boot/freeldr/bootsect/dosmbr.S

   1 /*
   2  * COPYRIGHT:       See COPYING in the top level directory
   3  * PROJECT:         ReactOS Bootsector for ISO file system
   4  * PURPOSE:         Normal DOS boot sector
   5  *                  Ported to NASM from FreeDOS fdisk 1.2.0 by:
   6  * PROGRAMMERS:     Casper Hornstrup (chorns@users.sourceforge.net)
   7  */
   8
   9 /* INCLUDES ******************************************************************/
  10
  11 #include <asm.inc>
  12
  13 //-----------------------------------------------------------------------
  14 //   ENTRY (copied from freedos bootsector)
  15 //
  16 // IN: DL = boot drive
  17 // OUT: DL = boot drive
  18 //
  19 //-----------------------------------------------------------------------
  20
  21 .code16
  22 real_start:
  23     cli
  24     cld
  25     xor ax, ax
  26     mov ss, ax          // initialize stack
  27     mov ds, ax
  28     mov bp, HEX(7c00)
  29     lea sp, [bp-32]
  30     sti
  31
  32     /* Copy 512 bytes of MBR to 1fe0:7c00 */
  33     mov ax, HEX(1FE0)
  34     mov es, ax
  35     mov si, bp
  36     mov di, bp
  37     mov cx, 256
  38     rep movsw
  39
  40     /* Jump into relocated code */
  41     ljmp16 HEX(1FE0), cont
  42 cont:
  43
  44     /* Setup segment registers */
  45     mov ds, ax
  46     mov ss, ax
  47     xor ax, ax
  48     mov es, ax
  49
  50     /* Search for active partition */
  51     lea di, [bp + HEX(1be)] // start of partition table
  52 test_next_for_active:
  53     test byte ptr ds:[di], HEX(80)
  54     jne active_partition_found
  55     add di, 16                    // next table
  56     cmp di, HEX(07c00) + HEX(1fe) // scanned beyond end of table ??
  57     jb test_next_for_active
  58
  59 /*****************************************************************/
  60     call print
  61     .asciz "No active partition found"
  62
  63 WAIT_FOR_REBOOT:
  64     jmp WAIT_FOR_REBOOT
  65
  66
  67 /*****************************************************************/
  68 trouble_reading_drive:
  69     call print
  70     .asciz "Read error while reading drive"
  71     jmp WAIT_FOR_REBOOT
  72
  73 /*****************************************************************/
  74
  75 invalid_partition_code:
  76     call print
  77     .asciz "Partition signature != 55AA"
  78
  79     jmp WAIT_FOR_REBOOT
  80
  81 /*****************************************************************/
  82
  83 active_partition_found:
  84 //  call print
  85 //  .asciz "Loading active partition"
  86
  87     call read_boot_sector
  88
  89     jc  trouble_reading_drive
  90
  91     cmp word ptr es:[HEX(7c00)+HEX(1fe)], HEX(0aa55)
  92     jne invalid_partition_code
  93
  94     ljmp16 0, HEX(7c00)            // and jump to boot sector code
  95
  96 /*****************************
  97  * read_boot_sector
  98  *
  99  * IN: DI--> partition info
 100  * OUT:CARRY
 101  *****************************/
 102 read_boot_sector:
 103     /* check for LBA support */
 104     mov bx, HEX(55aa)
 105     mov ah, HEX(41)
 106     int HEX(13)
 107
 108     jc  StandardBios    //  if (regs.b.x != 0xaa55 || (regs.flags & 0x01))
 109     cmp bx, HEX(0aa55)  //    goto StandardBios;
 110     jne StandardBios
 111
 112     /* if DAP cannot be used, don't use LBA
 113        if ((regs.c.x & 1) == 0)
 114            goto StandardBios; */
 115     test cl, 1
 116     jz StandardBios
 117
 118     jmp short LBABios
 119
 120
 121 _bios_LBA_address_packet:
 122     .byte 16
 123     .byte 0
 124     .byte 4         // read four sectors - why not
 125     .byte 0
 126     .word HEX(7c00) // fixed boot address for DOS sector
 127     .word HEX(0000)
 128
 129 _bios_LBA_low:
 130     .word 0
 131 _bios_LBA_high:
 132     .word 0
 133     .word 0,0
 134
 135
 136 LBABios:
 137     // copy start address of partition to DAP
 138     mov ax, [di + 8]
 139     mov word ptr ds:[_bios_LBA_low], ax
 140     mov ax,[di + 8 + 2]
 141     mov word ptr ds:[_bios_LBA_high], ax
 142
 143     mov ax, HEX(4200)    //  regs.a.x = LBA_READ;
 144     mov si, offset _bios_LBA_address_packet // regs.si = FP_OFF(&dap);
 145
 146     int HEX(13)
 147     ret
 148
 149 /*****************************************************************
 150  * read disk, using standard BIOS
 151  */
 152 StandardBios:
 153     mov ax, HEX(0204)               //  regs.a.x = 0x0201;
 154     mov bx, HEX(7c00)               //  regs.b.x = FP_OFF(buffer);
 155
 156     /* regs.c.x =
 157        ((chs.Cylinder & 0xff) << 8) + ((chs.Cylinder & 0x300) >> 2) +
 158         ; chs.Sector;
 159          that was easy ;-) */
 160     mov cx, word ptr ds:[di + 2]
 161     mov dh, byte ptr ds:[di + 1]    //  regs.d.b.h = chs.Head;
 162                                     //  regs.es = FP_SEG(buffer);
 163     int HEX(13)
 164     ret
 165
 166 /****** PRINT
 167  * prints text after call to this function.
 168  */
 169 print_1char:
 170     xor bx, bx                   // video page 0
 171     mov ah, HEX(0E)              // else print it
 172     int HEX(10)                  // via TTY mode
 173 print:
 174     pop si                       // this is the first character
 175 print1:
 176     lodsb                        // get token
 177     push si                      // stack up potential return address
 178     cmp al, 0                    // end of string?
 179     jne print_1char              // until done
 180     ret                          // and jump to it
 181
 182 .org 510
 183     .byte HEX(55), HEX(0aa)
 184
 185 .endcode16
 186
 187 END