boot/bootblock.s

   1 !       Bootblock 1.5 - Minix boot block.               Author: Kees J. Bot
   2 !                                                               21 Dec 1991
   3 !
   4 ! When the PC is powered on, it will try to read the first sector of floppy
   5 ! disk 0 at address 0x7C00.  If this fails due to the absence of flexible
   6 ! magnetic media, it will read the master boot record from the first sector
   7 ! of the hard disk.  This sector not only contains executable code, but also
   8 ! the partition table of the hard disk.  When executed, it will select the
   9 ! active partition and load the first sector of that at address 0x7C00.
  10 ! This file contains the code that is eventually read from either the floppy
  11 ! disk, or the hard disk partition.  It is just smart enough to load /boot
  12 ! from the boot device into memory at address 0x10000 and execute that.  The
  13 ! disk addresses for /boot are patched into this code by installboot as 24-bit
  14 ! sector numbers and 8-bit sector counts above enddata upwards.  /boot is in
  15 ! turn smart enough to load the different parts of the Minix kernel into
  16 ! memory and execute them to finally get Minix started.
  17 !
  18
  19         LOADOFF    =    0x7C00  ! 0x0000:LOADOFF is where this code is loaded
  20         BOOTSEG    =    0x1000  ! Secondary boot code segment.
  21         BOOTOFF    =    0x0030  ! Offset into /boot above header
  22         BUFFER     =    0x0600  ! First free memory
  23         LOWSEC     =         8  ! Offset of logical first sector in partition
  24                                 ! table
  25
  26         ! Variables addressed using bp register
  27         device     =         0  ! The boot device
  28         lowsec     =         2  ! Offset of boot partition within drive
  29         secpcyl    =         6  ! Sectors per cylinder = heads * sectors
  30
  31 .text
  32
  33 ! Start boot procedure.
  34
  35 boot:
  36         xor     ax, ax          ! ax = 0x0000, the vector segment
  37         mov     ds, ax
  38         cli                     ! Ignore interrupts while setting stack
  39         mov     ss, ax          ! ss = ds = vector segment
  40         mov     sp, #LOADOFF    ! Usual place for a bootstrap stack
  41         sti
  42
  43         push    ax
  44         push    ax              ! Push a zero lowsec(bp)
  45
  46         push    dx              ! Boot device in dl will be device(bp)
  47         mov     bp, sp          ! Using var(bp) is one byte cheaper then var.
  48
  49         push    es
  50         push    si              ! es:si = partition table entry if hard disk
  51
  52         mov     di, #LOADOFF+sectors    ! char *di = sectors;
  53
  54         testb   dl, dl          ! Winchester disks if dl >= 0x80
  55         jge     floppy
  56
  57 winchester:
  58
  59 ! Get the offset of the first sector of the boot partition from the partition
  60 ! table.  The table is found at es:si, the lowsec parameter at offset LOWSEC.
  61
  62         eseg
  63         les     ax, LOWSEC(si)    ! es:ax = LOWSEC+2(si):LOWSEC(si)
  64         mov     lowsec+0(bp), ax  ! Low 16 bits of partition's first sector
  65         mov     lowsec+2(bp), es  ! High 16 bits of partition's first sector
  66
  67 ! Get the drive parameters, the number of sectors is bluntly written into the
  68 ! floppy disk sectors/track array.
  69
  70         movb    ah, #0x08       ! Code for drive parameters
  71         int     0x13            ! dl still contains drive
  72         andb    cl, #0x3F       ! cl = max sector number (1-origin)
  73         movb    (di), cl        ! Number of sectors per track
  74         incb    dh              ! dh = 1 + max head number (0-origin)
  75         jmp     loadboot
  76
  77 ! Floppy:
  78 ! Execute three read tests to determine the drive type.  Test for each floppy
  79 ! type by reading the last sector on the first track.  If it fails, try a type
  80 ! that has less sectors.  Therefore we start with 1.44M (18 sectors) then 1.2M
  81 ! (15 sectors) ending with 720K/360K (both 9 sectors).
  82
  83 next:   inc     di              ! Next number of sectors per track
  84
  85 floppy: xorb    ah, ah          ! Reset drive
  86         int     0x13
  87
  88         movb    cl, (di)        ! cl = number of last sector on track
  89
  90         cmpb    cl, #9          ! No need to do the last 720K/360K test
  91         je      success
  92
  93 ! Try to read the last sector on track 0
  94
  95         mov     es, lowsec(bp)  ! es = vector segment (lowsec = 0)
  96         mov     bx, #BUFFER     ! es:bx buffer = 0x0000:0x0600
  97         mov     ax, #0x0201     ! Read sector, #sectors = 1
  98         xorb    ch, ch          ! Track 0, last sector
  99         xorb    dh, dh          ! Drive dl, head 0
 100         int     0x13
 101         jc      next            ! Error, try the next floppy type
 102
 103 success:movb    dh, #2          ! Load number of heads for multiply
 104
 105 loadboot:
 106 ! Load /boot from the boot device
 107
 108         movb    al, (di)        ! al = (di) = sectors per track
 109         mulb    dh              ! dh = heads, ax = heads * sectors
 110         mov     secpcyl(bp), ax ! Sectors per cylinder = heads * sectors
 111
 112         mov     ax, #BOOTSEG    ! Segment to load /boot into
 113         mov     es, ax
 114         xor     bx, bx          ! Load first sector at es:bx = BOOTSEG:0x0000
 115         mov     si, #LOADOFF+addresses  ! Start of the boot code addresses
 116 load:
 117         mov     ax, 1(si)       ! Get next sector number: low 16 bits
 118         movb    dl, 3(si)       ! Bits 16-23 for your up to 8GB partition
 119         xorb    dh, dh          ! dx:ax = sector within partition
 120         add     ax, lowsec+0(bp)
 121         adc     dx, lowsec+2(bp)! dx:ax = sector within drive
 122         cmp     dx, #[1024*255*63-255]>>16  ! Near 8G limit?
 123         jae     bigdisk
 124         div     secpcyl(bp)     ! ax = cylinder, dx = sector within cylinder
 125         xchg    ax, dx          ! ax = sector within cylinder, dx = cylinder
 126         movb    ch, dl          ! ch = low 8 bits of cylinder
 127         divb    (di)            ! al = head, ah = sector (0-origin)
 128         xorb    dl, dl          ! About to shift bits 8-9 of cylinder into dl
 129         shr     dx, #1
 130         shr     dx, #1          ! dl[6..7] = high cylinder
 131         orb     dl, ah          ! dl[0..5] = sector (0-origin)
 132         movb    cl, dl          ! cl[0..5] = sector, cl[6..7] = high cyl
 133         incb    cl              ! cl[0..5] = sector (1-origin)
 134         movb    dh, al          ! dh = al = head
 135         movb    dl, device(bp)  ! dl = device to read
 136         movb    al, (di)        ! Sectors per track - Sector number (0-origin)
 137         subb    al, ah          ! = Sectors left on this track
 138         cmpb    al, (si)        ! Compare with # sectors to read
 139         jbe     read            ! Can't read past the end of a cylinder?
 140         movb    al, (si)        ! (si) < sectors left on this track
 141 read:   push    ax              ! Save al = sectors to read
 142         movb    ah, #0x02       ! Code for disk read (all registers in use now!)
 143         int     0x13            ! Call the BIOS for a read
 144         pop     cx              ! Restore al in cl
 145         jmp     rdeval
 146 bigdisk:
 147         movb    cl, (si)        ! Number of sectors to read
 148         push    si              ! Save si
 149         mov     si, #LOADOFF+ext_rw ! si = extended read/write parameter packet
 150         movb    2(si), cl       ! Fill in # blocks to transfer
 151         mov     4(si), bx       ! Buffer address
 152         mov     8(si), ax       ! Starting block number = dx:ax
 153         mov     10(si), dx
 154         movb    dl, device(bp)  ! dl = device to read
 155         movb    ah, #0x42       ! Extended read
 156         int     0x13
 157         pop     si              ! Restore si to point to the addresses array
 158         !jmp    rdeval
 159 rdeval:
 160         jc      error           ! Jump on disk read error
 161         movb    al, cl          ! Restore al = sectors read
 162         addb    bh, al          ! bx += 2 * al * 256 (add bytes read)
 163         addb    bh, al          ! es:bx = where next sector must be read
 164         add     1(si), ax       ! Update address by sectors read
 165         adcb    3(si), ah       ! Don't forget bits 16-23 (add ah = 0)
 166         subb    (si), al        ! Decrement sector count by sectors read
 167         jnz     load            ! Not all sectors have been read
 168         add     si, #4          ! Next (address, count) pair
 169         cmpb    ah, (si)        ! Done when no sectors to read
 170         jnz     load            ! Read next chunk of /boot
 171
 172 done:
 173
 174 ! Call /boot, assuming a long a.out header (48 bytes).  The a.out header is
 175 ! usually short (32 bytes), but to be sure /boot has two entry points:
 176 ! One at offset 0 for the long, and one at offset 16 for the short header.
 177 ! Parameters passed in registers are:
 178 !
 179 !       dl      = Boot-device.
 180 !       es:si   = Partition table entry if hard disk.
 181 !
 182         pop     si              ! Restore es:si = partition table entry
 183         pop     es              ! dl is still loaded
 184         jmpf    BOOTOFF, BOOTSEG  ! jmp to sec. boot (skipping header).
 185
 186 ! Read error: print message, hang forever
 187 error:
 188         mov     si, #LOADOFF+errno+1
 189 prnum:  movb    al, ah          ! Error number in ah
 190         andb    al, #0x0F       ! Low 4 bits
 191         cmpb    al, #10         ! A-F?
 192         jb      digit           ! 0-9!
 193         addb    al, #7          ! 'A' - ':'
 194 digit:  addb    (si), al        ! Modify '0' in string
 195         dec     si
 196         movb    cl, #4          ! Next 4 bits
 197         shrb    ah, cl
 198         jnz     prnum           ! Again if digit > 0
 199
 200         mov     si, #LOADOFF+rderr  ! String to print
 201 print:  lodsb                   ! al = *si++ is char to be printed
 202         testb   al, al          ! Null byte marks end
 203 hang:   jz      hang            ! Hang forever waiting for CTRL-ALT-DEL
 204         movb    ah, #0x0E       ! Print character in teletype mode
 205         mov     bx, #0x0001     ! Page 0, foreground color
 206         int     0x10            ! Call BIOS VIDEO_IO
 207         jmp     print
 208
 209 .data
 210 rderr:  .ascii  "Read error "
 211 errno:  .ascii  "00 \0"
 212 errend:
 213
 214 ! Floppy disk sectors per track for the 1.44M, 1.2M and 360K/720K types:
 215 sectors:
 216         .data1  18, 15, 9
 217
 218 ! Extended read/write commands require a parameter packet.
 219 ext_rw:
 220         .data1  0x10            ! Length of extended r/w packet
 221         .data1  0               ! Reserved
 222         .data2  0               ! Blocks to transfer (to be filled in)
 223         .data2  0               ! Buffer address offset (tbfi)
 224         .data2  BOOTSEG         ! Buffer address segment
 225         .data4  0               ! Starting block number low 32 bits (tbfi)
 226         .data4  0               ! Starting block number high 32 bits
 227
 228         .align  2
 229 addresses:
 230 ! The space below this is for disk addresses for a 38K /boot program (worst
 231 ! case, i.e. file is completely fragmented).  It should be enough.