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jhb 2000-06-26 23:58:04 +00:00
parent 9fd6a32919
commit 4e0ad3c333
1 changed files with 36 additions and 5 deletions
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41
sys/boot/i386/mbr/mbr.s
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@ -15,7 +15,7 @@
# $FreeBSD$
# Master boot record
# A 512 byte MBR boot manager that simply boots the active partition.
.set LOAD,0x7c00 # Load address
.set EXEC,0x600 # Execution address
@ -27,6 +27,9 @@
.globl start # Entry point
.code16
#
# Setup the segment registers for flat addressing and setup the stack.
#
start: cld # String ops inc
xorw %ax,%ax # Zero
movw %ax,%es # Address
@ -35,13 +38,25 @@ start: cld # String ops inc
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.
#
movw $main-EXEC+LOAD,%si # Source
movw $main,%di # Destination
movw $0x200-(main-start),%cx # Byte count
rep # Relocate
movsb # code
#
# Jump to the relocated code.
#
jmp main-LOAD+EXEC # To relocated code
#
# Scan the partition table looking for an active entry. Note that %ch is
# zero from the repeated string instruction above. We save the offset of
# the active partition in %si and scan the entire table to ensure that only
# one partition is marked active.
#
main: xorw %si,%si # No active partition
movw $partbl,%bx # Partition table
movb $0x4,%cl # Number of entries
@ -56,11 +71,19 @@ main.2: addb $0x10,%bl # Till
testw %si,%si # Active found?
jnz main.3 # Yes
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.
#
main.3: cmpb $0x80,%dl # Drive valid?
jb main.4 # No
cmpb $0x80+NDRIVE,%dl # Within range?
jb main.5 # Yes
#
# 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
movw 0x2(%si),%cx # Load cylinder:sector
@ -68,10 +91,16 @@ main.5: movb 0x1(%si),%dh # Load head
movw $0x201,%ax # BIOS: Read from
int $0x13 # disk
jc err_rd # If error
#
# Now that we've loaded the bootstrap, check for the 0xaa55 signature. If it
# is present, execute the bootstrap we just loaded.
#
cmpw $MAGIC,0x1fe(%bx) # Bootable?
jne err_os # No
jmp *%bx # Invoke bootstrap
#
# Various error message entry points.
#
err_pt: movw $msg_pt,%si # "Invalid partition
jmp putstr # table"
@ -80,7 +109,9 @@ err_rd: movw $msg_rd,%si # "Error loading
err_os: movw $msg_os,%si # "Missing operating
jmp putstr # system"
#
# Output an ASCIZ string to the console via the BIOS.
#
putstr.0: movw $0x7,%bx # Page:attribute
movb $0xe,%ah # BIOS: Display
int $0x10 # character