minix/boot/masterboot/masterboot.s

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2005-04-21 16:53:53 +02:00
! 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