source: bootcd/isolinux/syslinux-6.03/mbr/oldmbr.asm

; -----------------------------------------------------------------------
;
;   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