Minix2.0/src/boot/bootblock.s
! Bootblock 1.2 - Minix boot block. Author: Kees J. Bot
! Floppy sensing code: Guy Helmer
!
! When the PC is powered on, it will try to read the first sector of floppy
! disk 0 into address 0x7C00. If this fails due to the absence of flexible
! magnetic media, it will read the master boot record from the first sector
! of the hard disk. This sector not only contains executable code, but also
! the partition table of the hard disk. When executed, it will select the
! active partition and load the first sector of that into address 0x7C00.
! This file contains the code that is eventually read from either the floppy
! disk, or the hard disk partition. It is just smart enough to load the
! secondary boot code from the boot device into memory at address 0x10000 and
! execute that. The disk addresses for this secondary boot code are patched
! into this code by installboot as 24-bit sector numbers and 8-bit sector
! counts above enddata upwards. The secondary boot code is in turn smart
! enough to load the different parts of the Minix kernel into memory and
! execute them to finally get Minix started.
!
! Kees J. Bot - 91/12/21:
! Adapted Guy Helmers code and added hard disk support for my boot monitor
! package.
!
LOADOFF = 0x7C00 ! 0x0000:LOADOFF is where this code is loaded
BOOTSEG = 0x1000 ! Secondary boot code segment.
BOOTOFF = 0x0030 ! Offset into secondary boot above header
BUFFER = 0x0600 ! First free memory
DSKBASE = 0x1E ! Floppy disk parameter vector
DSKPARSIZE = 11 ! 11 bytes of floppy parameters
SECTORS = 4 ! Offset into parameters to sectors per track
LOWSEC = 8 ! Offset of logical first sector in partition
! table
! Variables addressed using bp register
device = 0 ! The boot device
lowsec = 2 ! Offset of boot partition within drive
secpcyl = 6 ! Sectors per cylinder = heads * sectors
.define begtext, begdata, begbss, endtext, enddata, endbss, _main
.data
begdata:
.bss
begbss:
.text
begtext:
_main:
! Start boot procedure.
boot:
xor ax, ax ! ax = 0x0000, the vector segment
mov ds, ax
cli ! Ignore interrupts while setting stack
mov ss, ax ! ss = ds = vector segment
mov sp, #LOADOFF ! Usual place for a bootstrap stack
sti
push ax
push ax ! Push a zero lowsec(bp)
push dx ! Boot device in dl will be device(bp)
mov bp, sp ! Using var(bp) is one byte cheaper then var.
push es
push si ! es:si = partition table entry if hard disk
mov di, #LOADOFF+parameters ! char (*di)[DSKPARSIZE] = parameters;
testb dl, dl ! Winchester disks if dl >= 0x80
jge floppy
winchester:
! Get the offset of the first sector of the boot partition from the partition
! table. The table is found at es:si, the lowsec parameter at offset LOWSEC.
eseg
les ax, LOWSEC(si) ! es:ax = LOWSEC+2(si):LOWSEC(si)
mov lowsec+0(bp), ax ! Low 16 bits of partition's first sector
mov lowsec+2(bp), es ! High 16 bits of partition's first sector
! Get the drive parameters, the number of sectors is bluntly written into the
! floppy disk parameters.
movb ah, #0x08 ! Code for drive parameters
int 0x13 ! dl still contains drive
andb cl, #0x3F ! cl = max sector number (1-origin)
movb SECTORS(di), cl ! Number of sectors per track
incb dh ! dh = 1 + max head number (0-origin)
jmp loadboot
! Floppy:
! Execute three read tests to determine the drive type. Test for each floppy
! type by reading the last sector on the first track. If it fails, try a type
! that has less sectors. Therefore we start with 1.44M (18 sectors) then 1.2M
! (15 sectors) ending with 720K/360K (both 9 sectors). (The floppy parameters
! of the last two are equal, apart from the motor start time. This saves us
! the rather painful "try to read track 41" test.)
next: add di, #DSKPARSIZE ! Next set of parameters
floppy: mov DSKBASE*4+0, di ! Load offset of disk parameters
mov DSKBASE*4+2, ds ! Load segment of disk parameters
xorb ah, ah ! Reset drive
int 0x13
movb cl, SECTORS(di) ! cl = number of last sector on track
cmp di, #LOADOFF+dsdd3 ! No need to do the last 720K/360K test
jz success
! Try to read the last sector on track 0
mov es, lowsec(bp) ! es = vector segment (lowsec = 0)
mov bx, #BUFFER ! es:bx buffer = 0x0000:0x0600
mov ax, #0x0201 ! Read sector, #sectors = 1
xorb ch, ch ! Track 0, last sector
xorb dh, dh ! Drive dl, head 0
int 0x13
jb next ! Error, try the next floppy type
success:movb dh, #2 ! Load number of heads for multiply
! Number of sectors is still in cl
loadboot:
! Load the secondary boot code from the boot device
movb al, cl ! al = cl = sectors per track
mulb dh ! dh = heads, ax = heads * sectors
mov secpcyl(bp), ax ! Sectors per cylinder = heads * sectors
mov ax, #BOOTSEG ! Segment to load secondary boot code into
mov es, ax
xor bx, bx ! Load first sector at es:bx = BOOTSEG:0x0000
mov si, #LOADOFF+addresses ! Start of the boot code addresses
load:
mov ax, 1(si) ! Get next sector number: low 16 bits
movb dl, 3(si) ! Bits 16-23 for your 8GB disk
xorb dh, dh ! dx:ax = sector within partition
add ax, lowsec+0(bp)
adc dx, lowsec+2(bp)! dx:ax = sector within drive
div secpcyl(bp) ! 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 SECTORS(di) ! 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)
movb dh, al ! dh = al = head
movb dl, device(bp) ! dl = device to read
movb al, SECTORS(di) ! Sectors per track - Sector number (0-origin)
subb al, ah ! = Sectors left on this track
cmpb al, (si) ! Compare with # sectors to read
jbe read ! Can't read past the end of a cylinder?
movb al, (si) ! (si) < sectors left on this track
read: push ax ! Save al = sectors to read
movb ah, #2 ! Code for disk read (all registers in use now!)
int 0x13 ! Call the BIOS for a read
pop cx ! Restore al in cl
jb error ! Jump on disk read error
movb al, cl ! Restore al = sectors read
addb bh, al ! bx += 2 * al * 256 (add bytes read)
addb bh, al ! es:bx = where next sector must be read
add 1(si), ax ! Update address by sectors read
adcb 3(si), ah ! Don't forget bits 16-23 (add ah = 0)
subb (si), al ! Decrement sector count by sectors read
jnz load ! Not all sectors have been read
add si, #4 ! Next (address, count) pair
cmpb ah, (si) ! Done when no sectors to read
jnz load ! Read next chunk of secondary boot code
done:
! Call secondary boot, assuming a long a.out header (48 bytes). The a.out
! header is usually short (32 bytes), but secondary boot has two entry points:
! One at offset 0 for the long, and one at offset 16 for the short header.
! Parameters passed in registers are:
!
! dl = Boot-device.
! es:si = Partition table entry if hard disk.
!
pop si ! Restore es:si = partition table entry
pop es ! dl is still loaded
jmpf BOOTOFF, BOOTSEG ! jmp to sec. boot (skipping header).
! Read error: print message, hang forever
error:
! mov si, #LOADOFF+errno+1 ! Uncomment this at disaster time
!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
mov si, #LOADOFF+rderr ! String to print
print: lodsb ! al = *si++ is char to be printed
movb ah, #14 ! 14 = print char
mov bx, #0x0001 ! Page 0, foreground color
int 0x10 ! Call BIOS VIDEO_IO
cmp si, #LOADOFF+errend ! End of string reached?
jb print
! Hang forever waiting for CTRL-ALT-DEL
hang: jmp hang
.data
rderr: .ascii "Read error "
!errno: .ascii "00 "
errend:
parameters:
! Floppy disk parameters sorted down by sectors per track. (The format gap
! length params of the 3.5" disks might be wrong, but that won't matter.)
! 1.44M 3.5"
dshd3: .data1 0xAF, 0x02, 25, 2, 18, 0x1B, 0xFF, 0x54, 0xF6, 15, 8
! 1.2M 5.25"
dshd5: .data1 0xDF, 0x02, 25, 2, 15, 0x1B, 0xFF, 0x54, 0xF6, 15, 8
! 720K 3.5", also used for 360K 5.25"
dsdd3: .data1 0xDF, 0x02, 25, 2, 9, 0x2A, 0xFF, 0x50, 0xF6, 15, 8
! Just for completeness, here are the real 360K params.
!dsdd5: .data1 0xDF, 0x02, 25, 2, 9, 0x2A, 0xFF, 0x54, 0xF6, 15, 3
.text
endtext:
.data
enddata:
addresses:
! The space below this is for disk addresses for a 62K secondary boot
! program (worst case, i.e. file is fragmented). It should be enough.
.bss
endbss: