boot/masterboot.s

   1 !       masterboot 2.0 - Master boot block code         Author: Kees J. Bot
   2 !
   3 ! This code may be placed in the first sector (the boot sector) of a floppy,
   4 ! hard disk or hard disk primary partition.  There it will perform the
   5 ! following actions at boot time:
   6 !
   7 ! - If the booted device is a hard disk and one of the partitions is active
   8 !   then the active partition is booted.
   9 !
  10 ! - Otherwise the next floppy or hard disk device is booted, trying them one
  11 !   by one.
  12 !
  13 ! To make things a little clearer, the boot path might be:
  14 !       /dev/fd0        - Floppy disk containing data, tries fd1 then d0
  15 !       [/dev/fd1]      - Drive empty
  16 !       /dev/c0d0       - Master boot block, selects active partition 2
  17 !       /dev/c0d0p2     - Submaster, selects active subpartition 0
  18 !       /dev/c0d0p2s0   - Minix bootblock, reads Boot Monitor /boot
  19 !       Minix           - Started by /boot from a kernel image in /minix
  20
  21         LOADOFF    =    0x7C00  ! 0x0000:LOADOFF is where this code is loaded
  22         BUFFER     =    0x0600  ! First free memory
  23         PART_TABLE =       446  ! Location of partition table within this code
  24         PENTRYSIZE =        16  ! Size of one partition table entry
  25         MAGIC      =       510  ! Location of the AA55 magic number
  26
  27         ! <ibm/partition>.h:
  28         bootind    =         0
  29         sysind     =         4
  30         lowsec     =         8
  31
  32
  33 .text
  34
  35 ! Find active (sub)partition, load its first sector, run it.
  36
  37 master:
  38         xor     ax, ax
  39         mov     ds, ax
  40         mov     es, ax
  41         cli
  42         mov     ss, ax                  ! ds = es = ss = Vector segment
  43         mov     sp, #LOADOFF
  44         sti
  45
  46 ! Copy this code to safety, then jump to it.
  47         mov     si, sp                  ! si = start of this code
  48         push    si                      ! Also where we'll return to eventually
  49         mov     di, #BUFFER             ! Buffer area
  50         mov     cx, #512/2              ! One sector
  51         cld
  52   rep   movs
  53         jmpf    BUFFER+migrate, 0       ! To safety
  54 migrate:
  55
  56 ! Find the active partition
  57 findactive:
  58         testb   dl, dl
  59         jns     nextdisk                ! No bootable partitions on floppies
  60         mov     si, #BUFFER+PART_TABLE
  61 find:   cmpb    sysind(si), #0          ! Partition type, nonzero when in use
  62         jz      nextpart
  63         testb   bootind(si), #0x80      ! Active partition flag in bit 7
  64         jz      nextpart                ! It's not active
  65 loadpart:
  66         call    load                    ! Load partition bootstrap
  67         jc      error1                  ! Not supposed to fail
  68 bootstrap:
  69         ret                             ! Jump to the master bootstrap
  70 nextpart:
  71         add     si, #PENTRYSIZE
  72         cmp     si, #BUFFER+PART_TABLE+4*PENTRYSIZE
  73         jb      find
  74 ! No active partition, tell 'em
  75         call    print
  76         .ascii  "No active partition\0"
  77         jmp     reboot
  78
  79 ! There are no active partitions on this drive, try the next drive.
  80 nextdisk:
  81         incb    dl                      ! Increment dl for the next drive
  82         testb   dl, dl
  83         js      nexthd                  ! Hard disk if negative
  84         int     0x11                    ! Get equipment configuration
  85         shl     ax, #1                  ! Highest floppy drive # in bits 6-7
  86         shl     ax, #1                  ! Now in bits 0-1 of ah
  87         andb    ah, #0x03               ! Extract bits
  88         cmpb    dl, ah                  ! Must be dl <= ah for drive to exist
  89         ja      nextdisk                ! Otherwise try disk 0 eventually
  90         call    load0                   ! Read the next floppy bootstrap
  91         jc      nextdisk                ! It failed, next disk please
  92         ret                             ! Jump to the next master bootstrap
  93 nexthd: call    load0                   ! Read the hard disk bootstrap
  94 error1: jc      error                   ! No disk?
  95         ret
  96
  97
  98 ! Load sector 0 from the current device.  It's either a floppy bootstrap or
  99 ! a hard disk master bootstrap.
 100 load0:
 101         mov     si, #BUFFER+zero-lowsec ! si = where lowsec(si) is zero
 102         !jmp    load
 103
 104 ! Load sector lowsec(si) from the current device.  The obvious head, sector,
 105 ! and cylinder numbers are ignored in favour of the more trustworthy absolute
 106 ! start of partition.
 107 load:
 108         mov     di, #3          ! Three retries for floppy spinup
 109 retry:  push    dx              ! Save drive code
 110         push    es
 111         push    di              ! Next call destroys es and di
 112         movb    ah, #0x08       ! Code for drive parameters
 113         int     0x13
 114         pop     di
 115         pop     es
 116         andb    cl, #0x3F       ! cl = max sector number (1-origin)
 117         incb    dh              ! dh = 1 + max head number (0-origin)
 118         movb    al, cl          ! al = cl = sectors per track
 119         mulb    dh              ! dh = heads, ax = heads * sectors
 120         mov     bx, ax          ! bx = sectors per cylinder = heads * sectors
 121         mov     ax, lowsec+0(si)
 122         mov     dx, lowsec+2(si)! dx:ax = sector within drive
 123         cmp     dx, #[1024*255*63-255]>>16  ! Near 8G limit?
 124         jae     bigdisk
 125         div     bx              ! ax = cylinder, dx = sector within cylinder
 126         xchg    ax, dx          ! ax = sector within cylinder, dx = cylinder
 127         movb    ch, dl          ! ch = low 8 bits of cylinder
 128         divb    cl              ! al = head, ah = sector (0-origin)
 129         xorb    dl, dl          ! About to shift bits 8-9 of cylinder into dl
 130         shr     dx, #1
 131         shr     dx, #1          ! dl[6..7] = high cylinder
 132         orb     dl, ah          ! dl[0..5] = sector (0-origin)
 133         movb    cl, dl          ! cl[0..5] = sector, cl[6..7] = high cyl
 134         incb    cl              ! cl[0..5] = sector (1-origin)
 135         pop     dx              ! Restore drive code in dl
 136         movb    dh, al          ! dh = al = head
 137         mov     bx, #LOADOFF    ! es:bx = where sector is loaded
 138         mov     ax, #0x0201     ! Code for read, just one sector
 139         int     0x13            ! Call the BIOS for a read
 140         jmp     rdeval          ! Evaluate read result
 141 bigdisk:
 142         mov     bx, dx          ! bx:ax = dx:ax = sector to read
 143         pop     dx              ! Restore drive code in dl
 144         push    si              ! Save si
 145         mov     si, #BUFFER+ext_rw ! si = extended read/write parameter packet
 146         mov     8(si), ax       ! Starting block number = bx:ax
 147         mov     10(si), bx
 148         movb    ah, #0x42       ! Extended read
 149         int     0x13
 150         pop     si              ! Restore si to point to partition entry
 151         !jmp    rdeval
 152 rdeval:
 153         jnc     rdok            ! Read succeeded
 154         cmpb    ah, #0x80       ! Disk timed out?  (Floppy drive empty)
 155         je      rdbad
 156         dec     di
 157         jl      rdbad           ! Retry count expired
 158         xorb    ah, ah
 159         int     0x13            ! Reset
 160         jnc     retry           ! Try again
 161 rdbad:  stc                     ! Set carry flag
 162         ret
 163 rdok:   cmp     LOADOFF+MAGIC, #0xAA55
 164         jne     nosig           ! Error if signature wrong
 165         ret                     ! Return with carry still clear
 166 nosig:  call    print
 167         .ascii  "Not bootable\0"
 168         jmp     reboot
 169
 170 ! A read error occurred, complain and hang
 171 error:
 172         mov     si, #LOADOFF+errno+1
 173 prnum:  movb    al, ah          ! Error number in ah
 174         andb    al, #0x0F       ! Low 4 bits
 175         cmpb    al, #10         ! A-F?
 176         jb      digit           ! 0-9!
 177         addb    al, #7          ! 'A' - ':'
 178 digit:  addb    (si), al        ! Modify '0' in string
 179         dec     si
 180         movb    cl, #4          ! Next 4 bits
 181         shrb    ah, cl
 182         jnz     prnum           ! Again if digit > 0
 183         call    print
 184         .ascii  "Read error "
 185 errno:  .ascii  "00\0"
 186         !jmp    reboot
 187
 188 reboot:
 189         call    print
 190         .ascii  ".  Hit any key to reboot.\0"
 191         xorb    ah, ah          ! Wait for keypress
 192         int     0x16
 193         call    print
 194         .ascii  "\r\n\0"
 195         int     0x19
 196
 197 ! Print a message.
 198 print:  pop     si              ! si = String following 'call print'
 199 prnext: lodsb                   ! al = *si++ is char to be printed
 200         testb   al, al          ! Null marks end
 201         jz      prdone
 202         movb    ah, #0x0E       ! Print character in teletype mode
 203         mov     bx, #0x0001     ! Page 0, foreground color
 204         int     0x10
 205         jmp     prnext
 206 prdone: jmp     (si)            ! Continue after the string
 207
 208 .data
 209
 210 ! Extended read/write commands require a parameter packet.
 211 ext_rw:
 212         .data1  0x10            ! Length of extended r/w packet
 213         .data1  0               ! Reserved
 214         .data2  1               ! Blocks to transfer (just one)
 215         .data2  LOADOFF         ! Buffer address offset
 216         .data2  0               ! Buffer address segment
 217         .data4  0               ! Starting block number low 32 bits (tbfi)
 218 zero:   .data4  0               ! Starting block number high 32 bits