blob: 708f0934c4277935629ad38bf7ec17e71f7c7ea7 [file] [log] [blame]
* Copyright (c) 2008 Luigi Rizzo (mostly documentation)
* Copyright (c) 2002 Bruce M. Simpson
* Copyright (c) 1998 Robert Nordier
* All rights reserved.
* Redistribution and use in source and binary forms are freely
* permitted provided that the above copyright notice and this
* paragraph and the following disclaimer are duplicated in all
* such forms.
* This software is provided "AS IS" and without any express or
* implied warranties, including, without limitation, the implied
* warranties of merchantability and fitness for a particular
* purpose.
* $FreeBSD$
/* build options: */
#ifdef SIO /* use serial console on COM1. */
#ifdef PXE /* enable PXE/INT18 booting with F6 */
#ifdef CHECK_DRIVE /* make sure we boot from a HD. */
#ifdef ONLY_F_KEYS /* Only F1..F6, no digits on console */
#ifdef VOLUME_SERIAL /* support Volume serial number */
#define B0_BASE 0x1ae /* move the internal data area */
#define B0_BASE 0x1b2
#ifdef TEST /* enable some test code */
* Note - this code uses many tricks to save space and fit in one sector.
* This includes using side effects of certain instructions, reusing
* register values from previous operations, etc.
* Be extremely careful when changing the code, even for simple things.
* This code implements a Master Boot Record (MBR) for an Intel/PC disk.
* It is 512 bytes long and it is normally loaded by the BIOS (or another
* bootloader) at 0:0x7c00. This code depends on %cs:%ip being 0:0x7c00
* The initial chunk of instructions is used as a signature by external
* tools (e.g. boot0cfg) which can manipulate the block itself.
* The area at offset 0x1b2 contains a magic string ('Drive '), also
* used as a signature to detect the block, and some variables that can
* be updated by boot0cfg (and optionally written back to the disk).
* These variables control the operation of the bootloader itself,
* e.g. which partitions to enable, the timeout, the use of LBA
* (called 'packet') or CHS mode, whether to force a drive number,
* and whether to write back the user's selection back to disk.
* As in every Master Boot Record, the partition table is at 0x1be,
* made of four 16-byte entries each containing:
* 0 1 status (0x80: bootable, 0: non bootable)
* 1 3 start sector CHS
* 8:head, 6:sector, 2:cyl bit 9..8, 8:cyl bit 7..0
* 4 1 partition type
* 5 3 end sector CHS
* 8 4 LBA of first sector
* 12 4 partition size in sectors
* and followed by the two bytes 0x55, 0xAA (MBR signature).
* On entry, the registers contain the following values:
* %cs:%ip 0:0x7c00
* %dl drive number (0x80, 0x81, ... )
* %si pointer to the partition table from which we were loaded.
* Some boot code (e.g. syslinux) use this info to relocate
* themselves, so we want to pass a valid one to the next stage.
* NOTE: the use of %si is not a standard.
* This boot block first relocates itself at a different address (0:0x600),
* to free the space at 0:0x7c00 for the next stage boot block.
* It then initializes some memory at 0:0x800 and above (pointed by %bp)
* to store the original drive number (%dl) passed to us, and to construct a
* fake partition entry. The latter is used by the disk I/O routine and,
* in some cases, passed in %si to the next stage boot code.
* The variables at 0x1b2 are accessed as negative offsets from %bp.
* After the relocation, the code scans the partition table printing
* out enabled partition or disks, and waits for user input.
* When a partition is selected, or a timeout expires, the currently
* selected partition is used to load the next stage boot code,
* %dl and %si are set appropriately as when we were called, and
* control is transferred to the newly loaded code at 0:0x7c00.
* NHRDRV is the address in segment 0 where the BIOS writes the
* total number of hard disks in the system.
* LOAD is the original load address and cannot be changed.
* ORIGIN is the relocation address. If you change it, you also need
* to change the value passed to the linker in the Makefile
* PRT_OFF is the location of the partition table (from the MBR standard).
* B0_OFF is the location of the data area, known to boot0cfg so
* it cannot be changed. Computed as a negative offset from 0x200
* MAGIC is the signature of a boot block.
.set NHRDRV,0x475 # Number of hard drives
.set ORIGIN,0x600 # Execution address
.set LOAD,0x7c00 # Load address
.set PRT_OFF,0x1be # Partition table
.set B0_OFF,(B0_BASE-0x200) # Offset of boot0 data
.set MAGIC,0xaa55 # Magic: bootable
.set KEY_ENTER,0x1c # Enter key scan code
.set KEY_F1,0x3b # F1 key scan code
.set KEY_1,0x02 # #1 key scan code
.set ASCII_BEL,'#' # ASCII code for <BEL>
.set ASCII_CR,0x0D # ASCII code for <CR>
* Offsets of variables in the block at B0_OFF, and in the volatile
* data area, computed as displacement from %bp.
* We need to define them as constant as the assembler cannot
* compute them in its single pass.
.set _NXTDRV, B0_OFF+6 # Next drive
.set _OPT, B0_OFF+7 # Default option
.set _SETDRV, B0_OFF+8 # Drive to force
.set _FLAGS, B0_OFF+9 # Flags
.set SETDRV, 0x20 # the 'setdrv' flag
.set NOUPDATE, 0x40 # the 'noupdate' flag
.set USEPACKET, 0x80 # the 'packet' flag
/* ticks is at a fixed position */
.set _TICKS, (PRT_OFF - 0x200 - 2) # Timeout ticks
.set _MNUOPT, 0x10 # Saved menu entries
.set TLEN, (desc_ofs - bootable_ids) # size of bootable ids
.globl start # Entry point
.code16 # This runs in real mode
* Initialise segments and registers to known values.
* segments start at 0.
* The stack is immediately below the address we were loaded to.
* NOTE: the initial section of the code (up to movw $LOAD,%sp)
* is used by boot0cfg, together with the 'Drive ' string and
* the 0x55, 0xaa at the end, as an identifier for version 1.0
* of the boot code. Do not change it.
* In version 1.0 the parameter table (_NEXTDRV etc) is at 0x1b9
start: cld # String ops inc
xorw %ax,%ax # Zero
movw %ax,%es # Address
movw %ax,%ds # data
movw %ax,%ss # Set up
movw $LOAD,%sp # stack
* Copy this code to the address it was linked for, 0x600 by default.
movw %sp,%si # Source
movw $start,%di # Destination
movw $0x100,%cx # Word count
rep # Relocate
movsw # code
* After the code, (i.e. at %di+0, 0x800) create a partition entry,
* initialized to LBA 0 / CHS 0:0:1.
* Set %bp to point to the partition and also, with negative offsets,
* to the variables embedded in the bootblock (nextdrv and so on).
movw %di,%bp # Address variables
movb $0x8,%cl # Words to clear
rep # Zero
stosw # them
incb -0xe(%di) # Set the S field to 1
jmp main-LOAD+ORIGIN # Jump to relocated code
#if defined(SIO) && COMSPEED != 0
* Init the serial port. bioscom preserves the driver number in DX.
movw $COMSPEED,%ax # defined by Makefile
callw bioscom
* If the 'setdrv' flag is set in the boot sector, use the drive
* number from the boot sector at 'setdrv_num'.
* Optionally, do the same if the BIOS gives us an invalid number
* (note though that the override prevents booting from a floppy
* or a ZIP/flash drive in floppy emulation).
* The test costs 4 bytes of code so it is disabled by default.
testb $SETDRV,_FLAGS(%bp) # Set drive number?
#ifndef CHECK_DRIVE /* disable drive checks */
jz save_curdrive # no, use the default
jnz disable_update # Yes
testb %dl,%dl # Drive number valid?
js save_curdrive # Possibly (0x80 set)
* Disable updates if the drive number is forced.
disable_update: orb $NOUPDATE,_FLAGS(%bp) # Disable updates
movb _SETDRV(%bp),%dl # Use stored drive number
* Whatever drive we decided to use, store it at (%bp). The byte
* is normally used for the state of the partition (0x80 or 0x00),
* but we abuse it as it is very convenient to access at offset 0.
* The value is read back after 'check_selection'
save_curdrive: movb %dl, (%bp) # Save drive number
pushw %dx # Also in the stack
#ifdef TEST /* test code, print internal bios drive */
rolb $1, %dl
movw $drive, %si
call putkey
callw putn # Print a newline
* Start out with a pointer to the 4th byte of the first table entry
* so that after 4 iterations it's beyond the end of the sector
* and beyond a 256 byte boundary. We use the latter trick to check for
* end of the loop without using an extra register (see start.5).
movw $(partbl+0x4),%bx # Partition table (+4)
xorw %dx,%dx # Item number
* Loop around on the partition table, printing values until we
* pass a 256 byte boundary.
read_entry: movb %ch,-0x4(%bx) # Zero active flag (ch == 0)
btw %dx,_FLAGS(%bp) # Entry enabled?
jnc next_entry # No
movb (%bx),%al # Load type
test %al, %al # skip empty partition
jz next_entry
* Scan the table of bootable ids, which starts at %di and has
* length TLEN. On a match, %di points to the element following the
* match; the corresponding offset to the description is $(TLEN-1)
* bytes ahead. We use a count of TLEN+1 so if we don't find a match
* within the first TLEN entries, we hit the 'unknown' entry.
movw $bootable_ids,%di # Lookup tables
movb $(TLEN+1),%cl # Number of entries
repne # Locate
scasb # type
* Get the matching element in the next array.
* The byte at $(TLEN-1)(%di) contains the offset of the description
* string from %di, so we add the number and print the string.
addw $(TLEN-1), %di # Adjust
movb (%di),%cl # Partition
addw %cx,%di # description
callw putx # Display it
next_entry: incw %dx # Next item
addb $0x10,%bl # Next entry
jnc read_entry # Till done
* We are past a 256 byte boundary: the partition table is finished.
* Add one to the drive number and check it is valid.
* Note that if we started from a floppy, %dl was 0 so we still
* get an entry for the next drive, which is the first Hard Disk.
popw %ax # Drive number
subb $0x80-0x1,%al # Does next
cmpb NHRDRV,%al # drive exist? (from BIOS?)
jb print_drive # Yes
* If this is the only drive, don't display it as an option.
decw %ax # Already drive 0?
jz print_prompt # Yes
* If it was illegal or we cycled through them, go back to drive 0.
xorb %al,%al # Drive 0
* Whatever drive we selected, make it an ascii digit and save it
* back to the "nxtdrv" location in case we want to save it to disk.
* This digit is also part of the printed drive string, so add 0x80
* to indicate end of string.
print_drive: addb $'0'|0x80,%al # Save next
movb %al,_NXTDRV(%bp) # drive number
movw $drive,%di # Display
callw putx # item
* Menu is complete, display a prompt followed by current selection.
* 'decw %si' makes the register point to the space after 'Boot: '
* so we do not see an extra CRLF on the screen.
print_prompt: movw $prompt,%si # Display
callw putstr # prompt
movb _OPT(%bp),%dl # Display
decw %si # default
callw putkey # key
jmp start_input # Skip beep
* Here we have the code waiting for user input or a timeout.
beep: movb $ASCII_BEL,%al # Input error, print or beep
callw putchr
* Actual Start of input loop. Take note of time
xorb %ah,%ah # BIOS: Get
int $0x1a # system time
movw %dx,%di # Ticks when
addw _TICKS(%bp),%di # timeout
* Busy loop, looking for keystrokes but keeping one eye on the time.
#ifndef SIO
movb $0x1,%ah # BIOS: Check
int $0x16 # for keypress
#else /* SIO */
movb $0x03,%ah # BIOS: Read COM
call bioscom
testb $0x01,%ah # Check line status
# (bit 1 indicates input)
#endif /* SIO */
jnz got_key # Have input
xorb %ah,%ah # BIOS: int 0x1a, 00
int $0x1a # get system time
cmpw %di,%dx # Timeout?
jb read_key # No
* Timed out or default selection
use_default: movb _OPT(%bp),%al # Load default
orb $NOUPDATE,_FLAGS(%bp) # Disable updates
jmp check_selection # Join common code
* Get the keystroke.
* ENTER or CR confirm the current selection (same as a timeout).
* Otherwise convert F1..F6 (or '1'..'6') to 0..5 and check if the
* selection is valid.
* The SIO code uses ascii chars, the console code uses scancodes.
#ifndef SIO
xorb %ah,%ah # BIOS: int 0x16, 00
int $0x16 # get keypress
movb %ah,%al # move scan code to %al
cmpb $KEY_ENTER,%al
movb $0x02,%ah # BIOS: Receive
call bioscom
cmpb $ASCII_CR,%al
je use_default # enter -> default
* Check if the key is acceptable, and loop back if not.
* The console (non-SIO) code looks at scancodes and accepts
* both F1..F6 and 1..6 (the latter costs 6 bytes of code),
* relying on the fact that F1..F6 have higher scancodes than 1..6
* The SIO code only takes 1..6
#ifdef SIO /* SIO mode, use ascii values */
subb $'1',%al # Subtract '1' ascii code
#else /* console mode -- use scancodes */
subb $KEY_F1,%al /* Subtract F1 scan code */
#if !defined(ONLY_F_KEYS)
cmpb $0x5,%al # F1..F6
jna 3f # Yes
subb $(KEY_1 - KEY_F1),%al # Less #1 scan code
#endif /* ONLY_F_KEYS */
#endif /* SIO */
cmpb $0x5,%al # F1..F6 or 1..6 ?
#ifdef PXE /* enable PXE/INT18 using F6 */
jne 1f;
int $0x18 # found F6, try INT18
#endif /* PXE */
jae beep # Not in F1..F5, beep
* We have a selection. If it's a bad selection go back to complain.
* The bits in MNUOPT were set when the options were printed.
* Anything not printed is not an option.
cbtw # Extend (%ah=0 used later)
btw %ax,_MNUOPT(%bp) # Option enabled?
jnc beep # No
* Save the info in the original tables
* for rewriting to the disk.
movb %al,_OPT(%bp) # Save option
* Make %si and %bx point to the fake partition at LBA 0 (CHS 0:0:1).
* Because the correct address is already in %bp, just use it.
* Set %dl with the drive number saved in byte 0.
* If we have pressed F5 or 5, then this is a good, fake value
* to present to the next stage boot code.
movw %bp,%si # Partition for write
movb (%si),%dl # Drive number, saved above
movw %si,%bx # Partition for read
cmpb $0x4,%al # F5/#5 pressed?
pushf # Save results for later
je 1f # Yes, F5
* F1..F4 was pressed, so make %bx point to the currently
* selected partition, and leave the drive number unchanged.
shlb $0x4,%al # Point to
addw $partbl,%ax # selected
xchgw %bx,%ax # partition
movb $0x80,(%bx) # Flag active
* If not asked to do a write-back (flags 0x40) don't do one.
* Around the call, save the partition pointer to %bx and
* restore to %si which is where the next stage expects it.
1: pushw %bx # Save
testb $NOUPDATE,_FLAGS(%bp) # No updates?
jnz 2f # skip update
movw $start,%bx # Data to write
movb $0x3,%ah # Write sector
callw intx13 # to disk
2: popw %si # Restore
* If going to next drive, replace drive with selected one.
* Remember to un-ascii it. Hey 0x80 is already set, cool!
popf # Restore %al test results
jne 3f # If not F5/#5
movb _NXTDRV(%bp),%dl # Next drive
subb $'0',%dl # number
* Load selected bootsector to the LOAD location in RAM. If read
* fails or there is no 0x55aa marker, treat it as a bad selection.
3: movw $LOAD,%bx # Address for read
movb $0x2,%ah # Read sector
callw intx13 # from disk
jc beep # If error
cmpw $MAGIC,0x1fe(%bx) # Bootable?
jne beep # No
pushw %si # Save ptr to selected part.
callw putn # Leave some space
popw %si # Restore, next stage uses it
jmp *%bx # Invoke bootstrap
* Display routines
* putkey prints the option selected in %dl (F1..F5 or 1..5) followed by
* the string at %si
* putx: print the option in %dl followed by the string at %di
* also record the drive as valid.
* putn: print a crlf
* putstr: print the string at %si
* putchr: print the char in al
* Display the option and record the drive as valid in the options.
* That last point is done using the btsw instruction which does
* a test and set. We don't care for the test part.
putx: btsw %dx,_MNUOPT(%bp) # Enable menu option
movw $item,%si # Display
callw putkey # key
movw %di,%si # Display the rest
callw putstr # Display string
putn: movw $crlf,%si # To next line
jmp putstr
#ifndef SIO
movb $'F',%al # Display
callw putchr # 'F'
movb $'1',%al # Prepare
addb %dl,%al # digit
putstr.1: callw putchr # Display char
putstr: lodsb # Get byte
testb $0x80,%al # End of string?
jz putstr.1 # No
andb $~0x80,%al # Clear MSB then print last
#ifndef SIO
pushw %bx # Save
movw $0x7,%bx # Page:attribute
movb $0xe,%ah # BIOS: Display
int $0x10 # character
popw %bx # Restore
#else /* SIO */
movb $0x01,%ah # BIOS: Send character
pushw %dx # Save
xorw %dx,%dx # Use COM1
int $0x14 # BIOS: Serial I/O
popw %dx # Restore
#endif /* SIO */
retw # To caller
/* One-sector disk I/O routine */
* %dl: drive, %si partition entry, %es:%bx transfer buffer.
* Load the CHS values and possibly the LBA address from the block
* at %si, and use the appropriate method to load the sector.
* Don't use packet mode for a floppy.
intx13: # Prepare CHS parameters
movb 0x1(%si),%dh # Load head
movw 0x2(%si),%cx # Load cylinder:sector
movb $0x1,%al # Sector count
pushw %si # Save
movw %sp,%di # Save
#ifndef CHECK_DRIVE /* floppy support */
testb %dl, %dl # is this a floppy ?
jz 1f # Yes, use CHS mode
testb $USEPACKET,_FLAGS(%bp) # Use packet interface?
jz 1f # No
pushl $0x0 # Set the
pushl 0x8(%si) # LBA address
pushw %es # Set the transfer
pushw %bx # buffer address
push $0x1 # Block count
push $0x10 # Packet size
movw %sp,%si # Packet pointer
decw %ax # Verify off
orb $0x40,%ah # Use disk packet
1: int $0x13 # BIOS: Disk I/O
movw %di,%sp # Restore
popw %si # Restore
retw # To caller
* Various menu strings. 'item' goes after 'prompt' to save space.
* Also use shorter versions to make room for the PXE/INT18 code.
#ifdef PXE
.ascii "\nF6 PXE\r"
.ascii "\nBoot:"
item: .ascii " "; .byte ' '|0x80
crlf: .ascii "\r"; .byte '\n'|0x80
/* Partition type tables */
* These values indicate bootable types we know about.
* Corresponding descriptions are at desc_ofs:
* Entries don't need to be sorted.
.byte 0x83, 0xa5, 0xa6, 0xa9, 0x06, 0x07, 0x0b
.byte 0x05 # extended partition
#ifndef SAVE_MEMORY /* other DOS partitions */
.byte 0x01 # FAT12
.byte 0x04 # FAT16 < 32M
* Offsets that match the known types above, used to point to the
* actual partition name. The last entry must point to os_misc,
* which is used for non-matching names.
.byte os_linux-. # 131, Linux
.byte os_freebsd-. # 165, FreeBSD
.byte os_bsd-. # 166, OpenBSD
.byte os_bsd-. # 169, NetBSD
.byte os_dos-. # 6, FAT16 >= 32M
.byte os_win-. # 7, NTFS
.byte os_win-. # 11, FAT32
.byte os_ext-. # 5, DOS Ext
.byte os_dos-. # 1, FAT12 DOS
.byte os_dos-. # 4, FAT16 <32M
.byte os_misc-. # Unknown
* And here are the strings themselves. The last byte of
* the string has bit 7 set.
os_misc: .byte '?'|0x80
#ifndef SAVE_MORE_MEMORY /* 'DOS' remapped to 'WIN' if no room */
.ascii "DO"; .byte 'S'|0x80
os_win: .ascii "Wi"; .byte 'n'|0x80
os_linux: .ascii "Linu"; .byte 'x'|0x80
os_freebsd: .ascii "Free"
os_bsd: .ascii "BS"; .byte 'D'|0x80
os_ext: .ascii "EX"; .byte 'T'|0x80
.org (0x200 + B0_OFF),0x90
* The boot0 version 1.0 parameter table.
* Do not move it nor change the "Drive " string, boot0cfg
* uses its offset and content to identify the boot sector.
* The other fields are sometimes changed before writing back to the drive
* Be especially careful that nxtdrv: must come after drive:, as it
* is part of the same string.
drive: .ascii "Drive "
nxtdrv: .byte 0x0 # Next drive number
opt: .byte 0x0 # Option
setdrv_num: .byte 0x80 # Drive to force
flags: .byte FLAGS # Flags
.byte 0xa8,0xa8,0xa8,0xa8 # Volume Serial Number
ticks: .word TICKS # Delay
.org PRT_OFF
* Here is the 64 byte partition table that fdisk would fiddle with.
partbl: .fill 0x40,0x1,0x0 # Partition table
.word MAGIC # Magic number
.org 0x200 # again, safety check