; -----------------------------------------------------------------------
;
; Copyright 2003-2008 H. Peter Anvin - All Rights Reserved
;
; Permission is hereby granted, free of charge, to any person
; obtaining a copy of this software and associated documentation
; files (the "Software"), to deal in the Software without
; restriction, including without limitation the rights to use,
; copy, modify, merge, publish, distribute, sublicense, and/or
; sell copies of the Software, and to permit persons to whom
; the Software is furnished to do so, subject to the following
; conditions:
;
; The above copyright notice and this permission notice shall
; be included in all copies or substantial portions of the Software.
;
; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
; EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
; OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
; NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
; HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
; WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
; FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
; OTHER DEALINGS IN THE SOFTWARE.
;
; -----------------------------------------------------------------------
;
; mbr.asm
;
; Simple Master Boot Record, including support for EBIOS extensions.
;
; The MBR lives in front of the boot sector, and is responsible for
; loading the boot sector of the active partition. The EBIOS support
; is needed if the active partition starts beyond cylinder 1024.
;
; This MBR determines all geometry info at runtime. It uses only the
; linear block field in the partition table. It does, however, pass
; the partition table information unchanged to the target OS.
;
; This MBR should be "8086-clean", i.e. not require a 386.
;
%include "bios.inc"
;
; Note: The MBR is actually loaded at 0:7C00h, but we quickly move it down to
; 0600h.
;
section .text
cpu 8086
org 0600h
_start: cli
xor ax,ax
mov ds,ax
mov es,ax
mov ss,ax
mov sp,7C00h
sti
cld
mov si,sp ; Start address
mov di,0600h ; Destination address
mov cx,512/2
rep movsw
;
; Now, jump to the copy at 0600h so we can load the boot sector at 7C00h.
; Since some BIOSes seem to think 0000:7C00h and 07C0:0000h are the same
; thing, use a far jump to canonicalize the address. This also makes
; sure that it is a code speculation barrier.
;
jmp 0:next ; Jump to copy at 0600h
next:
mov [DriveNo], dl ; Drive number stored in DL
;
; Check for CHS parameters. This doesn't work on floppy disks,
; but for an MBR we don't care.
;
mov ah,08h ; Get drive parameters
int 13h
and cx,3Fh ; Max sector number
mov [Sectors],cx
xor ax,ax
mov al,dh
inc ax ; From 0-based to count
mul cx ; Heads*Sectors
mov [SecPerCyl],ax
; Note: we actually don't care about the number of
; cylinders, since that's the highest-order division
;
; Now look for one (and only one) active partition.
;
mov si,PartitionTable
xor ax,ax
mov cx,4
checkpartloop:
test byte [si],80h
jz .notactive
inc ax
mov di,si
.notactive: add si,byte 16
loop checkpartloop
cmp ax,byte 1 ; Better be only one
jnz not_one_partition
;
; Now we have the active partition partition information in DS:DI.
; Check to see if we support EBIOS.
;
mov dl,[DriveNo]
mov ax,4100h
mov bx,055AAh
xor cx,cx
xor dh,dh
stc
int 13h
jc no_ebios
cmp bx,0AA55h
jne no_ebios
test cl,1 ; LBA device access
jz no_ebios
;
; We have EBIOS. Load the boot sector using LBA.
;
push di
mov si,dapa
mov bx,[di+8] ; Copy the block address
mov [si+8],bx
mov bx,[di+10]
mov [si+10],bx
mov dl,[DriveNo]
mov ah,42h ; Extended Read
jmp short common_tail
;
; No EBIOS. Load the boot sector using CHS.
;
no_ebios:
push di
mov ax,[di+8]
mov dx,[di+10]
div word [SecPerCyl] ; AX = cylinder DX = sec in cyl
ror ah,1
ror ah,1
mov cl,ah
mov ch,al ; CL = cyl[9:8], CH = cyl[7:0]
mov ax,dx
div byte [Sectors] ; AL = head AH = sector
mov dh,al
inc ah
or cl,ah ; CX = cylinder and sector
mov dl,[DriveNo]
mov bx,7C00h
mov ax,0201h ; Read one sector
common_tail:
int 13h
jc disk_error
pop si ; DS:SI -> partition table entry
;
; Verify that we have a boot sector, jump
;
cmp word [7C00h+510],0AA55h
jne missing_os
cli
jmp 0:7C00h ; Jump to boot sector; far
; jump is speculation barrier
; (Probably not neecessary, but
; there is plenty of space.)
not_one_partition:
ja too_many_os
missing_os:
mov si,missing_os_msg
jmp short die
too_many_os:
disk_error:
mov si,bad_disk_msg
die:
.msgloop:
lodsb
and al,al
jz .now
mov ah,0Eh ; TTY output
mov bh,[BIOS_page] ; Current page
mov bl,07h
int 10h
jmp short .msgloop
.now:
jmp short .now
align 4, db 0 ; Begin data area
;
; EBIOS disk address packet
;
dapa:
dw 16 ; Packet size
.count: dw 1 ; Block count
.off: dw 7C00h ; Offset of buffer
.seg: dw 0 ; Segment of buffer
.lba: dd 0 ; LBA (LSW)
dd 0 ; LBA (MSW)
; CHS information
SecPerCyl: dw 0 ; Heads*Sectors
Sectors: dw 0
; Error messages
missing_os_msg db 'Missing operating system', 13, 10, 0
bad_disk_msg db 'Operating system loading error', 13, 10, 0
;
; Maximum MBR size: 446 bytes; end-of-boot-sector signature also needed.
; Note that some operating systems (NT, DR-DOS) put additional stuff at
; the end of the MBR, so shorter is better. Location 440 is known to
; have a 4-byte attempt-at-unique-ID for some OSes.
;
PartitionTable equ $$+446 ; Start of partition table
;
; BSS data; put at 800h
;
DriveNo equ 0800h
