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- Don't blindly assume that there are 8 hard drives installed. Instead,

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use the BIOS Equipment List to determine how many hard drives are
  installed and if the drive number we received in %dl is valid.
- Don't bother to disable interrupts when setting up the stack.  The 8086
  and beyond implicitly disable interrupts after an instruction that sets
  %ss (for example, a pop or a mov) so that you can safely set %ss and %sp
  in two consecutive instructions.  An exception to this is the lss
  instruction, which can set both registers simultaneously and thus doesn't
  need this hack.
- Add support for EDD BIOS extensions to support booting off of hard drives
  of nearly arbitrary length.
This commit is contained in:
John Baldwin 2000-06-27 20:04:10 +00:00
parent 28445eef28
commit bce816b9cc
1 changed files with 37 additions and 10 deletions
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47
sys/boot/i386/mbr/mbr.s
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@ -22,7 +22,7 @@
.set PT_OFF,0x1be # Partition table
.set MAGIC,0xaa55 # Magic: bootable
.set NDRIVE,0x8 # Drives to support
.set NHRDRV,0x475 # Number of hard drives
.globl start # Entry point
.code16
@ -34,10 +34,8 @@ start: cld # String ops inc
xorw %ax,%ax # Zero
movw %ax,%es # Address
movw %ax,%ds # data
cli # Disable interrupts
movw %ax,%ss # Set up
movw $LOAD,%sp # stack
sti # Enable interrupts
#
# Relocate ourself to a lower address so that we are out of the way when
# we load in the bootstrap from the partition to boot.
@ -73,23 +71,52 @@ main.2: addb $0x10,%bl # Till
int $0x18 # BIOS: Diskless boot
#
# Ok, we've found a possible active partition. Check to see that the drive
# is a valid hard drive number. XXX - We assume that there are up to 8 valid
# hard drives, regardless of how many are actually installed. Yuck.
# is a valid hard drive number.
#
main.3: cmpb $0x80,%dl # Drive valid?
jb main.4 # No
cmpb $0x80+NDRIVE,%dl # Within range?
movb NHRDRV,%dh # Calculate the highest
addb $0x80,%dh # drive number available
cmpb %dh,%dl # Within range?
jb main.5 # Yes
main.4: movb (%si),%dl # Load drive
#
# Ok, now that we have a valid drive and partition entry, load the CHS from
# the partition entry and read the sector from the disk.
#
main.4: movb (%si),%dl # Load drive
main.5: movb 0x1(%si),%dh # Load head
main.5: movw %sp,%di # Save stack pointer
movb 0x1(%si),%dh # Load head
movw 0x2(%si),%cx # Load cylinder:sector
movw $LOAD,%bx # Transfer buffer
movw $0x201,%ax # BIOS: Read from
int $0x13 # disk
cmpb $0xff,%dh # Might we need to use LBA?
jnz main.7 # No.
cmpw $0xffff,%cx # Do we need to use LBA?
jnz main.7 # No.
pushw %cx # Save %cx
pushw %bx # Save %bx
movw $0x55aa,%bx # Magic
movb $0x41,%ah # BIOS: EDD extensions
int $0x13 # present?
jc main.6 # No.
cmpw $0xaa55,%bx # Magic ok?
jne main.6 # No.
testb $0x1,%cl # Packet mode present?
jz main.6 # No.
popw %bx # Restore %bx
pushl $0x0 # Set the LBA
pushl 0x8(%si) # address
pushw %es # Set the address of
pushw %bx # the transfer buffer
pushw $0x1 # Read 1 sector
pushw $0x10 # Packet length
movw %sp,%si # Packer pointer
movw $0x4200,%ax # BIOS: LBA Read from disk
jmp main.8 # Skip the CHS setup
main.6: popw %bx # Restore %bx
popw %cx # Restore %cx
main.7: movw $0x201,%ax # BIOS: Read from disk
main.8: int $0x13 # Call the BIOS
movw %di,%sp # Restore stack
jc err_rd # If error
#
# Now that we've loaded the bootstrap, check for the 0xaa55 signature. If it