minix/boot/bootblock/bootblock.s

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#
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! Bootblock 1.5 - Minix boot block. Author: Kees J. Bot
! 21 Dec 1991
!
! When the PC is powered on, it will try to read the first sector of floppy
! disk 0 at 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 at 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 /boot
! from the boot device into memory at address 0x10000 and execute that. The
! disk addresses for /boot are patched into this code by installboot as 24-bit
! sector numbers and 8-bit sector counts above enddata upwards. /boot is in
! turn smart enough to load the different parts of the Minix kernel into
! memory and execute them to finally get Minix started.
!
LOADOFF = 0x7C00 ! 0x0000:LOADOFF is where this code is loaded
BOOTSEG = 0x1000 ! Secondary boot code segment.
#ifdef CDBOOT
BOOTOFF = 0x0050 ! Offset into /boot above header
#else
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BOOTOFF = 0x0030 ! Offset into /boot above header
#endif
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BUFFER = 0x0600 ! First free memory
#ifndef CDBOOT /* just constants, but make no sense for CDs */
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LOWSEC = 8 ! Offset of logical first sector in partition
! table
! Variables addressed using bp register
lowsec = 2 ! Offset of boot partition within drive
secpcyl = 6 ! Sectors per cylinder = heads * sectors
#endif
device = 0 ! The boot device
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.text
! 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+sectors ! char *di = sectors;
#ifndef CDBOOT
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testb dl, dl ! Winchester disks if dl >= 0x80
jge floppy
#endif
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winchester:
#ifndef CDBOOT
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! 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 partitions first sector
mov lowsec+2(bp), es ! High 16 bits of partitions first sector
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! Get the drive parameters, the number of sectors is bluntly written into the
! floppy disk sectors/track array.
movb ah, #0x08 ! Code for drive parameters
int 0x13 ! dl still contains drive
andb cl, #0x3F ! cl = max sector number (1-origin)
movb (di), cl ! Number of sectors per track
incb dh ! dh = 1 + max head number (0-origin)
#endif
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jmp loadboot
#ifndef CDBOOT
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! 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).
next: inc di ! Next number of sectors per track
floppy: xorb ah, ah ! Reset drive
int 0x13
movb cl, (di) ! cl = number of last sector on track
cmpb cl, #9 ! No need to do the last 720K/360K test
je 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
jc next ! Error, try the next floppy type
success:movb dh, #2 ! Load number of heads for multiply
#endif
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loadboot:
! Load /boot from the boot device
#ifndef CDBOOT
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movb al, (di) ! al = (di) = sectors per track
mulb dh ! dh = heads, ax = heads * sectors
mov secpcyl(bp), ax ! Sectors per cylinder = heads * sectors
#endif
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mov ax, #BOOTSEG ! Segment to load /boot 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 up to 8GB partition
xorb dh, dh ! dx:ax = sector within partition
#ifndef CDBOOT
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add ax, lowsec+0(bp)
adc dx, lowsec+2(bp)! dx:ax = sector within drive
cmp dx, #[1024*255*63-255]>>16 ! Near 8G limit?
jae bigdisk
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 (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, (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 ! Cant read past the end of a cylinder?
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movb al, (si) ! (si) < sectors left on this track
read: push ax ! Save al = sectors to read
movb ah, #0x02 ! Code for disk read (all registers in use now!)
int 0x13 ! Call the BIOS for a read
pop cx ! Restore al in cl
jmp rdeval
#endif
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bigdisk:
movb cl, (si) ! Number of sectors to read
push si ! Save si
mov si, #LOADOFF+ext_rw ! si = extended read/write parameter packet
movb 2(si), cl ! Fill in # blocks to transfer
mov 4(si), bx ! Buffer address
mov 8(si), ax ! Starting block number = dx:ax
mov 10(si), dx
movb dl, device(bp) ! dl = device to read
movb ah, #0x42 ! Extended read
int 0x13
pop si ! Restore si to point to the addresses array
!jmp rdeval
rdeval:
jc 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
#ifdef CDBOOT
addb bh, al ! For CDs, a sector is 2048 bytes, so
addb bh, al ! do this 6 more times to get byte count.
addb bh, al
addb bh, al
addb bh, al
addb bh, al
#endif
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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 /boot
done:
! Call /boot, assuming a long a.out header (48 bytes). The a.out header is
! usually short (32 bytes), but to be sure /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
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
testb al, al ! Null byte marks end
hang: jz hang ! Hang forever waiting for CTRL-ALT-DEL
movb ah, #0x0E ! Print character in teletype mode
mov bx, #0x0001 ! Page 0, foreground color
int 0x10 ! Call BIOS VIDEO_IO
jmp print
.data
rderr: .ascii "Read error "
errno: .ascii "00 \0"
errend:
! Floppy disk sectors per track for the 1.44M, 1.2M and 360K/720K types:
sectors:
.data1 18, 15, 9
! Extended read/write commands require a parameter packet.
ext_rw:
.data1 0x10 ! Length of extended r/w packet
.data1 0 ! Reserved
.data2 0 ! Blocks to transfer (to be filled in)
.data2 0 ! Buffer address offset (tbfi)
.data2 BOOTSEG ! Buffer address segment
.data4 0 ! Starting block number low 32 bits (tbfi)
.data4 0 ! Starting block number high 32 bits
.align 2
addresses:
! The space below this is for disk addresses for a 38K /boot program (worst
! case, i.e. file is completely fragmented). It should be enough.