boot.asm

用C和汇编写的Dos操作系统的源码

;
; 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.
;
; $Logfile:   C:/dos-c/src/boot/boot.asv  $
;
; $Header:   C:/dos-c/src/boot/boot.asv   1.5   10 Jan 1997  4:58:06   patv  $
;
; $Log:   C:/dos-c/src/boot/boot.asv  $
;	
;	   Rev 1.5   10 Jan 1997  4:58:06   patv
;	Corrected copyright
;	
;	   Rev 1.4   10 Jan 1997  4:52:50   patv
;	Re-written to support C drive and eliminate restrictions on IPL.SYS
;	
;	   Rev 1.3   29 Aug 1996 13:06:50   patv
;	Bug fixes for v0.91b
;	
;	   Rev 1.2   01 Sep 1995 17:56:44   patv
;	First GPL release.
;	
;	   Rev 1.1   30 Jul 1995 20:37:38   patv
;	Initialized stack before use.
;	
;	   Rev 1.0   02 Jul 1995 10:57:52   patv
;	Initial revision.
;

		page	60,132
		title	DOS-C boot


IFDEF DEBUG
TEXT SEGMENT WORD PUBLIC 'TEXT'
TEXT ENDS

LOAD SEGMENT PARA PUBLIC 'LOAD'
LOAD ENDS

TEXT SEGMENT WORD PUBLIC 'TEXT'
ASSUME CS:TEXT, DS:TEXT

LOAD SEGMENT PARA PUBLIC 'LOAD'
		org     0
loadBuffer      dw      32767 dup (?)
LOAD ENDS

ELSE

TEXT SEGMENT WORD PUBLIC 'TEXT'
ASSUME CS:TEXT, DS:TEXT
ENDIF

IFDEF DEBUG
		mov     dl, BOOTDRIVE
		jmp     Entry
BASE            equ     7c00h
ELSE
BASE            equ     0
ENDIF


		org     BASE
Entry:          jmp     real_start

;	bp is initialized to 7c00h
oem             equ     [bp+3]
bytesPerSector  equ     [bp+0bh]
sectPerCluster  equ     [bp+0dh]
resSectors      equ     [bp+0eh]
nFats           equ     [bp+10h]
nRootDir        equ     [bp+11h]
nSectors        equ     [bp+13h]
MID             equ     [bp+15h]
sectPerFat      equ     [bp+16h]
sectPerTrack    equ     [bp+18h]
nHeads          equ     [bp+1ah]
nHidden         equ     [bp+1ch]
nSectorHuge     equ     [bp+20h]
drive           equ     [bp+24h]
extBoot         equ     [bp+26h]
volid           equ     [bp+27h]
vollabel        equ     [bp+2bh]
filesys         equ     [bp+36h]

IFDEF           DEBUG
		db      'FreeDOS '
		dw      512                     ; bytes/sector
		db      2                       ; sectors/allocation unit
		dw      1                       ; # reserved sectors
		db      2                       ; # of fats
		dw      112                     ; # of root directories
		dw      720                     ; # sectors total in image
		db      0fdh                    ; media descrip: fd=2side9sec, etc...
		dw      2                       ; # sectors in a fat
		dw      9                       ; # sectors/track
		dw      2                       ; # heads
		dd      0                       ; # hidden sectors
		dd      0                       ; # sectors if > 65536
		db      00h                     ; drive number
		db      00h
		db      29h                     ; extended boot signature
		dd      0
		db      'DOS-C BOOT '
		db      'FAT12   '


LOADSEG         equ     seg LOAD
ELSE
LOADSEG         equ     2000h
ENDIF

FATBUF          equ     4000h           ; offset of temporary buffer for FAT 
					; chain
RETRYCOUNT      equ     5               ; number of retries on disk errors

;	Some extra variables that are created on the stack frame

fat_start       equ     [bp-4]          ; first FAT sector
root_dir_start  equ     [bp-8]          ; first root directory sector
data_start      equ     [bp-12]         ; first data sector


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


;	FINDFILE: Searches for the file in the root directory.
;
;	Returns:
;
;	If file not found:      CF set
;	
;	If file found:          CF clear
;	                        AX = first cluster of file


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

		mov     ax, word ptr root_dir_start
		mov     dx, word ptr root_dir_start+2
		mov     di, nRootDir
		xor     bx, bx
		mov     es, tempbuf
		call    readDisk
		jc      ffDone

		xor     di, di

next_entry:     mov     cx, 11
		mov     si, offset filename+7c00h
		push    di
		repe    cmpsb
		pop     di
		mov     ax, es:[di][1ah]    ; get cluster number from directory entry
		clc
		je      ffDone

		add     di, 20h             ; go to next directory entry
		cmp     byte ptr es:[di], 0     ; if the first byte of the name is 0,
		jnz     next_entry              ; there is no more files in the directory

		stc
ffDone:
		ENDM

;	GETDRIVEPARMS:  Calculate start of some disk areas.

GETDRIVEPARMS   MACRO
		mov     si, word ptr nHidden
		mov     di, word ptr nHidden+2
		add     si, word ptr resSectors
		adc     di, 0                   ; DI:SI = first FAT sector

		mov     word ptr fat_start, si
		mov     word ptr fat_start+2, di

		mov     al, nFats
		xor     ah, ah
		mul     word ptr sectPerFat     ; DX:AX = total number of FAT sectors

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

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

		mov     ax, nRootDir
		xor     dx, dx
		div     bx                      

		mov     nRootDir, ax            ; AX = sectors per root directory

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

		mov     data_start, si
		mov     data_start+2, di
		ENDM

;	GETFATCHAIN:    
;
;	Reads the FAT chain and stores it in a temporary buffer in the first
;	64 kb.  The FAT chain is stored an array of 16-bit cluster numbers,
;	ending with 0.
;	
;	The file must fit in conventional memory, so it can't be larger than
;	640 kb. The sector size must be at least 512 bytes, so the FAT chain
;	can't be larger than around 3 kb.
;
;	Call with:      AX = first cluster in chain
;
;	Returns:        CF clear on success, set on error

GETFATCHAIN     MACRO
		push    ax                      ; store first cluster number

		; Load the complete FAT into memory. The FAT can't be larger
		; than 128 kb, so it should fit in the temporary buffer.

		mov     es, tempbuf
		xor     bx, bx
		mov     di, sectPerFat
		mov     ax, word ptr fat_start
		mov     dx, word ptr fat_start+2
		call    readDisk
		pop     ax                      ; restore first cluster number
		jc      boot_error

		; Set ES:DI to the temporary storage for the FAT chain.
		push    ds
		push    es
		pop     ds
		pop     es
		mov     di, FATBUF