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; -----------------------------------------------------------------------
; Copyright 1994-2009 H. Peter Anvin - All Rights Reserved
; Copyright 2009-2011 Intel Corporation; author: H. Peter Anvin
; This program is free software; you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
; Boston MA 02110-1301, USA; either version 2 of the License, or
; (at your option) any later version; incorporated herein by reference.
; -----------------------------------------------------------------------
; Common boot sector code for harddisk-based Syslinux derivatives.
; Requires macros z[bwd], labels ldlinux_ent, ldlinux_magic, ldlinux_sys
; and constants BS_MAGIC_VER, LDLINUX_MAGIC, retry_count, Sect1Ptr[01]_VAL,
section .init
; Some of the things that have to be saved very early are saved
; "close" to the initial stack pointer offset, in order to
; reduce the code size...
global StackBuf, PartInfo, Hidden, OrigESDI, DriveNumber
global OrigFDCTabPtr
StackBuf equ STACK_TOP-44-92 ; Start the stack here (grow down - 4K)
PartInfo equ StackBuf
.mbr equ PartInfo
.gptlen equ PartInfo+16
.gpt equ PartInfo+20
FloppyTable equ PartInfo+76
; Total size of PartInfo + FloppyTable == 76+16 = 92 bytes
Hidden equ StackBuf-24 ; Partition offset (qword)
OrigFDCTabPtr equ StackBuf-16 ; Original FDC table
OrigDSSI equ StackBuf-12 ; DS:SI -> partinfo
OrigESDI equ StackBuf-8 ; ES:DI -> $PnP structure
DriveNumber equ StackBuf-4 ; Drive number
StackHome equ Hidden ; The start of the canonical stack
; Primary entry point. Tempting as though it may be, we can't put the
; initial "cli" here; the jmp opcode in the first byte is part of the
; "magic number" (using the term very loosely) for the DOS superblock.
bootsec equ $
_start: jmp short start ; 2 bytes
nop ; 1 byte
; "Superblock" follows -- it's in the boot sector, so it's already
; loaded and ready for us
bsOemName db MY_NAME ; The SYS command sets this, so...
zb 8-($-bsOemName)
; These are the fields we actually care about. We end up expanding them
; all to dword size early in the code, so generate labels for both
; the expanded and unexpanded versions.
%macro superb 1
bx %+ %1 equ SuperInfo+($-superblock)*8+4
bs %+ %1 equ $
zb 1
%macro superw 1
bx %+ %1 equ SuperInfo+($-superblock)*8
bs %+ %1 equ $
zw 1
%macro superd 1
bx %+ %1 equ $ ; no expansion for dwords
bs %+ %1 equ $
zd 1
superblock equ $
superw BytesPerSec
superb SecPerClust
superw ResSectors
superb FATs
superw RootDirEnts
superw Sectors
superb Media
superw FATsecs
superw SecPerTrack
superw Heads
superinfo_size equ ($-superblock)-1 ; How much to expand
superd Hidden
superd HugeSectors
; This is as far as FAT12/16 and FAT32 are consistent
; FAT12/16 need 26 more bytes,
; FAT32 need 54 more bytes
superblock_len_fat16 equ $-superblock+26
superblock_len_fat32 equ $-superblock+54
zb 54 ; Maximum needed size
superblock_max equ $-superblock
SecPerClust equ bxSecPerClust
; Note we don't check the constraints above now; we did that at install
; time (we hope!)
cli ; No interrupts yet, please
cld ; Copy upwards
; Set up the stack
xor cx,cx
mov ss,cx
mov sp,StackBuf-2 ; Just below BSS (-2 for alignment)
push dx ; Save drive number (in DL)
push es ; Save initial ES:DI -> $PnP pointer
push di
push ds ; Save original DS:SI -> partinfo
push si
mov es,cx
; DS:SI may contain a partition table entry and possibly a GPT entry.
; Preserve it for us. This saves 56 bytes of the GPT entry, which is
; currently the maximum we care about. Total is 76 bytes.
mov cl,(16+4+56)/2 ; Save partition info
mov di,PartInfo
rep movsw ; This puts CX back to zero
mov ds,cx ; Now we can initialize DS...
; Now sautee the BIOS floppy info block to that it will support decent-
; size transfers; the floppy block is 11 bytes and is stored in the
; INT 1Eh vector (brilliant waste of resources, eh?)
; Of course, if BIOSes had been properly programmed, we wouldn't have
; had to waste precious space with this code.
mov bx,fdctab
lfs si,[bx] ; FS:SI -> original fdctab
push fs ; Save on stack in case we need to bail
push si
; Save the old fdctab even if hard disk so the stack layout
; is the same. The instructions above do not change the flags
and dl,dl ; If floppy disk (00-7F), assume no
; partition table
js harddisk
xor ax,ax
mov cl,6 ; 12 bytes (CX == 0)
; es:di -> FloppyTable already
; This should be safe to do now, interrupts are off...
mov [bx],di ; FloppyTable
mov [bx+2],ax ; Segment 0
fs rep movsw ; Faster to move words
mov cl,[bsSecPerTrack] ; Patch the sector count
mov [di-12+4],cl
push ax ; Partition offset == 0
push ax
push ax
push ax
int 13h ; Some BIOSes need this
; Using xint13 costs +1B
jmp short not_harddisk
; The drive number and possibly partition information was passed to us
; by the BIOS or previous boot loader (MBR). Current "best practice" is to
; trust that rather than what the superblock contains.
; Note: di points to beyond the end of PartInfo
; Note: false negatives might slip through the handover area's sanity checks,
; if the region is very close (less than a paragraph) to
; PartInfo ; no false positives are possible though
mov dx,[di-76-10] ; Original DS
mov si,[di-76-12] ; Original SI
shr si,4
add dx,si
cmp dx,4fh ; DS:SI < 50h:0 (BDA or IVT) ?
jbe .no_partition
cmp dx,(PartInfo-75)>>4 ; DS:SI in overwritten memory?
jae .no_partition
test byte [di-76],7Fh ; Sanity check: "active flag" should
jnz .no_partition ; be 00 or 80
cmp [di-76+4],cl ; Sanity check: partition type != 0
je .no_partition
cmp eax,'!GPT' ; !GPT signature?
jne .mbr
cmp byte [di-76+4],0EDh ; Synthetic GPT partition entry?
jne .mbr
.gpt: ; GPT-style partition info
push dword [di-76+20+36]
push dword [di-76+20+32]
jmp .gotoffs
.mbr: ; MBR-style partition info
push cx ; Upper half partition offset == 0
push cx
push dword [di-76+8] ; Partition offset (dword)
jmp .gotoffs
; No partition table given... assume that the Hidden field in the boot sector
; tells the truth (in particular, is zero if this is an unpartitioned disk.)
push cx
push cx
push dword [bsHidden]
; Get disk drive parameters (don't trust the superblock.) Don't do this for
; floppy drives -- INT 13:08 on floppy drives will (may?) return info about
; what the *drive* supports, not about the *media*. Fortunately floppy disks
; tend to have a fixed, well-defined geometry which is stored in the superblock.
; DL == drive # still
mov ah,08h
call xint13
jc no_driveparm
and ah,ah
jnz no_driveparm
shr dx,8
inc dx ; Contains # of heads - 1
mov [bsHeads],dx
and cx,3fh
mov [bsSecPerTrack],cx
; Ready to enable interrupts, captain
; Do we have EBIOS (EDD)?
mov bx,55AAh
mov ah,41h ; EDD existence query
call xint13
jc .noedd
cmp bx,0AA55h
jne .noedd
test cl,1 ; Extended disk access functionality set
jz .noedd
; We have EDD support...
mov byte [],(getonesec_ebios-(
; Load the first sector of LDLINUX.SYS; this used to be all proper
; with parsing the superblock and root directory; it doesn't fit
; together with EBIOS support, unfortunately.
mov eax,strict dword Sect1Ptr0_VAL ; 0xdeadbeef
Sect1Ptr0 equ $-4
mov edx,strict dword Sect1Ptr1_VAL ; 0xfeedface
Sect1Ptr1 equ $-4
mov bx,ldlinux_sys ; Where to load it
call getonesec
; Some modicum of integrity checking
cmp dword [ldlinux_magic+4],LDLINUX_MAGIC^HEXDATE
jne kaboom
; Go for it!
jmp ldlinux_ent
; getonesec: load a single disk linear sector EDX:EAX into the buffer
; at ES:BX.
; This routine assumes CS == DS == SS, and trashes most registers.
; Stylistic note: use "xchg" instead of "mov" when the source is a register
; that is dead from that point; this saves space. However, please keep
; the order to dst,src to keep things sane.
add eax,[Hidden] ; Add partition offset
adc edx,[Hidden+4]
mov cx,retry_count
.jmp: jmp strict short getonesec_cbios
; getonesec_ebios:
; getonesec implementation for EBIOS (EDD)
; Form DAPA on stack
push edx
push eax
push es
push bx
push word 1
push word 16
mov si,sp
mov ah,42h ; Extended Read
call xint13
lea sp,[si+16] ; Remove DAPA
jc .error
; Some systems seem to get "stuck" in an error state when
; using EBIOS. Doesn't happen when using CBIOS, which is
; good, since some other systems get timeout failures
; waiting for the floppy disk to spin up.
pushad ; Try resetting the device
xor ax,ax
call xint13
loop .retry ; CX-- and jump if not zero
; Total failure. Try falling back to CBIOS.
mov byte [],(getonesec_cbios-(
; getonesec_cbios:
; getlinsec implementation for legacy CBIOS
movzx esi,word [bsSecPerTrack]
movzx edi,word [bsHeads]
; Dividing by sectors to get (track,sector): we may have
; up to 2^18 tracks, so we need to use 32-bit arithmetric.
div esi
xor cx,cx
xchg cx,dx ; CX <- sector index (0-based)
; EDX <- 0
; eax = track #
div edi ; Convert track to head/cyl
cmp eax,1023 ; Outside the CHS range?
ja kaboom
; Now we have AX = cyl, DX = head, CX = sector (0-based),
; SI = bsSecPerTrack, ES:BX = data target
shl ah,6 ; Because IBM was STOOPID
; and thought 8 bits were enough
; then thought 10 bits were enough...
inc cx ; Sector numbers are 1-based, sigh
or cl,ah
mov ch,al
mov dh,dl
mov ax,0201h ; Read one sector
call xint13
jc .error
loop .retry
; Fall through to disk_error
; kaboom: write a message and bail out.
%ifdef BINFMT
global kaboom
global kaboom:function hidden
xor si,si
mov ss,si
mov sp,OrigFDCTabPtr ; Reset stack
mov ds,si ; Reset data segment
pop dword [fdctab] ; Restore FDC table
.patch: ; When we have full code, intercept here
mov si,bailmsg
.loop: lodsb
and al,al
jz .done
mov ah,0Eh ; Write to screen as TTY
mov bx,0007h ; Attribute
int 10h
jmp short .loop
xor ax,ax
.again: int 16h ; Wait for keypress
; NB: replaced by int 18h if
; chosen at install time..
int 19h ; And try once more to boot...
.norge: hlt ; If int 19h returned; this is the end
jmp short .norge
; INT 13h wrapper function
mov dl,[DriveNumber]
push es ; ES destroyed by INT 13h AH 08h
int 13h
pop es
; Error message on failure
bailmsg: db 'Boot error', 0Dh, 0Ah, 0
; This fails if the boot sector overflows
zb 1F8h-($-$$)
bs_magic dd LDLINUX_MAGIC
bs_link dw (Sect1Load - bootsec) | BS_MAGIC_VER
bootsignature dw 0xAA55
; ===========================================================================
; End of boot sector
; ===========================================================================