mbr/oldmbr.asm

   1 ; -----------------------------------------------------------------------
   2 ;
   3 ;   Copyright 2003-2008 H. Peter Anvin - All Rights Reserved
   4 ;
   5 ;   Permission is hereby granted, free of charge, to any person
   6 ;   obtaining a copy of this software and associated documentation
   7 ;   files (the "Software"), to deal in the Software without
   8 ;   restriction, including without limitation the rights to use,
   9 ;   copy, modify, merge, publish, distribute, sublicense, and/or
  10 ;   sell copies of the Software, and to permit persons to whom
  11 ;   the Software is furnished to do so, subject to the following
  12 ;   conditions:
  13 ;
  14 ;   The above copyright notice and this permission notice shall
  15 ;   be included in all copies or substantial portions of the Software.
  16 ;
  17 ;   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  18 ;   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
  19 ;   OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  20 ;   NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
  21 ;   HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
  22 ;   WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  23 ;   FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
  24 ;   OTHER DEALINGS IN THE SOFTWARE.
  25 ;
  26 ; -----------------------------------------------------------------------
  27
  28 ;
  29 ; mbr.asm
  30 ;
  31 ; Simple Master Boot Record, including support for EBIOS extensions.
  32 ;
  33 ; The MBR lives in front of the boot sector, and is responsible for
  34 ; loading the boot sector of the active partition.  The EBIOS support
  35 ; is needed if the active partition starts beyond cylinder 1024.
  36 ;
  37 ; This MBR determines all geometry info at runtime.  It uses only the
  38 ; linear block field in the partition table.  It does, however, pass
  39 ; the partition table information unchanged to the target OS.
  40 ;
  41 ; This MBR should be "8086-clean", i.e. not require a 386.
  42 ;
  43
  44 %include "bios.inc"
  45
  46 ;
  47 ; Note: The MBR is actually loaded at 0:7C00h, but we quickly move it down to
  48 ; 0600h.
  49 ;
  50                 section .text
  51                 cpu 8086
  52                 org 0600h
  53
  54 _start:         cli
  55                 xor ax,ax
  56                 mov ds,ax
  57                 mov es,ax
  58                 mov ss,ax
  59                 mov sp,7C00h
  60                 sti
  61                 cld
  62                 mov si,sp               ; Start address
  63                 mov di,0600h            ; Destination address
  64                 mov cx,512/2
  65                 rep movsw
  66
  67 ;
  68 ; Now, jump to the copy at 0600h so we can load the boot sector at 7C00h.
  69 ; Since some BIOSes seem to think 0000:7C00h and 07C0:0000h are the same
  70 ; thing, use a far jump to canonicalize the address.  This also makes
  71 ; sure that it is a code speculation barrier.
  72 ;
  73
  74                 jmp 0:next              ; Jump to copy at 0600h
  75
  76 next:
  77                 mov [DriveNo], dl               ; Drive number stored in DL
  78 ;
  79 ; Check for CHS parameters.  This doesn't work on floppy disks,
  80 ; but for an MBR we don't care.
  81 ;
  82                 mov ah,08h                      ; Get drive parameters
  83                 int 13h
  84                 and cx,3Fh                      ; Max sector number
  85                 mov [Sectors],cx
  86                 xor ax,ax
  87                 mov al,dh
  88                 inc ax                          ; From 0-based to count
  89                 mul cx                          ; Heads*Sectors
  90                 mov [SecPerCyl],ax
  91                 ; Note: we actually don't care about the number of
  92                 ; cylinders, since that's the highest-order division
  93
  94 ;
  95 ; Now look for one (and only one) active partition.
  96 ;
  97                 mov si,PartitionTable
  98                 xor ax,ax
  99                 mov cx,4
 100 checkpartloop:
 101                 test byte [si],80h
 102                 jz .notactive
 103                 inc ax
 104                 mov di,si
 105 .notactive:     add si,byte 16
 106                 loop checkpartloop
 107
 108                 cmp ax,byte 1                   ; Better be only one
 109                 jnz not_one_partition
 110
 111 ;
 112 ; Now we have the active partition partition information in DS:DI.
 113 ; Check to see if we support EBIOS.
 114 ;
 115                 mov dl,[DriveNo]
 116                 mov ax,4100h
 117                 mov bx,055AAh
 118                 xor cx,cx
 119                 xor dh,dh
 120                 stc
 121                 int 13h
 122                 jc no_ebios
 123                 cmp bx,0AA55h
 124                 jne no_ebios
 125                 test cl,1                       ; LBA device access
 126                 jz no_ebios
 127 ;
 128 ; We have EBIOS.  Load the boot sector using LBA.
 129 ;
 130                 push di
 131                 mov si,dapa
 132                 mov bx,[di+8]                   ; Copy the block address
 133                 mov [si+8],bx
 134                 mov bx,[di+10]
 135                 mov [si+10],bx
 136                 mov dl,[DriveNo]
 137                 mov ah,42h                      ; Extended Read
 138                 jmp short common_tail
 139 ;
 140 ; No EBIOS.  Load the boot sector using CHS.
 141 ;
 142 no_ebios:
 143                 push di
 144                 mov ax,[di+8]
 145                 mov dx,[di+10]
 146                 div word [SecPerCyl]    ; AX = cylinder DX = sec in cyl
 147                 ror ah,1
 148                 ror ah,1
 149                 mov cl,ah
 150                 mov ch,al                       ; CL = cyl[9:8], CH = cyl[7:0]
 151
 152                 mov ax,dx
 153                 div byte [Sectors]              ; AL = head AH = sector
 154                 mov dh,al
 155                 inc ah
 156                 or cl,ah                        ; CX = cylinder and sector
 157
 158                 mov dl,[DriveNo]
 159                 mov bx,7C00h
 160                 mov ax,0201h                    ; Read one sector
 161 common_tail:
 162                 int 13h
 163                 jc disk_error
 164                 pop si                          ; DS:SI -> partition table entry
 165 ;
 166 ; Verify that we have a boot sector, jump
 167 ;
 168                 cmp word [7C00h+510],0AA55h
 169                 jne missing_os
 170                 cli
 171                 jmp 0:7C00h                     ; Jump to boot sector; far
 172                                                 ; jump is speculation barrier
 173                                                 ; (Probably not neecessary, but
 174                                                 ; there is plenty of space.)
 175
 176 not_one_partition:
 177                 ja too_many_os
 178 missing_os:
 179                 mov si,missing_os_msg
 180                 jmp short die
 181 too_many_os:
 182 disk_error:
 183                 mov si,bad_disk_msg
 184 die:
 185 .msgloop:
 186                 lodsb
 187                 and al,al
 188                 jz .now
 189                 mov ah,0Eh                      ; TTY output
 190                 mov bh,[BIOS_page]              ; Current page
 191                 mov bl,07h
 192                 int 10h
 193                 jmp short .msgloop
 194 .now:
 195                 jmp short .now
 196
 197                 align 4, db 0                   ; Begin data area
 198
 199 ;
 200 ; EBIOS disk address packet
 201 ;
 202 dapa:
 203                 dw 16                           ; Packet size
 204 .count:         dw 1                            ; Block count
 205 .off:           dw 7C00h                        ; Offset of buffer
 206 .seg:           dw 0                            ; Segment of buffer
 207 .lba:           dd 0                            ; LBA (LSW)
 208                 dd 0                            ; LBA (MSW)
 209
 210 ; CHS information
 211 SecPerCyl:      dw 0                            ; Heads*Sectors
 212 Sectors:        dw 0
 213
 214 ; Error messages
 215 missing_os_msg  db 'Missing operating system', 13, 10, 0
 216 bad_disk_msg    db 'Operating system loading error', 13, 10, 0
 217
 218 ;
 219 ; Maximum MBR size: 446 bytes; end-of-boot-sector signature also needed.
 220 ; Note that some operating systems (NT, DR-DOS) put additional stuff at
 221 ; the end of the MBR, so shorter is better.  Location 440 is known to
 222 ; have a 4-byte attempt-at-unique-ID for some OSes.
 223 ;
 224
 225 PartitionTable  equ $$+446                      ; Start of partition table
 226
 227 ;
 228 ; BSS data; put at 800h
 229 ;
 230 DriveNo         equ 0800h