source: trunk/minix/boot/bootblock.s@ 12

!       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.
        BOOTOFF    =    0x0030  ! Offset into /boot above header
        BUFFER     =    0x0600  ! First free memory
        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

.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;

        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 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)
        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).

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

loadboot:
! Load /boot from the boot device

        movb    al, (di)        ! al = (di) = 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 /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
        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            ! 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, #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
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
        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.