boot/jumpboot.s

   1 !       jumpboot 1.0 - Jump to another bootstrap        Author: Kees J. Bot
   2 !                                                               14 Apr 1999
   3 !
   4 ! This code may be placed into any free boot sector, like the first sector
   5 ! of an extended partition, a file system partition other than the root,
   6 ! or even the master bootstrap.  It will load and run another bootstrap whose
   7 ! disk, partition, and slice number (not necessarily all three) are patched
   8 ! into this code by installboot.  If the ALT key is held down when this code
   9 ! is booted then you can type the disk, partition, and slice numbers manually.
  10 ! The manual interface is default if no numbers are patched in by installboot.
  11 !
  12
  13         o32        =      0x66  ! This assembler doesn't know 386 extensions
  14         LOADOFF    =    0x7C00  ! 0x0000:LOADOFF is where this code is loaded
  15         BUFFER     =    0x0600  ! First free memory
  16         PART_TABLE =       446  ! Location of partition table within master
  17         PENTRYSIZE =        16  ! Size of one partition table entry
  18         MAGIC      =       510  ! Location of the AA55 magic number
  19
  20         ! <ibm/partition.h>:
  21         MINIX_PART =      0x81
  22         sysind     =         4
  23         lowsec     =         8
  24
  25
  26 .text
  27
  28 ! Find and load another bootstrap and jump to it.
  29 jumpboot:
  30         xor     ax, ax
  31         mov     ds, ax
  32         mov     es, ax
  33         cli
  34         mov     ss, ax                  ! ds = es = ss = Vector segment
  35         mov     sp, #LOADOFF
  36         sti
  37
  38 ! Move this code to safety, then jump to it.
  39         mov     si, sp                  ! si = start of this code
  40         mov     di, #BUFFER             ! di = Buffer area
  41         mov     cx, #512/2              ! One sector
  42         cld
  43   rep   movs
  44         jmpf    BUFFER+migrate, 0       ! To safety
  45 migrate:
  46
  47         mov     bp, #BUFFER+guide       ! Patched guiding characters
  48 altkey:
  49         movb    ah, #0x02               ! Keyboard shift status
  50         int     0x16
  51         testb   al, #0x08               ! Bit 3 = ALT key
  52         jz      noalt                   ! ALT key pressed?
  53 again:
  54         mov     bp, #zero               ! Ignore patched stuff
  55 noalt:
  56
  57 ! Follow guide characters to find the boot partition.
  58         call    print
  59         .ascii  "d?\b\0"                ! Initial greeting
  60
  61 ! Disk number?
  62 disk:
  63         movb    dl, #0x80 - 0x30        ! Prepare to add an ASCII digit
  64         call    getch                   ! Get number to tell which disk
  65         addb    dl, al                  ! dl = 0x80 + (al - '0')
  66         jns     n0nboot                 ! Result should be >= 0x80
  67         mov     si, #BUFFER+zero-lowsec ! si = where lowsec(si) is zero
  68         cmpb    (bp), #0x23             ! Next guide character is '#'?
  69         jne     notlogical
  70         lea     si, 1-lowsec(bp)        ! Logical sector offset follows '#'
  71 notlogical:
  72         call    load                    ! Load chosen sector of chosen disk
  73         cmpb    (bp), #0x23
  74         je      boot                    ! Run bootstrap if a logical is chosen
  75
  76         call    print                   ! Intro to partition number
  77         .ascii  "p?\b\0"
  78
  79 part:
  80         call    getch                   ! Get character to tell partition
  81         call    gettable                ! Get partition table
  82         call    sort                    ! Sort partition table
  83         call    choose_load             ! Compute chosen entry and load
  84
  85         cmpb    sysind(si), #MINIX_PART ! Minix subpartition table possible?
  86         jne     waitboot
  87
  88         call    print                   ! Intro to slice number
  89         .ascii  "s?\b\0"
  90
  91 slice:
  92         call    getch                   ! Get character to tell slice
  93         call    gettable                ! Get partition table
  94         call    choose_load             ! Compute chosen entry and load
  95
  96 waitboot:
  97         call    print                   ! Intro to nothing
  98         .ascii  " ?\b\0"
  99         call    getch                   ! Supposed to type RETURN now
 100 n0nboot:jmp     nonboot                 ! Sorry, can't go further
 101
 102 ! Get a character, either the patched-in, or one from the keyboard.
 103 getch:
 104         movb    al, (bp)        ! Get patched-in character
 105         testb   al, al
 106         jz      getkey
 107         inc     bp
 108         jmp     gotkey
 109 getkey: xorb    ah, ah          ! Wait for keypress
 110         int     0x16
 111 gotkey: testb   dl, dl          ! Ignore CR if disk number not yet set
 112         jns     putch
 113         cmpb    al, #0x0D       ! Carriage return?
 114         je      boot
 115         !jmp    putch
 116
 117 ! Print a character
 118 putch:  movb    ah, #0x0E       ! Print character in teletype mode
 119         mov     bx, #0x0001     ! Page 0, foreground color
 120         int     0x10
 121         ret
 122
 123 ! Print a message.
 124 print:  mov     cx, si          ! Save si
 125         pop     si              ! si = String following 'call print'
 126 prnext: lodsb                   ! al = *si++ is char to be printed
 127         testb   al, al          ! Null marks end
 128         jz      prdone
 129         call    putch
 130         jmp     prnext
 131 prdone: xchg    si, cx          ! Restore si
 132         jmp     (cx)            ! Continue after the string
 133
 134 ! Return typed (or in patched data) means to run the bootstrap now in core!
 135 boot:
 136         call    print           ! Make line on screen look proper
 137         .ascii  "\b  \r\n\0"
 138         jmp     LOADOFF-BUFFER  ! Jump to LOADOFF
 139
 140 ! Compute address of chosen partition entry from choice al into si, then
 141 ! continue to load the boot sector of that partition.
 142 choose_load:
 143         subb    al, #0x30               ! al -= '0'
 144         cmpb    al, #4                  ! Only four partitions
 145         ja      n0nboot
 146         movb    ah, #PENTRYSIZE
 147         mulb    ah                      ! al *= PENTRYSIZE
 148         add     ax, #BUFFER+PART_TABLE
 149         mov     si, ax                  ! si = &part_table[al - '0']
 150         movb    al, sysind(si)          ! System indicator
 151         testb   al, al                  ! Unused partition?
 152         jz      n0nboot
 153         !jmp    load                    ! Continue to load boot sector
 154
 155 ! Load boot sector of the current partition.
 156 load:
 157         push    dx              ! Save drive code
 158         push    es              ! Next call sets es
 159         movb    ah, #0x08       ! Code for drive parameters
 160         int     0x13
 161         pop     es
 162         andb    cl, #0x3F       ! cl = max sector number (1-origin)
 163         incb    dh              ! dh = 1 + max head number (0-origin)
 164         movb    al, cl          ! al = cl = sectors per track
 165         mulb    dh              ! dh = heads, ax = heads * sectors
 166         mov     bx, ax          ! bx = sectors per cylinder = heads * sectors
 167         mov     ax, lowsec+0(si)
 168         mov     dx, lowsec+2(si) ! dx:ax = sector within drive
 169         cmp     dx, #[1024*255*63-255]>>16  ! Near 8G limit?
 170         jae     bigdisk
 171         div     bx              ! ax = cylinder, dx = sector within cylinder
 172         xchg    ax, dx          ! ax = sector within cylinder, dx = cylinder
 173         movb    ch, dl          ! ch = low 8 bits of cylinder
 174         divb    cl              ! al = head, ah = sector (0-origin)
 175         xorb    dl, dl          ! About to shift bits 8-9 of cylinder into dl
 176         shr     dx, #1
 177         shr     dx, #1          ! dl[6..7] = high cylinder
 178         orb     dl, ah          ! dl[0..5] = sector (0-origin)
 179         movb    cl, dl          ! cl[0..5] = sector, cl[6..7] = high cyl
 180         incb    cl              ! cl[0..5] = sector (1-origin)
 181         pop     dx              ! Restore drive code in dl
 182         movb    dh, al          ! dh = al = head
 183         mov     bx, #LOADOFF    ! es:bx = where sector is loaded
 184         mov     ax, #0x0201     ! ah = Code for read / al = one sector
 185         int     0x13
 186         jmp     rdeval          ! Evaluate read result
 187 bigdisk:
 188         mov     bx, dx          ! bx:ax = dx:ax = sector to read
 189         pop     dx              ! Restore drive code in dl
 190         push    si              ! Save si
 191         mov     si, #BUFFER+ext_rw ! si = extended read/write parameter packet
 192         mov     8(si), ax       ! Starting block number = bx:ax
 193         mov     10(si), bx
 194         movb    ah, #0x42       ! Extended read
 195         int     0x13
 196         pop     si              ! Restore si to point to partition entry
 197         !jmp    rdeval
 198 rdeval:
 199         jnc     rdok
 200 rderr:
 201         call    print
 202         .ascii  "\r\nRead error\r\n\0"
 203         jmp     again
 204 rdok:
 205         cmp     LOADOFF+MAGIC, #0xAA55
 206         je      sigok           ! Signature ok?
 207 nonboot:
 208         call    print
 209         .ascii  "\r\nNot bootable\r\n\0"
 210         jmp     again
 211 sigok:
 212         ret
 213
 214 ! Get the partition table into my space.
 215 gettable:
 216         mov     si, #LOADOFF+PART_TABLE
 217         mov     di, #BUFFER+PART_TABLE
 218         mov     cx, #4*PENTRYSIZE/2
 219   rep   movs
 220         ret
 221
 222 ! Sort the partition table.
 223 sort:
 224         mov     cx, #4                  ! Four times is enough to sort
 225 bubble: mov     si, #BUFFER+PART_TABLE  ! First table entry
 226 bubble1:lea     di, PENTRYSIZE(si)      ! Next entry
 227         cmpb    sysind(si), ch          ! Partition type, nonzero when in use
 228         jz      exchg                   ! Unused entries sort to the end
 229 inuse:  mov     bx, lowsec+0(di)
 230         sub     bx, lowsec+0(si)        ! Compute di->lowsec - si->lowsec
 231         mov     bx, lowsec+2(di)
 232         sbb     bx, lowsec+2(si)
 233         jae     order                   ! In order if si->lowsec <= di->lowsec
 234 exchg:  movb    bl, (si)
 235         xchgb   bl, PENTRYSIZE(si)      ! Exchange entries byte by byte
 236         movb    (si), bl
 237         inc     si
 238         cmp     si, di
 239         jb      exchg
 240 order:  mov     si, di
 241         cmp     si, #BUFFER+PART_TABLE+3*PENTRYSIZE
 242         jb      bubble1
 243         loop    bubble
 244         ret
 245
 246 .data
 247
 248 ! Extended read/write commands require a parameter packet.
 249 ext_rw:
 250         .data1  0x10            ! Length of extended r/w packet
 251         .data1  0               ! Reserved
 252         .data2  1               ! Blocks to transfer (just one)
 253         .data2  LOADOFF         ! Buffer address offset
 254         .data2  0               ! Buffer address segment
 255         .data4  0               ! Starting block number low 32 bits (tbfi)
 256 zero:   .data4  0               ! Starting block number high 32 bits
 257
 258         .align  2
 259 guide:
 260 ! Guide characters and possibly a logical partition number patched here by
 261 ! installboot, up to 6 bytes maximum.