reactos/boot/freeldr/bootsect/dosmbr.S

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/*
* COPYRIGHT: See COPYING in the top level directory
* PROJECT: ReactOS Bootsector for ISO file system
* PURPOSE: Normal DOS boot sector
* Ported to NASM from FreeDOS fdisk 1.2.0 by:
* PROGRAMMERS: Casper Hornstrup (chorns@users.sourceforge.net)
*/
/* INCLUDES ******************************************************************/
#include <asm.inc>
//-----------------------------------------------------------------------
// ENTRY (copied from freedos bootsector)
//
// IN: DL = boot drive
// OUT: DL = boot drive
//
//-----------------------------------------------------------------------
.code16
real_start:
cli
cld
xor ax, ax
mov ss, ax // initialize stack
mov ds, ax
mov bp, HEX(7c00)
lea sp, [bp-32]
sti
/* Copy 512 bytes of MBR to 1fe0:7c00 */
mov ax, HEX(1FE0)
mov es, ax
mov si, bp
mov di, bp
mov cx, 256
rep movsw
/* Jump into relocated code */
ljmp16 HEX(1FE0), cont
cont:
/* Setup segment registers */
mov ds, ax
mov ss, ax
xor ax, ax
mov es, ax
/* Search for active partition */
lea di, [bp + HEX(1be)] // start of partition table
test_next_for_active:
test byte ptr ds:[di], HEX(80)
jne active_partition_found
add di, 16 // next table
cmp di, HEX(07c00) + HEX(1fe) // scanned beyond end of table ??
jb test_next_for_active
/*****************************************************************/
call print
.asciz "No active partition found"
WAIT_FOR_REBOOT:
jmp WAIT_FOR_REBOOT
/*****************************************************************/
trouble_reading_drive:
call print
.asciz "Read error while reading drive"
jmp WAIT_FOR_REBOOT
/*****************************************************************/
invalid_partition_code:
call print
.asciz "Partition signature != 55AA"
jmp WAIT_FOR_REBOOT
/*****************************************************************/
active_partition_found:
// call print
// .asciz "Loading active partition"
call read_boot_sector
jc trouble_reading_drive
cmp word ptr es:[HEX(7c00)+HEX(1fe)], HEX(0aa55)
jne invalid_partition_code
ljmp16 0, HEX(7c00) // and jump to boot sector code
/*****************************
* read_boot_sector
*
* IN: DI--> partition info
* OUT:CARRY
*****************************/
read_boot_sector:
/* check for LBA support */
mov bx, HEX(55aa)
mov ah, HEX(41)
int HEX(13)
jc StandardBios // if (regs.b.x != 0xaa55 || (regs.flags & 0x01))
cmp bx, HEX(0aa55) // goto StandardBios;
jne StandardBios
/* if DAP cannot be used, don't use LBA
if ((regs.c.x & 1) == 0)
goto StandardBios; */
test cl, 1
jz StandardBios
jmp short LBABios
_bios_LBA_address_packet:
.byte 16
.byte 0
.byte 4 // read four sectors - why not
.byte 0
.word HEX(7c00) // fixed boot address for DOS sector
.word HEX(0000)
_bios_LBA_low:
.word 0
_bios_LBA_high:
.word 0
.word 0,0
LBABios:
// copy start address of partition to DAP
mov ax, [di + 8]
mov word ptr ds:[_bios_LBA_low], ax
mov ax,[di + 8 + 2]
mov word ptr ds:[_bios_LBA_high], ax
mov ax, HEX(4200) // regs.a.x = LBA_READ;
mov si, offset _bios_LBA_address_packet // regs.si = FP_OFF(&dap);
int HEX(13)
ret
/*****************************************************************
* read disk, using standard BIOS
*/
StandardBios:
mov ax, HEX(0204) // regs.a.x = 0x0201;
mov bx, HEX(7c00) // regs.b.x = FP_OFF(buffer);
/* regs.c.x =
((chs.Cylinder & 0xff) << 8) + ((chs.Cylinder & 0x300) >> 2) +
; chs.Sector;
that was easy ;-) */
mov cx, word ptr ds:[di + 2]
mov dh, byte ptr ds:[di + 1] // regs.d.b.h = chs.Head;
// regs.es = FP_SEG(buffer);
int HEX(13)
ret
/****** PRINT
* prints text after call to this function.
*/
print_1char:
xor bx, bx // video page 0
mov ah, HEX(0E) // else print it
int HEX(10) // via TTY mode
print:
pop si // this is the first character
print1:
lodsb // get token
push si // stack up potential return address
cmp al, 0 // end of string?
jne print_1char // until done
ret // and jump to it
.org 510
.byte HEX(55), HEX(0aa)
.endcode16
END