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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
! before the boot blocks are checked.
!
! Or
!
! Space for 12 extra partitions in the 'DiskManager' form that Linux
! _does_ understand.
!
! NB: This needs Dev86 0.15.2 or later
! Lowest available is $0500, MSDOS appears to use $0600 ... I wonder why?
ORGADDR=$0500
preboot=0 ! Include the pre-boot loader.
mbrkey=0 ! Option to choose the boot record base on keystroke
message=1 ! Display boot message
diskman=0 ! Disk manager partitions, allows 16 partitions but
! don't overwrite this with a LILO BB.
linear=0 ! Use the linear addresses not the CHS ones
partition_start=ORGADDR+0x1BE
partition_size=0x10
partition_end=ORGADDR+0x1FE
if diskman
! Partition table start ...
table_start=ORGADDR+0xFC
low_partition=table_start+2
else
table_start=partition_start
endif
org ORGADDR
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 or a keypress
if message
call disp_message
endif
if preboot
call preboot_code
endif
if mbrkey
call key_wait
endif
! Now check the partition table, must use SI as pointer cause that's what the
! partition boot blocks expect.
mov si,#partition_start
check_active:
cmp byte [si],#$80 ! Flag for activated partition
jz found_active
if mbrkey=0
bad_boot:
endif
add si,#partition_size
cmp si,#partition_end
jnz check_active
! Check for Disk manager partitions in the order that Linux numbers them.
if diskman
cmp word ptr diskman_magic,#$55AA
jnz no_diskman
mov si,#partition_start
check_next:
sub si,#partition_size
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:
if linear
call linearise
else
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 at $7C00
endif
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
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 #$7C00,#0 ! Go!
! Fatal errors ...........
if mbrkey
bad_boot:
mov si,#no_bootpart
no_boot: ! SI now has pointer to error message
call puts
mov si,#crlf
call puts
tick:
call key_wait
j tick
else
no_boot: ! SI now has pointer to error message
lodsb
cmp al,#0
jz EOS
mov bx,#7
mov ah,#$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 rarely does anything useful...
jmpi $0,$FFFF ! Wam! Try or die!
press_key:
.asciz "\r\nPress return:"
press_end:
endif
no_bootpart:
.asciz "No active partition"
disk_read_error:
.asciz "Disk read error"
!-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
! Instead of loading using the CHS data in the ptbl use the linear addr
!
if linear
linearise:
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 at $7C00
fail! Todo ...
ret
endif
!-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
! Choose the partition based on a pressed key ...
if mbrkey
key_wait:
mov si,#Prompt
call puts
mov di,#19 ! Wait for 18-19 ticks
next_loop:
mov ah,#1
int $16
jnz Got_key
mov ah,#0
int $1A ! Get current tick
cmp dx,si ! If changed DEC our counter.
jz next_loop
mov si,dx
dec di
jnz next_loop
mov si,#Unprompt ! Nothing has happened, return.
call puts
bad_key:
ret
Got_key:
cmp al,#$20
jnz not_space
mov si,#Pause
j showit
not_space:
mov Showkey,al
mov si,#Showkey
showit:
call puts
mov ah,#0 ! Clean the kbd buffer.
int $16
! ... Now we use our key ...
! 0 => Floppy
! 1 .. 4 => Hard disk partition.
mov di,#-1
cmp al,#$20
jz next_loop
and ax,#0xF
jnz not_floppy
mov si,#floppy_part
br found_active
not_floppy:
dec ax
test ax,#0xC
jnz bad_key
mov cl,#4
shl ax,cl
add ax,#partition_start
mov si,ax
! Set active flag for disk interrupt.
or byte [si],#$80
br found_active
puts:
lodsb
cmp al,#0
jz EOS
push bx
mov bx,#7
mov ah,#$E ! Can't use $13 cause that's AT+ only!
int $10
pop bx
jmp puts
EOS:
ret
Prompt:
.asciz "MBR: "
Unprompt:
.asciz "\b\b\b\b\b \b\b\b\b\b"
Pause:
.asciz "\b\b> "
Showkey:
.ascii " "
crlf:
.asciz "\r\n"
floppy_part:
.word 0,1
endif
!-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
!
if message
disp_message:
mov si,#Banner
if mbrkey
br puts
else
puts:
lodsb
cmp al,#0
jz .EOS
push bx
mov bx,#7
mov ah,#$E ! Can't use $13 cause that's AT+ only!
int $10
pop bx
jmp puts
.EOS:
ret
endif
export Banner
Banner:
.asciz ""
endif
!-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
! This is the pre-boot loader it uses CHS but that's ok for track 0
!
if preboot
public preboot_code
preboot_code:
push bx
mov si,#pre_boot_table
lodsw
mov di,ax ! First word is execute address
more_boot:
lodsw
test ah,ah
jz load_done
mov bx,ax ! word 1 address
lodsw
mov cx,ax ! word 2 CX, cylinder/sector
lodsw
mov dx,ax ! word 3 DX, drive, head
lodsw ! word 4 AX, $02, sector count
int $13
jnc more_boot ! This doesn't retry, with a HD it shouldn't be bad.
mov si,#disk_read_error
br no_boot ! Sorry it ain't gonna work.
load_done:
call di
exec_done:
pop bx
return:
ret
export pre_boot_table
pre_boot_table:
! Example: Do nothing.
.word return,0
! Labels
! .word <execute address>
! Then repeat ..
! .word <BX>, <CX>, <DX>, <AX>
! Or.
! .word <Load Address>
! .byte <sector> + (<cyl> & $300)>>2), <cyl> & $FF, <Drive>, <Head>, <cnt>, 2
! Finally
! .word 0
! Example: Load rest of H0,C0 into mem at $7C00 (8k).
! .word $7C00
! .word $7C00,$0002,$8000,$0210
! .word $0000
endif
!-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
! Now make sure this isn't too big!
if *>table_start
fail! Partition table overlaps
endif
!-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
! The diskman magic number and empty DM partitions.
if diskman
org ORGADDR+0xFC
public diskman_magic
diskman_magic:
.word 0xAA55
.blkb 12*partition_size-1
.byte 0
endif
!-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
! And finally a copyright message if there's room.
if *<ORGADDR+0x180
org ORGADDR+0x180
.asciz "ELKS MBR "
.asciz "Robert de Bath,"
.asciz "Copyright 1996-2000."
org partition_start-1
.byte 0xFF
endif
!THE END
|