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|
!
! This is a 'Master Boot Record' following the MSDOS 'standards'.
! This BB successfully boots MSDOS or Linux.
!
! In addition it has the facility to load and execute a small program
! (of 8 extents) before the boot blocks are checked.
!
! Or
!
! Space for 12 extra partitions in a form that Linux _does_ understand.
!
! Lowest available is $0500, MSDOS appears to use $0600 ... I wonder why?
ORGADDR=$0500
preboot=0 ! Include the pre-boot loader.
diskman=1 ! Disk manager partitions, allows 16 partitions but
! don't overwrite this with a LILO BB.
! Include standard layout
org ORGADDR
include sysboot.s
public partition_1
public partition_2
public partition_3
public partition_4
if diskman=0
org ORGADDR+$3
.ascii "ELKS MBR Copyright 1996, Robert de Bath"
! Start after dos fsstat data, not strictly required.
org codestart
else
org ORGADDR
endif
cli ! Assume _nothing_!
cld
mov bx,#$7C00 ! Pointer to start of BB.
xor ax,ax ! Segs all to zero
mov ds,ax
mov es,ax
mov ss,ax
mov sp,bx ! SP Just below BB
mov cx,#$100 ! Move 256 words
mov si,bx ! From default BB
mov di,#ORGADDR ! To the correct address.
rep
movsw
jmpi cont,#0 ! Set CS:IP correct.
cont:
sti ! Let the interrupts back in.
!
! Next check for a pre-boot load.
if preboot
push bx
mov si,#pre_boot_table
lodsw
mov di,ax ! First word is execute address
more_boot:
lodsw
test ax,ax
jz load_done
mov bx,ax ! word 1 address
lodsw
mov cx,ax ! word 2 CX, head/sector
lodsw
mov dx,ax ! word 3 DX, drive, cylinder
lodsw ! word 4 AX, $02, sector count
int $13
jnc more_boot ! This doesn't retry, with a HD it shouldn't be bad.
jc disk_error
load_done:
call di
exec_done:
pop bx
endif
! Now check the partition table, must use SI as pointer cause that's what the
! partition boot blocks expect.
mov si,#partition_1
check_active:
cmp byte [si],#$80 ! Flag for activated partition
jz found_active
bad_boot:
add si,#partition_2-partition_1
cmp si,#bootblock_magic
jnz check_active
! Check for Disk manager partitions (12 more!)
if diskman
cmp word ptr diskman_magic,#$55AA
jnz no_diskman
mov si,#partition_1
check_next:
sub si,#partition_2-partition_1
cmp byte [si],#$80 ! Flag for activated partition
jz found_active
cmp si,#low_partition
jnz check_next
no_diskman:
endif
mov si,#no_bootpart ! Message & boot
jmp no_boot
found_active:
mov di,#6 ! Max retries, int list says 3 ... double it
mov dx,[si] ! dh = Drive head, dl = $80 ie HD drive 0
mov cx,[si+2] ! cx = Sector & head encoded for int $13
! bx is correct
retry:
movb [$7DFE],#0 ! Clear magic for dosemu
mov ax,#$0201 ! Read 1 sector
int $13 ! Disk read.
jnc sector_loaded
! Error, reset and retry
xor ax,ax
int $13 ! Disk reset
dec di
jnz retry ! Try again
disk_error:
mov si,#disk_read_error
jmp no_boot ! Sorry it ain't gonna work.
sector_loaded:
mov di,#$7DFE ! End of sector loaded
cmp [di],#$AA55 ! Check for magic
jnz bad_boot ! No? Try next partition.
mov bp,si ! LILO says some BBs use bp rather than si
jmpi #$7C00,#0 ! Go!
! Fatal errors ...........
no_boot: ! SI now has pointer to error message
lodsb
cmp al,#0
jz EOS
mov bx,#7
mov ah,#$E ! Can't use $13 cause that's AT+ only!
int $10
jmp no_boot
EOS:
cmp si,#press_end ! After msg output 'press key' message
jz keyboot
mov si,#press_key
jmp no_boot
keyboot: ! Wait for a key then reboot
xor ax,ax
int $16
int $19 ! This should be OK as we haven't touched anything.
jmpi $0,$FFFF ! Wam! Try or die!
no_bootpart:
.asciz "No bootable partition"
disk_read_error:
.asciz "Error loading system"
press_key:
.asciz "\r\nPress a key to reboot"
press_end:
if preboot
return:
ret
export pre_boot_table
pre_boot_table:
! Example: Load rest of H0,C0 into mem at $7C00 (8k).
! .word $7C00, $7C00,$0002,$0000,$0210, $0000
.word return
.word 0
endif
if diskman
org ORGADDR+0xFC
public diskman_magic
diskman_magic:
.word 0xAA55
low_partition:
public partition_16
partition_16 = low_partition+0x00
public partition_15
partition_15 = low_partition+0x10
public partition_14
partition_14 = low_partition+0x20
public partition_13
partition_13 = low_partition+0x30
public partition_12
partition_12 = low_partition+0x40
public partition_11
partition_11 = low_partition+0x50
public partition_10
partition_10 = low_partition+0x60
public partition_9
partition_9 = low_partition+0x70
public partition_8
partition_8 = low_partition+0x80
public partition_7
partition_7 = low_partition+0x90
public partition_6
partition_6 = low_partition+0xA0
public partition_5
partition_5 = low_partition+0xB0
endif
! Now make sure this isn't to big!
if *>partition_1
fail! Partition overlap
endif
!THE END
|
