_SEG        LBA_PACKET+6
%define LBA_SECTOR_0   word [LBA_PACKET+8 ]
%define LBA_SECTOR_16  word [LBA_PACKET+10]
%define LBA_SECTOR_32  word [LBA_PACKET+12]
%define LBA_SECTOR_48  word [LBA_PACKET+14]



%define PARAMS LBA_PACKET+0x10
%define RootDirSecs     PARAMS+0x0         ; # of sectors root dir uses

%define fat_start       PARAMS+0x2         ; first FAT sector

%define root_dir_start  PARAMS+0x6         ; first root directory sector

%define data_start      PARAMS+0x0a        ; first data sector


;-----------------------------------------------------------------------
;   ENTRY
;-----------------------------------------------------------------------

real_start:
		cli
		cld
		xor	ax, ax
		mov	ds, ax
		mov     bp, 0x7c00


					; a reset should not be needed here
;		int     0x13            ; reset drive

;		int	0x12		; get memory available in AX
;		mov	ax, 0x01e0
;		mov	cl, 6		; move boot sector to higher memory
;		shl	ax, cl
;		sub	ax, 0x07e0

		mov	ax, 0x1FE0
		mov	es, ax
		mov	si, bp
		mov	di, bp
		mov	cx, 0x0100
		rep	movsw
                jmp     word 0x1FE0:cont

loadseg_off	dw	0
loadseg_seg	dw	LOADSEG

cont:
		mov     ds, ax
		mov	ss, ax
		lea     sp, [bp-0x60]
		sti
;
; Some BIOS don't pass drive number in DL, so don't use it if [drive] is known
;
		cmp     byte [drive], 0xff ; impossible number written by SYS
		jne     dont_use_dl     ; was SYS drive: other than A or B?
		mov     [drive], dl     ; yes, rely on BIOS drive number in DL
dont_use_dl:				; no,  rely on [drive] written by SYS

		mov     LBA_SIZE, 10h
		mov     LBA_SECNUM,1    ; initialise LBA packet constants

                call    print
                db      "FreeDOS",0

                GETDRIVEPARMS


;       FINDFILE: Searches for the file in the root directory.
;
;       Returns:
;                               AX = first cluster of file

                ; First, read the whole root directory
                ; into the temporary buffer.

                mov     ax, word [root_dir_start]
                mov     dx, word [root_dir_start+2]
                mov     di, word [RootDirSecs]
                les     bx, [loadsegoff_60] ; es:bx = 60:0
                call    readDisk
                jc      jmp_boot_error

		les     di, [loadsegoff_60] ; es:di = 60:0


		; Search for KERNEL.SYS file name, and find start cluster.

next_entry:     mov     cx, 11
                mov     si, filename
                push    di
                repe    cmpsb
                pop     di
                mov     ax, [es:di+0x1A]; get cluster number from directory entry
                je      ffDone

                add     di, byte 0x20   ; go to next directory entry
                cmp     byte [es:di], 0	; if the first byte of the name is 0,
                jnz     next_entry	; there is no more files in the directory

                jc      boot_error	; fail if not found
ffDone:
                push    ax              ; store first cluster number

;                call    print
;                db      " FAT",0