bootblock-1.4 源代码

! Bootblock 1.4 - 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 the

! secondary boot code from the boot device into memory at address 0x10000 and

! execute that. The disk addresses for this secondary boot code are patched

! into this code by installboot as 24-bit sector numbers and 8-bit sector

! counts above enddata upwards. The secondary boot code 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 secondary 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 the secondary boot code 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 secondary boot code 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 8GB disk

xorb dh, dh ! dx:ax = sector within partition

add ax, lowsec+0(bp)

adc dx, lowsec+2(bp)! dx:ax = sector within drive

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, #2 ! Code for disk read (all registers in use now!)

int 0x13 ! Call the BIOS for a read

pop cx ! Restore al in cl

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 secondary boot code

done:

! Call secondary boot, assuming a long a.out header (48 bytes). The a.out

! header is usually short (32 bytes), but secondary 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

movb ah, #0x0E ! Print character in teletype mode

mov bx, #0x0001 ! Page 0, foreground color

int 0x10 ! Call BIOS VIDEO_IO

cmp si, #LOADOFF+errend ! End of string reached?

jb print

! Hang forever waiting for CTRL-ALT-DEL

hang: jmp hang

.data

rderr: .ascii "Read error "

errno: .ascii "00 "

errend:

! Floppy disk sectors per track for the 1.44M, 1.2M and 360K/720K types:

sectors:

.data1 18, 15, 9

.align 2

addresses:

! The space below this is for disk addresses for a 66K secondary boot

! program (worst case, i.e. file is fragmented). It should be enough.

posted on 2011-03-07 14:24 The A 阅读(481) 评论(0) 编辑收藏引用所属分类: MINIX

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