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source: bootcd/isolinux/syslinux-6.03/core/isolinux.asm

Last change on this file was e16e8f2, checked in by Edwin Eefting <edwin@datux.nl>, 3 years ago
bootstuff
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File size: 30.0 KB

Rev	Line
[e16e8f2]	1	; -- fundamental -- (asm-mode sucks)
	2	; ****************************************************************************
	3	;
	4	; isolinux.asm
	5	;
	6	; A program to boot Linux kernels off a CD-ROM using the El Torito
	7	; boot standard in "no emulation" mode, making the entire filesystem
	8	; available. It is based on the SYSLINUX boot loader for MS-DOS
	9	; floppies.
	10	;
	11	; Copyright 1994-2009 H. Peter Anvin - All Rights Reserved
	12	; Copyright 2009 Intel Corporation; author: H. Peter Anvin
	13	;
	14	; This program is free software; you can redistribute it and/or modify
	15	; it under the terms of the GNU General Public License as published by
	16	; the Free Software Foundation, Inc., 53 Temple Place Ste 330,
	17	; Boston MA 02111-1307, USA; either version 2 of the License, or
	18	; (at your option) any later version; incorporated herein by reference.
	19	;
	20	; ****************************************************************************
	21
	22	%define IS_ISOLINUX 1
	23	%include "head.inc"
	24
	25	;
	26	; Some semi-configurable constants... change on your own risk.
	27	;
	28	my_id equ isolinux_id
	29	NULLFILE equ 0 ; Zero byte == null file name
	30	NULLOFFSET equ 0 ; Position in which to look
	31	retry_count equ 6 ; How patient are we with the BIOS?
	32	%assign HIGHMEM_SLOP 128*1024 ; Avoid this much memory near the top
	33	SECTOR_SHIFT equ 11 ; 2048 bytes/sector (El Torito requirement)
	34	SECTOR_SIZE equ (1 << SECTOR_SHIFT)
	35
	36	ROOT_DIR_WORD equ 0x002F
	37
	38	; ---------------------------------------------------------------------------
	39	; BEGIN CODE
	40	; ---------------------------------------------------------------------------
	41
	42	;
	43	; Memory below this point is reserved for the BIOS and the MBR
	44	;
	45	section .earlybss
	46	global trackbuf
	47	trackbufsize equ 8192
	48	trackbuf resb trackbufsize ; Track buffer goes here
	49	; ends at 2800h
	50
	51	; Some of these are touched before the whole image
	52	; is loaded. DO NOT move this to .bss16/.uibss.
	53	section .earlybss
	54	global BIOSName
	55	alignb 4
	56	FirstSecSum resd 1 ; Checksum of bytes 64-2048
	57	ImageDwords resd 1 ; isolinux.bin size, dwords
	58	InitStack resd 1 ; Initial stack pointer (SS:SP)
	59	DiskSys resw 1 ; Last INT 13h call
	60	ImageSectors resw 1 ; isolinux.bin size, sectors
	61	; These following two are accessed as a single dword...
	62	GetlinsecPtr resw 1 ; The sector-read pointer
	63	BIOSName resw 1 ; Display string for BIOS type
	64	%define HAVE_BIOSNAME 1
	65	global BIOSType
	66	BIOSType resw 1
	67	DiskError resb 1 ; Error code for disk I/O
	68	global DriveNumber
	69	DriveNumber resb 1 ; CD-ROM BIOS drive number
	70	ISOFlags resb 1 ; Flags for ISO directory search
	71	RetryCount resb 1 ; Used for disk access retries
	72
	73	alignb 8
	74	global Hidden
	75	Hidden resq 1 ; Used in hybrid mode
	76	bsSecPerTrack resw 1 ; Used in hybrid mode
	77	bsHeads resw 1 ; Used in hybrid mode
	78
	79
	80	;
	81	; El Torito spec packet
	82	;
	83
	84	alignb 8
	85	_spec_start equ $
	86	global spec_packet
	87	spec_packet: resb 1 ; Size of packet
	88	sp_media: resb 1 ; Media type
	89	sp_drive: resb 1 ; Drive number
	90	sp_controller: resb 1 ; Controller index
	91	sp_lba: resd 1 ; LBA for emulated disk image
	92	sp_devspec: resw 1 ; IDE/SCSI information
	93	sp_buffer: resw 1 ; User-provided buffer
	94	sp_loadseg: resw 1 ; Load segment
	95	sp_sectors: resw 1 ; Sector count
	96	sp_chs: resb 3 ; Simulated CHS geometry
	97	sp_dummy: resb 1 ; Scratch, safe to overwrite
	98
	99	;
	100	; EBIOS drive parameter packet
	101	;
	102	alignb 8
	103	drive_params: resw 1 ; Buffer size
	104	dp_flags: resw 1 ; Information flags
	105	dp_cyl: resd 1 ; Physical cylinders
	106	dp_head: resd 1 ; Physical heads
	107	dp_sec: resd 1 ; Physical sectors/track
	108	dp_totalsec: resd 2 ; Total sectors
	109	dp_secsize: resw 1 ; Bytes per sector
	110	dp_dpte: resd 1 ; Device Parameter Table
	111	dp_dpi_key: resw 1 ; 0BEDDh if rest valid
	112	dp_dpi_len: resb 1 ; DPI len
	113	resb 1
	114	resw 1
	115	dp_bus: resb 4 ; Host bus type
	116	dp_interface: resb 8 ; Interface type
	117	db_i_path: resd 2 ; Interface path
	118	db_d_path: resd 2 ; Device path
	119	resb 1
	120	db_dpi_csum: resb 1 ; Checksum for DPI info
	121
	122	;
	123	; EBIOS disk address packet
	124	;
	125	alignb 8
	126	dapa: resw 1 ; Packet size
	127	.count: resw 1 ; Block count
	128	.off: resw 1 ; Offset of buffer
	129	.seg: resw 1 ; Segment of buffer
	130	.lba: resd 2 ; LBA (LSW, MSW)
	131
	132	;
	133	; Spec packet for disk image emulation
	134	;
	135	alignb 8
	136	dspec_packet: resb 1 ; Size of packet
	137	dsp_media: resb 1 ; Media type
	138	dsp_drive: resb 1 ; Drive number
	139	dsp_controller: resb 1 ; Controller index
	140	dsp_lba: resd 1 ; LBA for emulated disk image
	141	dsp_devspec: resw 1 ; IDE/SCSI information
	142	dsp_buffer: resw 1 ; User-provided buffer
	143	dsp_loadseg: resw 1 ; Load segment
	144	dsp_sectors: resw 1 ; Sector count
	145	dsp_chs: resb 3 ; Simulated CHS geometry
	146	dsp_dummy: resb 1 ; Scratch, safe to overwrite
	147
	148	alignb 4
	149	_spec_end equ $
	150	_spec_len equ _spec_end - _spec_start
	151
	152	section .init
	153	;;
	154	;; Primary entry point. Because BIOSes are buggy, we only load the first
	155	;; CD-ROM sector (2K) of the file, so the number one priority is actually
	156	;; loading the rest.
	157	;;
	158	global StackBuf
	159	StackBuf equ STACK_TOP-44 ; 44 bytes needed for
	160	; the bootsector chainloading
	161	; code!
	162	global OrigESDI
	163	OrigESDI equ StackBuf-4 ; The high dword on the stack
	164	StackHome equ OrigESDI
	165
	166	bootsec equ $
	167
	168	_start: ; Far jump makes sure we canonicalize the address
	169	cli
	170	jmp 0:_start1
	171	times 8-($-$$) nop ; Pad to file offset 8
	172
	173	; This table hopefully gets filled in by mkisofs using the
	174	; -boot-info-table option. If not, the values in this
	175	; table are default values that we can use to get us what
	176	; we need, at least under a certain set of assumptions.
	177	global iso_boot_info
	178	iso_boot_info:
	179	bi_pvd: dd 16 ; LBA of primary volume descriptor
	180	bi_file: dd 0 ; LBA of boot file
	181	bi_length: dd 0xdeadbeef ; Length of boot file
	182	bi_csum: dd 0xdeadbeef ; Checksum of boot file
	183	bi_reserved: times 10 dd 0xdeadbeef ; Reserved
	184	bi_end:
	185
	186	; Custom entry point for the hybrid-mode disk.
	187	; The following values will have been pushed onto the
	188	; entry stack:
	189	; - partition offset (qword)
	190	; - ES
	191	; - DI
	192	; - DX (including drive number)
	193	; - CBIOS Heads
	194	; - CBIOS Sectors
	195	; - EBIOS flag
	196	; (top of stack)
	197	;
	198	; If we had an old isohybrid, the partition offset will
	199	; be missing; we can check for that with sp >= 0x7c00.
	200	; Serious hack alert.
	201	%ifndef DEBUG_MESSAGES
	202	_hybrid_signature:
	203	dd 0x7078c0fb ; An arbitrary number...
	204
	205	_start_hybrid:
	206	pop cx ; EBIOS flag
	207	pop word [cs:bsSecPerTrack]
	208	pop word [cs:bsHeads]
	209	pop dx
	210	pop di
	211	pop es
	212	xor eax,eax
	213	xor ebx,ebx
	214	cmp sp,7C00h
	215	jae .nooffset
	216	pop eax
	217	pop ebx
	218	.nooffset:
	219	mov si,bios_cbios
	220	jcxz _start_common
	221	mov si,bios_ebios
	222	jmp _start_common
	223	%endif
	224
	225	_start1:
	226	mov si,bios_cdrom
	227	xor eax,eax
	228	xor ebx,ebx
	229	_start_common:
	230	mov [cs:InitStack],sp ; Save initial stack pointer
	231	mov [cs:InitStack+2],ss
	232	xor cx,cx
	233	mov ss,cx
	234	mov sp,StackBuf ; Set up stack
	235	push es ; Save initial ES:DI -> $PnP pointer
	236	push di
	237	mov ds,cx
	238	mov es,cx
	239	mov fs,cx
	240	mov gs,cx
	241	sti
	242	cld
	243
	244	mov [Hidden],eax
	245	mov [Hidden+4],ebx
	246
	247	mov [BIOSType],si
	248	mov eax,[si]
	249	mov [GetlinsecPtr],eax
	250
	251	; Show signs of life
	252	mov si,syslinux_banner
	253	call writestr_early
	254	%ifdef DEBUG_MESSAGES
	255	mov si,copyright_str
	256	%else
	257	mov si,[BIOSName]
	258	%endif
	259	call writestr_early
	260
	261	;
	262	; Before modifying any memory, get the checksum of bytes
	263	; 64-2048
	264	;
	265	initial_csum: xor edi,edi
	266	mov si,bi_end
	267	mov cx,(SECTOR_SIZE-64) >> 2
	268	.loop: lodsd
	269	add edi,eax
	270	loop .loop
	271	mov [FirstSecSum],edi
	272
	273	mov [DriveNumber],dl
	274	%ifdef DEBUG_MESSAGES
	275	mov si,startup_msg
	276	call writemsg
	277	mov al,dl
	278	call writehex2
	279	call crlf_early
	280	%endif
	281	;
	282	; Initialize spec packet buffers
	283	;
	284	mov di,_spec_start
	285	mov cx,_spec_len >> 2
	286	xor eax,eax
	287	rep stosd
	288
	289	; Initialize length field of the various packets
	290	mov byte [spec_packet],13h
	291	mov byte [drive_params],30
	292	mov byte [dapa],16
	293	mov byte [dspec_packet],13h
	294
	295	; Other nonzero fields
	296	inc word [dsp_sectors]
	297
	298	; Are we just pretending to be a CD-ROM?
	299	cmp word [BIOSType],bios_cdrom
	300	jne found_drive ; If so, no spec packet...
	301
	302	; Now figure out what we're actually doing
	303	; Note: use passed-in DL value rather than 7Fh because
	304	; at least some BIOSes will get the wrong value otherwise
	305	mov ax,4B01h ; Get disk emulation status
	306	mov dl,[DriveNumber]
	307	mov si,spec_packet
	308	call int13
	309	jc award_hack ; changed for BrokenAwardHack
	310	mov dl,[DriveNumber]
	311	cmp [sp_drive],dl ; Should contain the drive number
	312	jne spec_query_failed
	313
	314	%ifdef DEBUG_MESSAGES
	315	mov si,spec_ok_msg
	316	call writemsg
	317	mov al,byte [sp_drive]
	318	call writehex2
	319	call crlf_early
	320	%endif
	321
	322	found_drive:
	323	; Alright, we have found the drive. Now, try to find the
	324	; boot file itself. If we have a boot info table, life is
	325	; good; if not, we have to make some assumptions, and try
	326	; to figure things out ourselves. In particular, the
	327	; assumptions we have to make are:
	328	; - single session only
	329	; - only one boot entry (no menu or other alternatives)
	330
	331	cmp dword [bi_file],0 ; Address of code to load
	332	jne found_file ; Boot info table present :)
	333
	334	%ifdef DEBUG_MESSAGES
	335	mov si,noinfotable_msg
	336	call writemsg
	337	%endif
	338
	339	; No such luck. See if the spec packet contained one.
	340	mov eax,[sp_lba]
	341	and eax,eax
	342	jz set_file ; Good enough
	343
	344	%ifdef DEBUG_MESSAGES
	345	mov si,noinfoinspec_msg
	346	call writemsg
	347	%endif
	348
	349	; No such luck. Get the Boot Record Volume, assuming single
	350	; session disk, and that we're the first entry in the chain.
	351	mov eax,17 ; Assumed address of BRV
	352	mov bx,trackbuf
	353	call getonesec
	354
	355	mov eax,[trackbuf+47h] ; Get boot catalog address
	356	mov bx,trackbuf
	357	call getonesec ; Get boot catalog
	358
	359	mov eax,[trackbuf+28h] ; First boot entry
	360	; And hope and pray this is us...
	361
	362	; Some BIOSes apparently have limitations on the size
	363	; that may be loaded (despite the El Torito spec being very
	364	; clear on the fact that it must all be loaded.) Therefore,
	365	; we load it ourselves, and bleep the BIOS.
	366
	367	set_file:
	368	mov [bi_file],eax
	369
	370	found_file:
	371	; Set up boot file sizes
	372	mov eax,[bi_length]
	373	sub eax,SECTOR_SIZE-3 ; ... minus sector loaded
	374	shr eax,2 ; bytes->dwords
	375	mov [ImageDwords],eax ; boot file dwords
	376	add eax,((SECTOR_SIZE-1) >> 2)
	377	shr eax,SECTOR_SHIFT-2 ; dwords->sectors
	378	mov [ImageSectors],ax ; boot file sectors
	379
	380	mov eax,[bi_file] ; Address of code to load
	381	inc eax ; Don't reload bootstrap code
	382	%ifdef DEBUG_MESSAGES
	383	mov si,offset_msg
	384	call writemsg
	385	call writehex8
	386	call crlf_early
	387	%endif
	388
	389	; Load the rest of the file. However, just in case there
	390	; are still BIOSes with 64K wraparound problems, we have to
	391	; take some extra precautions. Since the normal load
	392	; address (TEXT_START) is not 2K-sector-aligned, we round
	393	; the target address upward to a sector boundary,
	394	; and then move the entire thing down as a unit.
	395	MaxLMA equ 384*1024 ; Reasonable limit (384K)
	396
	397	mov bx,((TEXT_START+2*SECTOR_SIZE-1) & ~(SECTOR_SIZE-1)) >> 4
	398	mov bp,[ImageSectors]
	399	push bx ; Load segment address
	400
	401	.more:
	402	push bx ; Segment address
	403	push bp ; Sector count
	404	mov es,bx
	405	mov cx,0xfff
	406	and bx,cx
	407	inc cx
	408	sub cx,bx
	409	shr cx,SECTOR_SHIFT - 4
	410	jnz .notaligned
	411	mov cx,0x10000 >> SECTOR_SHIFT ; Full 64K segment possible
	412	.notaligned:
	413	cmp bp,cx
	414	jbe .ok
	415	mov bp,cx
	416	.ok:
	417	xor bx,bx
	418	push bp
	419	push eax
	420	call getlinsec
	421	pop eax
	422	pop cx
	423	movzx edx,cx
	424	pop bp
	425	pop bx
	426
	427	shl cx,SECTOR_SHIFT - 4
	428	add bx,cx
	429	add eax,edx
	430	sub bp,dx
	431	jnz .more
	432
	433	; Move the image into place, and also verify the
	434	; checksum
	435	pop ax ; Load segment address
	436	mov bx,(TEXT_START + SECTOR_SIZE) >> 4
	437	mov ecx,[ImageDwords]
	438	mov edi,[FirstSecSum] ; First sector checksum
	439	xor si,si
	440
	441	move_verify_image:
	442	.setseg:
	443	mov ds,ax
	444	mov es,bx
	445	.loop:
	446	mov edx,[si]
	447	add edi,edx
	448	dec ecx
	449	mov [es:si],edx
	450	jz .done
	451	add si,4
	452	jnz .loop
	453	add ax,1000h
	454	add bx,1000h
	455	jmp .setseg
	456	.done:
	457	mov ax,cs
	458	mov ds,ax
	459	mov es,ax
	460
	461	; Verify the checksum on the loaded image.
	462	cmp [bi_csum],edi
	463	je integrity_ok
	464
	465	mov si,checkerr_msg
	466	call writemsg
	467	jmp kaboom
	468
	469	integrity_ok:
	470	%ifdef DEBUG_MESSAGES
	471	mov si,allread_msg
	472	call writemsg
	473	%endif
	474	jmp all_read ; Jump to main code
	475
	476	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	477	;; Start of BrokenAwardHack --- 10-nov-2002 Knut_Petersen@t-online.de
	478	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	479	;;
	480	;; There is a problem with certain versions of the AWARD BIOS ...
	481	;; the boot sector will be loaded and executed correctly, but, because the
	482	;; int 13 vector points to the wrong code in the BIOS, every attempt to
	483	;; load the spec packet will fail. We scan for the equivalent of
	484	;;
	485	;; mov ax,0201h
	486	;; mov bx,7c00h
	487	;; mov cx,0006h
	488	;; mov dx,0180h
	489	;; pushf
	490	;; call <direct far>
	491	;;
	492	;; and use <direct far> as the new vector for int 13. The code above is
	493	;; used to load the boot code into ram, and there should be no reason
	494	;; for anybody to change it now or in the future. There are no opcodes
	495	;; that use encodings relativ to IP, so scanning is easy. If we find the
	496	;; code above in the BIOS code we can be pretty sure to run on a machine
	497	;; with an broken AWARD BIOS ...
	498	;;
	499	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	500	;;
	501	%ifdef DEBUG_MESSAGES ;;
	502	;;
	503	award_notice db "Trying BrokenAwardHack first ...",CR,LF,0 ;;
	504	award_not_orig db "BAH: Original Int 13 vector : ",0 ;;
	505	award_not_new db "BAH: Int 13 vector changed to : ",0 ;;
	506	award_not_succ db "BAH: SUCCESS",CR,LF,0 ;;
	507	award_not_fail db "BAH: FAILURE" ;;
	508	award_not_crlf db CR,LF,0 ;;
	509	;;
	510	%endif ;;
	511	;;
	512	award_oldint13 dd 0 ;;
	513	award_string db 0b8h,1,2,0bbh,0,7ch,0b9h,6,0,0bah,80h,1,09ch,09ah ;;
	514	;;
	515	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	516	award_hack: mov si,spec_err_msg ; Moved to this place from
	517	call writemsg ; spec_query_failed
	518	;
	519	%ifdef DEBUG_MESSAGES ;
	520	;
	521	mov si,award_notice ; display our plan
	522	call writemsg ;
	523	mov si,award_not_orig ; display original int 13
	524	call writemsg ; vector
	525	%endif ;
	526	mov eax,[13h*4] ;
	527	mov [award_oldint13],eax ;
	528	;
	529	%ifdef DEBUG_MESSAGES ;
	530	;
	531	call writehex8 ;
	532	mov si,award_not_crlf ;
	533	call writestr_early ;
	534	%endif ;
	535	push es ; save ES
	536	mov ax,0f000h ; ES = BIOS Seg
	537	mov es,ax ;
	538	cld ;
	539	xor di,di ; start at ES:DI = f000:0
	540	award_loop: push di ; save DI
	541	mov si,award_string ; scan for award_string
	542	mov cx,7 ; length of award_string = 7dw
	543	repz cmpsw ; compare
	544	pop di ; restore DI
	545	jcxz award_found ; jmp if found
	546	inc di ; not found, inc di
	547	jno award_loop ;
	548	;
	549	award_failed: pop es ; No, not this way :-((
	550	award_fail2: ;
	551	;
	552	%ifdef DEBUG_MESSAGES ;
	553	;
	554	mov si,award_not_fail ; display failure ...
	555	call writemsg ;
	556	%endif ;
	557	mov eax,[award_oldint13] ; restore the original int
	558	or eax,eax ; 13 vector if there is one
	559	jz spec_query_failed ; and try other workarounds
	560	mov [13h*4],eax ;
	561	jmp spec_query_failed ;
	562	;
	563	award_found: mov eax,[es:di+0eh] ; load possible int 13 addr
	564	pop es ; restore ES
	565	;
	566	cmp eax,[award_oldint13] ; give up if this is the
	567	jz award_failed ; active int 13 vector,
	568	mov [13h4],eax ; otherwise change 0:13h4
	569	;
	570	;
	571	%ifdef DEBUG_MESSAGES ;
	572	;
	573	push eax ; display message and
	574	mov si,award_not_new ; new vector address
	575	call writemsg ;
	576	pop eax ;
	577	call writehex8 ;
	578	mov si,award_not_crlf ;
	579	call writestr_early ;
	580	%endif ;
	581	mov ax,4B01h ; try to read the spec packet
	582	mov dl,[DriveNumber] ; now ... it should not fail
	583	mov si,spec_packet ; any longer
	584	int 13h ;
	585	jc award_fail2 ;
	586	;
	587	%ifdef DEBUG_MESSAGES ;
	588	;
	589	mov si,award_not_succ ; display our SUCCESS
	590	call writemsg ;
	591	%endif ;
	592	jmp found_drive ; and leave error recovery code
	593	;
	594	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	595	;; End of BrokenAwardHack ---- 10-nov-2002 Knut_Petersen@t-online.de
	596	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	597
	598
	599	; INT 13h, AX=4B01h, DL=<passed in value> failed.
	600	; Try to scan the entire 80h-FFh from the end.
	601
	602	spec_query_failed:
	603
	604	; some code moved to BrokenAwardHack
	605
	606	mov dl,0FFh
	607	.test_loop: pusha
	608	mov ax,4B01h
	609	mov si,spec_packet
	610	mov byte [si],13h ; Size of buffer
	611	call int13
	612	popa
	613	jc .still_broken
	614
	615	mov si,maybe_msg
	616	call writemsg
	617	mov al,dl
	618	call writehex2
	619	call crlf_early
	620
	621	cmp byte [sp_drive],dl
	622	jne .maybe_broken
	623
	624	; Okay, good enough...
	625	mov si,alright_msg
	626	call writemsg
	627	.found_drive0: mov [DriveNumber],dl
	628	.found_drive: jmp found_drive
	629
	630	; Award BIOS 4.51 apparently passes garbage in sp_drive,
	631	; but if this was the drive number originally passed in
	632	; DL then consider it "good enough"
	633	.maybe_broken:
	634	mov al,[DriveNumber]
	635	cmp al,dl
	636	je .found_drive
	637
	638	; Intel Classic R+ computer with Adaptec 1542CP BIOS 1.02
	639	; passes garbage in sp_drive, and the drive number originally
	640	; passed in DL does not have 80h bit set.
	641	or al,80h
	642	cmp al,dl
	643	je .found_drive0
	644
	645	.still_broken: dec dx
	646	cmp dl, 80h
	647	jnb .test_loop
	648
	649	; No spec packet anywhere. Some particularly pathetic
	650	; BIOSes apparently don't even implement function
	651	; 4B01h, so we can't query a spec packet no matter
	652	; what. If we got a drive number in DL, then try to
	653	; use it, and if it works, then well...
	654	mov dl,[DriveNumber]
	655	cmp dl,81h ; Should be 81-FF at least
	656	jb fatal_error ; If not, it's hopeless
	657
	658	; Write a warning to indicate we're on very thin ice now
	659	mov si,nospec_msg
	660	call writemsg
	661	mov al,dl
	662	call writehex2
	663	call crlf_early
	664	mov si,trysbm_msg
	665	call writemsg
	666	jmp .found_drive ; Pray that this works...
	667
	668	fatal_error:
	669	mov si,nothing_msg
	670	call writemsg
	671
	672	.norge: jmp short .norge
	673
	674	; Information message (DS:SI) output
	675	; Prefix with "isolinux: "
	676	;
	677	writemsg: push ax
	678	push si
	679	mov si,isolinux_str
	680	call writestr_early
	681	pop si
	682	call writestr_early
	683	pop ax
	684	ret
	685
	686	writestr_early:
	687	pushfd
	688	pushad
	689	.top: lodsb
	690	and al,al
	691	jz .end
	692	call writechr
	693	jmp short .top
	694	.end: popad
	695	popfd
	696	ret
	697
	698	crlf_early: push ax
	699	mov al,CR
	700	call writechr
	701	mov al,LF
	702	call writechr
	703	pop ax
	704	ret
	705
	706	;
	707	; Write a character to the screen. There is a more "sophisticated"
	708	; version of this in the subsequent code, so we patch the pointer
	709	; when appropriate.
	710	;
	711
	712	writechr:
	713	.simple:
	714	pushfd
	715	pushad
	716	mov ah,0Eh
	717	xor bx,bx
	718	int 10h
	719	popad
	720	popfd
	721	ret
	722
	723	;
	724	; int13: save all the segment registers and call INT 13h.
	725	; Some CD-ROM BIOSes have been found to corrupt segment registers
	726	; and/or disable interrupts.
	727	;
	728	int13:
	729	pushf
	730	push bp
	731	push ds
	732	push es
	733	push fs
	734	push gs
	735	int 13h
	736	mov bp,sp
	737	setc [bp+10] ; Propagate CF to the caller
	738	pop gs
	739	pop fs
	740	pop es
	741	pop ds
	742	pop bp
	743	popf
	744	ret
	745
	746	;
	747	; Get one sector. Convenience entry point.
	748	;
	749	getonesec:
	750	mov bp,1
	751	; Fall through to getlinsec
	752
	753	;
	754	; Get linear sectors - EBIOS LBA addressing, 2048-byte sectors.
	755	;
	756	; Input:
	757	; EAX - Linear sector number
	758	; ES:BX - Target buffer
	759	; BP - Sector count
	760	;
	761	global getlinsec
	762	getlinsec: jmp word [cs:GetlinsecPtr]
	763
	764	%ifndef DEBUG_MESSAGES
	765
	766	;
	767	; First, the variants that we use when actually loading off a disk
	768	; (hybrid mode.) These are adapted versions of the equivalent routines
	769	; in ldlinux.asm.
	770	;
	771
	772	;
	773	; getlinsec_ebios:
	774	;
	775	; getlinsec implementation for floppy/HDD EBIOS (EDD)
	776	;
	777	getlinsec_ebios:
	778	xor edx,edx
	779	shld edx,eax,2
	780	shl eax,2 ; Convert to HDD sectors
	781	add eax,[Hidden]
	782	adc edx,[Hidden+4]
	783	shl bp,2
	784
	785	.loop:
	786	push bp ; Sectors left
	787	.retry2:
	788	call maxtrans ; Enforce maximum transfer size
	789	movzx edi,bp ; Sectors we are about to read
	790	mov cx,retry_count
	791	.retry:
	792
	793	; Form DAPA on stack
	794	push edx
	795	push eax
	796	push es
	797	push bx
	798	push di
	799	push word 16
	800	mov si,sp
	801	pushad
	802	mov dl,[DriveNumber]
	803	push ds
	804	push ss
	805	pop ds ; DS <- SS
	806	mov ah,42h ; Extended Read
	807	call int13
	808	pop ds
	809	popad
	810	lea sp,[si+16] ; Remove DAPA
	811	jc .error
	812	pop bp
	813	add eax,edi ; Advance sector pointer
	814	adc edx,0
	815	sub bp,di ; Sectors left
	816	shl di,9 ; 512-byte sectors
	817	add bx,di ; Advance buffer pointer
	818	and bp,bp
	819	jnz .loop
	820
	821	ret
	822
	823	.error:
	824	; Some systems seem to get "stuck" in an error state when
	825	; using EBIOS. Doesn't happen when using CBIOS, which is
	826	; good, since some other systems get timeout failures
	827	; waiting for the floppy disk to spin up.
	828
	829	pushad ; Try resetting the device
	830	xor ax,ax
	831	mov dl,[DriveNumber]
	832	call int13
	833	popad
	834	loop .retry ; CX-- and jump if not zero
	835
	836	;shr word [MaxTransfer],1 ; Reduce the transfer size
	837	;jnz .retry2
	838
	839	; Total failure. Try falling back to CBIOS.
	840	mov word [GetlinsecPtr], getlinsec_cbios
	841	;mov byte [MaxTransfer],63 ; Max possibe CBIOS transfer
	842
	843	pop bp
	844	jmp getlinsec_cbios.loop
	845
	846	;
	847	; getlinsec_cbios:
	848	;
	849	; getlinsec implementation for legacy CBIOS
	850	;
	851	getlinsec_cbios:
	852	xor edx,edx
	853	shl eax,2 ; Convert to HDD sectors
	854	add eax,[Hidden]
	855	shl bp,2
	856
	857	.loop:
	858	push edx
	859	push eax
	860	push bp
	861	push bx
	862
	863	movzx esi,word [bsSecPerTrack]
	864	movzx edi,word [bsHeads]
	865	;
	866	; Dividing by sectors to get (track,sector): we may have
	867	; up to 2^18 tracks, so we need to use 32-bit arithmetric.
	868	;
	869	div esi
	870	xor cx,cx
	871	xchg cx,dx ; CX <- sector index (0-based)
	872	; EDX <- 0
	873	; eax = track #
	874	div edi ; Convert track to head/cyl
	875
	876	; We should test this, but it doesn't fit...
	877	; cmp eax,1023
	878	; ja .error
	879
	880	;
	881	; Now we have AX = cyl, DX = head, CX = sector (0-based),
	882	; BP = sectors to transfer, SI = bsSecPerTrack,
	883	; ES:BX = data target
	884	;
	885
	886	call maxtrans ; Enforce maximum transfer size
	887
	888	; Must not cross track boundaries, so BP <= SI-CX
	889	sub si,cx
	890	cmp bp,si
	891	jna .bp_ok
	892	mov bp,si
	893	.bp_ok:
	894
	895	shl ah,6 ; Because IBM was STOOPID
	896	; and thought 8 bits were enough
	897	; then thought 10 bits were enough...
	898	inc cx ; Sector numbers are 1-based, sigh
	899	or cl,ah
	900	mov ch,al
	901	mov dh,dl
	902	mov dl,[DriveNumber]
	903	xchg ax,bp ; Sector to transfer count
	904	mov ah,02h ; Read sectors
	905	mov bp,retry_count
	906	.retry:
	907	pushad
	908	call int13
	909	popad
	910	jc .error
	911	.resume:
	912	movzx ecx,al ; ECX <- sectors transferred
	913	shl ax,9 ; Convert sectors in AL to bytes in AX
	914	pop bx
	915	add bx,ax
	916	pop bp
	917	pop eax
	918	pop edx
	919	add eax,ecx
	920	sub bp,cx
	921	jnz .loop
	922	ret
	923
	924	.error:
	925	dec bp
	926	jnz .retry
	927
	928	xchg ax,bp ; Sectors transferred <- 0
	929	shr word [MaxTransfer],1
	930	jnz .resume
	931	jmp disk_error
	932
	933	;
	934	; Truncate BP to MaxTransfer
	935	;
	936	maxtrans:
	937	cmp bp,[MaxTransfer]
	938	jna .ok
	939	mov bp,[MaxTransfer]
	940	.ok: ret
	941
	942	%endif
	943
	944	;
	945	; This is the variant we use for real CD-ROMs:
	946	; LBA, 2K sectors, some special error handling.
	947	;
	948	getlinsec_cdrom:
	949	mov si,dapa ; Load up the DAPA
	950	mov [si+4],bx
	951	mov [si+6],es
	952	mov [si+8],eax
	953	.loop:
	954	push bp ; Sectors left
	955	cmp bp,[MaxTransferCD]
	956	jbe .bp_ok
	957	mov bp,[MaxTransferCD]
	958	.bp_ok:
	959	mov [si+2],bp
	960	push si
	961	mov dl,[DriveNumber]
	962	mov ah,42h ; Extended Read
	963	call xint13
	964	pop si
	965	pop bp
	966	movzx eax,word [si+2] ; Sectors we read
	967	add [si+8],eax ; Advance sector pointer
	968	sub bp,ax ; Sectors left
	969	shl ax,SECTOR_SHIFT-4 ; 2048-byte sectors -> segment
	970	add [si+6],ax ; Advance buffer pointer
	971	and bp,bp
	972	jnz .loop
	973	mov eax,[si+8] ; Next sector
	974	ret
	975
	976	; INT 13h with retry
	977	xint13: mov byte [RetryCount],retry_count
	978	.try: pushad
	979	call int13
	980	jc .error
	981	add sp,byte 8*4 ; Clean up stack
	982	ret
	983	.error:
	984	mov [DiskError],ah ; Save error code
	985	popad
	986	mov [DiskSys],ax ; Save system call number
	987	dec byte [RetryCount]
	988	jz .real_error
	989	push ax
	990	mov al,[RetryCount]
	991	mov ah,[dapa+2] ; Sector transfer count
	992	cmp al,2 ; Only 2 attempts left
	993	ja .nodanger
	994	mov ah,1 ; Drop transfer size to 1
	995	jmp short .setsize
	996	.nodanger:
	997	cmp al,retry_count-2
	998	ja .again ; First time, just try again
	999	shr ah,1 ; Otherwise, try to reduce
	1000	adc ah,0 ; the max transfer size, but not to 0
	1001	.setsize:
	1002	mov [MaxTransferCD],ah
	1003	mov [dapa+2],ah
	1004	.again:
	1005	pop ax
	1006	jmp .try
	1007
	1008	.real_error: mov si,diskerr_msg
	1009	call writemsg
	1010	mov al,[DiskError]
	1011	call writehex2
	1012	mov si,oncall_str
	1013	call writestr_early
	1014	mov ax,[DiskSys]
	1015	call writehex4
	1016	mov si,ondrive_str
	1017	call writestr_early
	1018	mov al,dl
	1019	call writehex2
	1020	call crlf_early
	1021	; Fall through to kaboom
	1022
	1023	;
	1024	; kaboom: write a message and bail out. Wait for a user keypress,
	1025	; then do a hard reboot.
	1026	;
	1027	global kaboom
	1028	disk_error:
	1029	kaboom:
	1030	RESET_STACK_AND_SEGS AX
	1031	mov si,bailmsg
	1032	pm_call pm_writestr
	1033	pm_call pm_getchar
	1034	cli
	1035	mov word [BIOS_magic],0 ; Cold reboot
	1036	jmp 0F000h:0FFF0h ; Reset vector address
	1037
	1038	; -----------------------------------------------------------------------------
	1039	; Common modules needed in the first sector
	1040	; -----------------------------------------------------------------------------
	1041
	1042	%include "writehex.inc" ; Hexadecimal output
	1043
	1044	; -----------------------------------------------------------------------------
	1045	; Data that needs to be in the first sector
	1046	; -----------------------------------------------------------------------------
	1047
	1048	global syslinux_banner, copyright_str
	1049	syslinux_banner db CR, LF, MY_NAME, ' ', VERSION_STR, ' ', DATE_STR, ' ', 0
	1050	copyright_str db ' Copyright (C) 1994-'
	1051	asciidec YEAR
	1052	db ' H. Peter Anvin et al', CR, LF, 0
	1053	isolinux_str db 'isolinux: ', 0
	1054	%ifdef DEBUG_MESSAGES
	1055	startup_msg: db 'Starting up, DL = ', 0
	1056	spec_ok_msg: db 'Loaded spec packet OK, drive = ', 0
	1057	secsize_msg: db 'Sector size ', 0
	1058	offset_msg: db 'Main image LBA = ', 0
	1059	verify_msg: db 'Image csum verified.', CR, LF, 0
	1060	allread_msg db 'Image read, jumping to main code...', CR, LF, 0
	1061	%endif
	1062	noinfotable_msg db 'No boot info table, assuming single session disk...', CR, LF, 0
	1063	noinfoinspec_msg db 'Spec packet missing LBA information, trying to wing it...', CR, LF, 0
	1064	spec_err_msg: db 'Loading spec packet failed, trying to wing it...', CR, LF, 0
	1065	maybe_msg: db 'Found something at drive = ', 0
	1066	alright_msg: db 'Looks reasonable, continuing...', CR, LF, 0
	1067	nospec_msg db 'Extremely broken BIOS detected, last attempt with drive = ', 0
	1068	nothing_msg: db 'Failed to locate CD-ROM device; boot failed.', CR, LF
	1069	trysbm_msg db 'See http://syslinux.zytor.com/sbm for more information.', CR, LF, 0
	1070	diskerr_msg: db 'Disk error ', 0
	1071	oncall_str: db ', AX = ',0
	1072	ondrive_str: db ', drive ', 0
	1073	checkerr_msg: db 'Image checksum error, sorry...', CR, LF, 0
	1074
	1075	err_bootfailed db CR, LF, 'Boot failed: press a key to retry...'
	1076	bailmsg equ err_bootfailed
	1077	crlf_msg db CR, LF
	1078	null_msg db 0
	1079
	1080	bios_cdrom_str db 'ETCD', 0
	1081	%ifndef DEBUG_MESSAGES
	1082	bios_cbios_str db 'CHDD', 0
	1083	bios_ebios_str db 'EHDD' ,0
	1084	%endif
	1085
	1086	alignz 4
	1087	global bios_cdrom
	1088	bios_cdrom: dw getlinsec_cdrom, bios_cdrom_str
	1089	%ifndef DEBUG_MESSAGES
	1090	bios_cbios: dw getlinsec_cbios, bios_cbios_str
	1091	bios_ebios: dw getlinsec_ebios, bios_ebios_str
	1092	%endif
	1093
	1094	; Maximum transfer size
	1095	MaxTransfer dw 127 ; Hard disk modes
	1096	MaxTransferCD dw 32 ; CD mode
	1097
	1098	rl_checkpt equ $ ; Must be <= 800h
	1099
	1100	; This pads to the end of sector 0 and errors out on
	1101	; overflow.
	1102	times 2048-($-$$) db 0
	1103
	1104	; ----------------------------------------------------------------------------
	1105	; End of code and data that have to be in the first sector
	1106	; ----------------------------------------------------------------------------
	1107
	1108	section .text16
	1109
	1110	all_read:
	1111
	1112	; Test tracers
	1113	TRACER 'T'
	1114	TRACER '>'
	1115
	1116	;
	1117	; Common initialization code
	1118	;
	1119	%include "init.inc"
	1120
	1121	; Tell the user we got this far...
	1122	%ifndef DEBUG_MESSAGES ; Gets messy with debugging on
	1123	mov si,copyright_str
	1124	pm_call pm_writestr
	1125	%endif
	1126
	1127	;
	1128	; Now we're all set to start with our real business. First load the
	1129	; configuration file (if any) and parse it.
	1130	;
	1131	; In previous versions I avoided using 32-bit registers because of a
	1132	; rumour some BIOSes clobbered the upper half of 32-bit registers at
	1133	; random. I figure, though, that if there are any of those still left
	1134	; they probably won't be trying to install Linux on them...
	1135	;
	1136	; The code is still ripe with 16-bitisms, though. Not worth the hassle
	1137	; to take'm out. In fact, we may want to put them back if we're going
	1138	; to boot ELKS at some point.
	1139	;
	1140
	1141	;
	1142	; Now, we need to sniff out the actual filesystem data structures.
	1143	; mkisofs gave us a pointer to the primary volume descriptor
	1144	; (which will be at 16 only for a single-session disk!); from the PVD
	1145	; we should be able to find the rest of what we need to know.
	1146	;
	1147	init_fs:
	1148	pushad
	1149	mov eax,ROOT_FS_OPS
	1150	mov dl,[DriveNumber]
	1151	cmp word [BIOSType],bios_cdrom
	1152	sete dh ; 1 for cdrom, 0 for hybrid mode
	1153	jne .hybrid
	1154	movzx ebp,word [MaxTransferCD]
	1155	jmp .common
	1156	.hybrid:
	1157	movzx ebp,word [MaxTransfer]
	1158	.common:
	1159	mov ecx,[Hidden]
	1160	mov ebx,[Hidden+4]
	1161	mov si,[bsHeads]
	1162	mov di,[bsSecPerTrack]
	1163	pm_call pm_fs_init
	1164	pm_call load_env32
	1165	enter_command:
	1166	auto_boot:
	1167	jmp kaboom ; load_env32() should never return. If
	1168	; it does, then kaboom!
	1169	popad
	1170
	1171	section .rodata
	1172	alignz 4
	1173	ROOT_FS_OPS:
	1174	extern iso_fs_ops
	1175	dd iso_fs_ops
	1176	dd 0
	1177
	1178	section .text16
	1179
	1180	%ifdef DEBUG_TRACERS
	1181	;
	1182	; debug hack to print a character with minimal code impact
	1183	;
	1184	debug_tracer: pushad
	1185	pushfd
	1186	mov bp,sp
	1187	mov bx,[bp+9*4] ; Get return address
	1188	mov al,[cs:bx] ; Get data byte
	1189	inc word [bp+9*4] ; Return to after data byte
	1190	call writechr
	1191	popfd
	1192	popad
	1193	ret
	1194	%endif ; DEBUG_TRACERS
	1195
	1196	section .bss16
	1197	alignb 4
	1198	ThisKbdTo resd 1 ; Temporary holder for KbdTimeout
	1199	ThisTotalTo resd 1 ; Temporary holder for TotalTimeout
	1200	KernelExtPtr resw 1 ; During search, final null pointer
	1201	FuncFlag resb 1 ; Escape sequences received from keyboard
	1202	KernelType resb 1 ; Kernel type, from vkernel, if known
	1203	global KernelName
	1204	KernelName resb FILENAME_MAX ; Mangled name for kernel
	1205
	1206	section .text16
	1207	;
	1208	; COM32 vestigial data structure
	1209	;
	1210	%include "com32.inc"
	1211
	1212	;
	1213	; Common local boot code
	1214	;
	1215	%include "localboot.inc"
	1216
	1217	; -----------------------------------------------------------------------------
	1218	; Common modules
	1219	; -----------------------------------------------------------------------------
	1220
	1221	%include "common.inc" ; Universal modules
	1222
	1223	; -----------------------------------------------------------------------------
	1224	; Begin data section
	1225	; -----------------------------------------------------------------------------
	1226
	1227	section .data16
	1228	err_disk_image db 'Cannot load disk image (invalid file)?', CR, LF, 0
	1229
	1230	section .bss16
	1231	global OrigFDCTabPtr
	1232	OrigFDCTabPtr resd 1 ; Keep bios_cleanup_hardware() honest

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