219 lines
6.4 KiB
ArmAsm
219 lines
6.4 KiB
ArmAsm
! masterboot 2.0 - Master boot block code Author: Kees J. Bot
|
|
!
|
|
! This code may be placed in the first sector (the boot sector) of a floppy,
|
|
! hard disk or hard disk primary partition. There it will perform the
|
|
! following actions at boot time:
|
|
!
|
|
! - If the booted device is a hard disk and one of the partitions is active
|
|
! then the active partition is booted.
|
|
!
|
|
! - Otherwise the next floppy or hard disk device is booted, trying them one
|
|
! by one.
|
|
!
|
|
! To make things a little clearer, the boot path might be:
|
|
! /dev/fd0 - Floppy disk containing data, tries fd1 then d0
|
|
! [/dev/fd1] - Drive empty
|
|
! /dev/c0d0 - Master boot block, selects active partition 2
|
|
! /dev/c0d0p2 - Submaster, selects active subpartition 0
|
|
! /dev/c0d0p2s0 - Minix bootblock, reads Boot Monitor /boot
|
|
! Minix - Started by /boot from a kernel image in /minix
|
|
|
|
LOADOFF = 0x7C00 ! 0x0000:LOADOFF is where this code is loaded
|
|
BUFFER = 0x0600 ! First free memory
|
|
PART_TABLE = 446 ! Location of partition table within this code
|
|
PENTRYSIZE = 16 ! Size of one partition table entry
|
|
MAGIC = 510 ! Location of the AA55 magic number
|
|
|
|
! <ibm/partition>.h:
|
|
bootind = 0
|
|
sysind = 4
|
|
lowsec = 8
|
|
|
|
|
|
.text
|
|
|
|
! Find active (sub)partition, load its first sector, run it.
|
|
|
|
master:
|
|
xor ax, ax
|
|
mov ds, ax
|
|
mov es, ax
|
|
cli
|
|
mov ss, ax ! ds = es = ss = Vector segment
|
|
mov sp, #LOADOFF
|
|
sti
|
|
|
|
! Copy this code to safety, then jump to it.
|
|
mov si, sp ! si = start of this code
|
|
push si ! Also where we'll return to eventually
|
|
mov di, #BUFFER ! Buffer area
|
|
mov cx, #512/2 ! One sector
|
|
cld
|
|
rep movs
|
|
jmpf BUFFER+migrate, 0 ! To safety
|
|
migrate:
|
|
|
|
! Find the active partition
|
|
findactive:
|
|
testb dl, dl
|
|
jns nextdisk ! No bootable partitions on floppies
|
|
mov si, #BUFFER+PART_TABLE
|
|
find: cmpb sysind(si), #0 ! Partition type, nonzero when in use
|
|
jz nextpart
|
|
testb bootind(si), #0x80 ! Active partition flag in bit 7
|
|
jz nextpart ! It's not active
|
|
loadpart:
|
|
call load ! Load partition bootstrap
|
|
jc error1 ! Not supposed to fail
|
|
bootstrap:
|
|
ret ! Jump to the master bootstrap
|
|
nextpart:
|
|
add si, #PENTRYSIZE
|
|
cmp si, #BUFFER+PART_TABLE+4*PENTRYSIZE
|
|
jb find
|
|
! No active partition, tell 'em
|
|
call print
|
|
.ascii "No active partition\0"
|
|
jmp reboot
|
|
|
|
! There are no active partitions on this drive, try the next drive.
|
|
nextdisk:
|
|
incb dl ! Increment dl for the next drive
|
|
testb dl, dl
|
|
js nexthd ! Hard disk if negative
|
|
int 0x11 ! Get equipment configuration
|
|
shl ax, #1 ! Highest floppy drive # in bits 6-7
|
|
shl ax, #1 ! Now in bits 0-1 of ah
|
|
andb ah, #0x03 ! Extract bits
|
|
cmpb dl, ah ! Must be dl <= ah for drive to exist
|
|
ja nextdisk ! Otherwise try disk 0 eventually
|
|
call load0 ! Read the next floppy bootstrap
|
|
jc nextdisk ! It failed, next disk please
|
|
ret ! Jump to the next master bootstrap
|
|
nexthd: call load0 ! Read the hard disk bootstrap
|
|
error1: jc error ! No disk?
|
|
ret
|
|
|
|
|
|
! Load sector 0 from the current device. It's either a floppy bootstrap or
|
|
! a hard disk master bootstrap.
|
|
load0:
|
|
mov si, #BUFFER+zero-lowsec ! si = where lowsec(si) is zero
|
|
!jmp load
|
|
|
|
! Load sector lowsec(si) from the current device. The obvious head, sector,
|
|
! and cylinder numbers are ignored in favour of the more trustworthy absolute
|
|
! start of partition.
|
|
load:
|
|
mov di, #3 ! Three retries for floppy spinup
|
|
retry: push dx ! Save drive code
|
|
push es
|
|
push di ! Next call destroys es and di
|
|
movb ah, #0x08 ! Code for drive parameters
|
|
int 0x13
|
|
pop di
|
|
pop es
|
|
andb cl, #0x3F ! cl = max sector number (1-origin)
|
|
incb dh ! dh = 1 + max head number (0-origin)
|
|
movb al, cl ! al = cl = sectors per track
|
|
mulb dh ! dh = heads, ax = heads * sectors
|
|
mov bx, ax ! bx = sectors per cylinder = heads * sectors
|
|
mov ax, lowsec+0(si)
|
|
mov dx, lowsec+2(si)! dx:ax = sector within drive
|
|
cmp dx, #[1024*255*63-255]>>16 ! Near 8G limit?
|
|
jae bigdisk
|
|
div bx ! ax = cylinder, dx = sector within cylinder
|
|
xchg ax, dx ! ax = sector within cylinder, dx = cylinder
|
|
movb ch, dl ! ch = low 8 bits of cylinder
|
|
divb cl ! al = head, ah = sector (0-origin)
|
|
xorb dl, dl ! About to shift bits 8-9 of cylinder into dl
|
|
shr dx, #1
|
|
shr dx, #1 ! dl[6..7] = high cylinder
|
|
orb dl, ah ! dl[0..5] = sector (0-origin)
|
|
movb cl, dl ! cl[0..5] = sector, cl[6..7] = high cyl
|
|
incb cl ! cl[0..5] = sector (1-origin)
|
|
pop dx ! Restore drive code in dl
|
|
movb dh, al ! dh = al = head
|
|
mov bx, #LOADOFF ! es:bx = where sector is loaded
|
|
mov ax, #0x0201 ! Code for read, just one sector
|
|
int 0x13 ! Call the BIOS for a read
|
|
jmp rdeval ! Evaluate read result
|
|
bigdisk:
|
|
mov bx, dx ! bx:ax = dx:ax = sector to read
|
|
pop dx ! Restore drive code in dl
|
|
push si ! Save si
|
|
mov si, #BUFFER+ext_rw ! si = extended read/write parameter packet
|
|
mov 8(si), ax ! Starting block number = bx:ax
|
|
mov 10(si), bx
|
|
movb ah, #0x42 ! Extended read
|
|
int 0x13
|
|
pop si ! Restore si to point to partition entry
|
|
!jmp rdeval
|
|
rdeval:
|
|
jnc rdok ! Read succeeded
|
|
cmpb ah, #0x80 ! Disk timed out? (Floppy drive empty)
|
|
je rdbad
|
|
dec di
|
|
jl rdbad ! Retry count expired
|
|
xorb ah, ah
|
|
int 0x13 ! Reset
|
|
jnc retry ! Try again
|
|
rdbad: stc ! Set carry flag
|
|
ret
|
|
rdok: cmp LOADOFF+MAGIC, #0xAA55
|
|
jne nosig ! Error if signature wrong
|
|
ret ! Return with carry still clear
|
|
nosig: call print
|
|
.ascii "Not bootable\0"
|
|
jmp reboot
|
|
|
|
! A read error occurred, complain and hang
|
|
error:
|
|
mov si, #LOADOFF+errno+1
|
|
prnum: movb al, ah ! Error number in ah
|
|
andb al, #0x0F ! Low 4 bits
|
|
cmpb al, #10 ! A-F?
|
|
jb digit ! 0-9!
|
|
addb al, #7 ! 'A' - ':'
|
|
digit: addb (si), al ! Modify '0' in string
|
|
dec si
|
|
movb cl, #4 ! Next 4 bits
|
|
shrb ah, cl
|
|
jnz prnum ! Again if digit > 0
|
|
call print
|
|
.ascii "Read error "
|
|
errno: .ascii "00\0"
|
|
!jmp reboot
|
|
|
|
reboot:
|
|
call print
|
|
.ascii ". Hit any key to reboot.\0"
|
|
xorb ah, ah ! Wait for keypress
|
|
int 0x16
|
|
call print
|
|
.ascii "\r\n\0"
|
|
int 0x19
|
|
|
|
! Print a message.
|
|
print: pop si ! si = String following 'call print'
|
|
prnext: lodsb ! al = *si++ is char to be printed
|
|
testb al, al ! Null marks end
|
|
jz prdone
|
|
movb ah, #0x0E ! Print character in teletype mode
|
|
mov bx, #0x0001 ! Page 0, foreground color
|
|
int 0x10
|
|
jmp prnext
|
|
prdone: jmp (si) ! Continue after the string
|
|
|
|
.data
|
|
|
|
! Extended read/write commands require a parameter packet.
|
|
ext_rw:
|
|
.data1 0x10 ! Length of extended r/w packet
|
|
.data1 0 ! Reserved
|
|
.data2 1 ! Blocks to transfer (just one)
|
|
.data2 LOADOFF ! Buffer address offset
|
|
.data2 0 ! Buffer address segment
|
|
.data4 0 ! Starting block number low 32 bits (tbfi)
|
|
zero: .data4 0 ! Starting block number high 32 bits
|