core/diskboot.inc - pub/scm/boot/syslinux/syslinux - Git at Google

 ; -----------------------------------------------------------------------
 ;
 ;   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.
 ;
 ; -----------------------------------------------------------------------

 ;
 ; diskboot.inc
 ;
 ; 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,
 ; STACK_TOP
 ;

 		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
 %endmacro
 %macro		superw 1
 bx %+ %1	equ SuperInfo+($-superblock)*8
 bs %+ %1	equ $
 		zw 1
 %endmacro
 %macro		superd 1
 bx %+ %1	equ $			; no expansion for dwords
 bs %+ %1	equ $
 		zd 1
 %endmacro
 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!)
 ;
 start:
 		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

 floppy:
 		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
 ;
 harddisk:
 		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:
 ;
 ; 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]
 .gotoffs:
 ;
 ; 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
 no_driveparm:
 not_harddisk:
 ;
 ; Ready to enable interrupts, captain
 ;
 		sti

 ;
 ; Do we have EBIOS (EDD)?
 ;
 eddcheck:
 		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.jmp+1],(getonesec_ebios-(getonesec.jmp+2))
 .noedd:

 ;
 ; 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.
 ;
 Sect1Load:
 		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.
 ;
 getonesec:
 		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)
 ;
 getonesec_ebios:
 .retry:
 		; Form DAPA on stack
 		push edx
 		push eax
 		push es
 		push bx
 		push word 1
 		push word 16
 		mov si,sp
 		pushad
                 mov ah,42h                      ; Extended Read
 		call xint13
 		popad
 		lea sp,[si+16]			; Remove DAPA
 		jc .error
                 ret

 .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
 		popad
 		loop .retry			; CX-- and jump if not zero

 		; Total failure.  Try falling back to CBIOS.
 		mov byte [getonesec.jmp+1],(getonesec_cbios-(getonesec.jmp+2))

 ;
 ; getonesec_cbios:
 ;
 ; getlinsec implementation for legacy CBIOS
 ;
 getonesec_cbios:
 .retry:
 		pushad

 		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
 		popad
 		jc .error
 		ret

 .error:
 		loop .retry
 		; Fall through to disk_error

 ;
 ; kaboom: write a message and bail out.
 ;
 %ifdef BINFMT
 		global kaboom
 %else
 		global kaboom:function hidden
 %endif
 disk_error:
 kaboom:
 		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

 .done:
 		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
 ;
 xint13:
 		mov dl,[DriveNumber]
 		push es		; ES destroyed by INT 13h AH 08h
 		int 13h
 		pop es
 		ret

 ;
 ; 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
 ; ===========================================================================
	; -----------------------------------------------------------------------
	;
	; 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.
	;
	; -----------------------------------------------------------------------

	;
	; diskboot.inc
	;
	; 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,
	; STACK_TOP
	;

	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
	%endmacro
	%macro superw 1
	bx %+ %1 equ SuperInfo+($-superblock)*8
	bs %+ %1 equ $
	zw 1
	%endmacro
	%macro superd 1
	bx %+ %1 equ $ ; no expansion for dwords
	bs %+ %1 equ $
	zd 1
	%endmacro
	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!)
	;
	start:
	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

	floppy:
	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
	;
	harddisk:
	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:
	;
	; 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]
	.gotoffs:
	;
	; 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
	no_driveparm:
	not_harddisk:
	;
	; Ready to enable interrupts, captain
	;
	sti

	;
	; Do we have EBIOS (EDD)?
	;
	eddcheck:
	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.jmp+1],(getonesec_ebios-(getonesec.jmp+2))
	.noedd:

	;
	; 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.
	;
	Sect1Load:
	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.
	;
	getonesec:
	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)
	;
	getonesec_ebios:
	.retry:
	; Form DAPA on stack
	push edx
	push eax
	push es
	push bx
	push word 1
	push word 16
	mov si,sp
	pushad
	mov ah,42h ; Extended Read
	call xint13
	popad
	lea sp,[si+16] ; Remove DAPA
	jc .error
	ret

	.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
	popad
	loop .retry ; CX-- and jump if not zero

	; Total failure. Try falling back to CBIOS.
	mov byte [getonesec.jmp+1],(getonesec_cbios-(getonesec.jmp+2))

	;
	; getonesec_cbios:
	;
	; getlinsec implementation for legacy CBIOS
	;
	getonesec_cbios:
	.retry:
	pushad

	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
	popad
	jc .error
	ret

	.error:
	loop .retry
	; Fall through to disk_error

	;
	; kaboom: write a message and bail out.
	;
	%ifdef BINFMT
	global kaboom
	%else
	global kaboom:function hidden
	%endif
	disk_error:
	kaboom:
	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

	.done:
	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
	;
	xint13:
	mov dl,[DriveNumber]
	push es ; ES destroyed by INT 13h AH 08h
	int 13h
	pop es
	ret

	;
	; 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
	; ===========================================================================