[syslinux] "isolinux.bin missing or corrupt" when booting USB flash drive in old PC

[Martin Str|mberg]

Thomas Schmitt wrote:
> My main point of interest is which block is really loaded as first one
> of isolinux.bin.
[Program snipped.]
> (It could be shorter if i did not insist in big endian words.)

Why insisting?

Anyway appended is a dumper version. I could put up a binery version
if needed. Let me know if so.

-- 
MartinS

/* -----------------------------------------------------------------------
 *
 *   Copyright 2007-2009 H. Peter Anvin - All Rights Reserved
 *   Copyright 2009 Intel Corporation; author: H. Peter Anvin
 *
 *   Permission is hereby granted, free of charge, to any person
 *   obtaining a copy of this software and associated documentation
 *   files (the "Software"), to deal in the Software without
 *   restriction, including without limitation the rights to use,
 *   copy, modify, merge, publish, distribute, sublicense, and/or
 *   sell copies of the Software, and to permit persons to whom
 *   the Software is furnished to do so, subject to the following
 *   conditions:
 *
 *   The above copyright notice and this permission notice shall
 *   be included in all copies or substantial portions of the Software.
 *
 *   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 *   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 *   OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 *   NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 *   HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 *   WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 *   FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 *   OTHER DEALINGS IN THE SOFTWARE.
 *
 * ----------------------------------------------------------------------- */

/*
 * Modified MBR code used on an ISO image in hybrid mode.
 *
 * This doesn't follow the El Torito spec at all -- it is just a stub
 * loader of a hard-coded offset, but that's good enough to load
 * ISOLINUX.
 */

#define DEBUG_PRINT 1

#include "adjust.h"

	.code16
	.text

HYBRID_MAGIC			= 0x7078c0fb
isolinux_hybrid_signature	= 0x7c00+64
isolinux_start_hybrid		= 0x7c00+64+4

	.globl	bootsec
/* Important: the top 6 words on the stack are passed to isolinux.bin */
stack		= 0x7c00
partoffset	= (stack-8)
driveno		= (stack-14)
heads		= (stack-16)
sectors		= (stack-18)
ebios_flag	= (stack-20)
secpercyl	= (stack-24)

BIOS_kbdflags	= 0x417
BIOS_page	= 0x462

	/* gas/ld has issues with doing this as absolute addresses... */
	.section ".bootsec", "a", @nobits
	.globl	bootsec
bootsec:
	.space	512

	.text
	.globl	_start
_start:
	.byte	0x33, 0xed	/* xorw	%bp, %bp */

	cli
	movw	%bp, %ss
	movw	$stack, %sp
	sti
	cld

	/* Check to see if we have a partition table entry */
	xorl	%ebx, %ebx
	xorl	%ecx, %ecx
#ifdef PARTITION_SUPPORT
	andw	%si, %si		/* %si == 0 -> no partition data */
	jz	1f
	testb	$0x7f, (%si)		/* Invalid active flag field? */
	jnz	1f
	cmpb	%cl, 4(%si)		/* Partition type zero == invalid? */
	je	1f
	cmpl	$0x58504721, %eax	/* !GPT signature in EAX? */
	jne	2f
	cmpb	$0xed, 4(%si)		/* EFI partition type? */
	jne	2f
	
	/* We have GPT partition information */
	movl	(32+20)(%si), %ecx
	movl	(36+20)(%si), %ebx

#ifdef DEBUG_PRINT
	push	%si
	call	print_string
	.ascii	"G\r\n"
	pop	%si
#endif
	jmp	1f

	/* We have non-GPT partition information */
2:
#ifdef DEBUG_PRINT
	push	%si
	call	print_string
	.ascii	"M\r\n"
	pop	%si
#endif

	movl	8(%si), %ecx
#endif
1:
	/* We have no partition information */
	pushl	%ebx			/*  -4: partoffset_hi */
	pushl	%ecx			/*  -8: partoffset_lo */
	pushw	%es			/* -10: es:di -> $PnP header */
	pushw	%di			/* -12: es:di -> $PnP header */

	movw	%bp, %ds
	movw	%bp, %es
	
	ADJUST_DRIVE
	pushw	%dx			/* -14: dl -> drive number */

	/* Copy down to 0:0x600 */
	movw	$0x7c00, %si
	movw	$_start, %di
	movw	$(512/2), %cx
	rep; movsw

	ljmpw	$0, $next
next:

	/* Check to see if we have EBIOS */
	pushw	%dx		/* drive number */
	movb	$0x41, %ah	/* %al == 0 already */
	movw	$0x55aa, %bx
	xorw	%cx, %cx
	xorb	%dh, %dh
	stc
	int	$0x13
	jc	1f
	cmpw	$0xaa55, %bx
	jne	1f
	andw	$1,%cx		/* Bit 0 = fixed disk subset */
	jz	1f

	/* We have EBIOS; patch in the following code at
	   read_sector_cbios: movb $0x42, %ah ;  jmp read_common */
	movl	$0xeb42b4+((read_common-read_sector_cbios-4) << 24), \
		(read_sector_cbios)

#ifdef	DEBUG_PRINT
	push	%si
	call	print_string
	.ascii	"E\r\n"
	jmp	2f
1:
	push	%si
	call	print_string
	.ascii	"C\r\n"
2:
	pop	%si
#else
1:	
#endif

	popw	%dx
	pushw	%cx		/* EBIOS flag */

	/* Get (C)HS geometry */
	movb	$0x08, %ah
	int	$0x13
	
	/* here we computer CHS values or just do some dummy computation for EBIOS */
	andw	$0x3f, %cx	/* Sector count */
	popw	%bx		/* EBIOS flag */
	pushw	%cx		/* -16: Save sectors on the stack */
	movzbw	%dh, %ax	/* dh = max head */
	incw	%ax		/* From 0-based max to count */
	pushw	%ax		/* -18: Save heads on the stack */
	mulw	%cx		/* Heads*sectors -> sectors per cylinder */

	pushw	%bx		/* -20: EBIOS flag */
	
	/* Save sectors/cylinder on the stack */
	pushw	%dx		/* -22: High word */
	pushw	%ax		/* -24: Low word */

	/*
	 * Load sectors.  We do this one at a time mostly to avoid
	 * pitfalls and to share code with the stock MBR code.
	 */
	movw	$0x7c00, %bx
	movw	$4, %cx		/* Sector count */
	movl	(lba_offset), %eax

2:
	call	read_sector
	jc	disk_error
	incl	%eax
	addb	$(512 >> 8), %bh
	loopw	2b

	/* Dump out the bytes of the first sector. */
	pusha
	
	mov	$0x7c00, %di
	mov	$16, %dx
2:	
	mov	$16, %cx
1:	
	mov	(%di), %al
	inc	%di
	call	print_al
	loop	1b
	call	print_string
	.ascii	"\r\n"
	dec	%dx
	jnz	2b
	
	call	print_string
	.ascii	"Key?\r\n"
	xor	%ax, %ax
	int	$0x16

	popa

	/*
	 * Okay, that actually worked... update the stack pointer
	 * and jump into isolinux.bin...
	 */
	cmpl	$HYBRID_MAGIC,(isolinux_hybrid_signature)
	jne	bad_signature

	cli
	movw	$ebios_flag, %sp

	/*
	 * Use a ljmpw here to work around a bug in some unknown version
	 * of gas or ld when it comes to jumping to an absolute symbol...
	 *
	 * Look more closely into it if we ever are short on space.
	 */
	ljmpw	$0, $isolinux_start_hybrid

bad_signature:
	call	print_string
	.ascii	"isolinux.bin missing or corrupt.\r\n"

boot_failure:
	int	$0x18		/* Boot failure */
die:
	hlt
	jmp	die

/*
 * read_sector: read a single sector pointed to by %eax to %es:%bx.
 * CF is set on error.  All registers saved.
 */
read_sector:
	pushal
	xorl	%edx, %edx
	addl	(partoffset), %eax
	adcl	(partoffset+4), %edx
	pushl	%edx	/* MSW of LBA */
	pushl	%eax	/* LSW of LBA */
	pushw	%es	/* Buffer segment */
	pushw	%bx	/* Buffer offset */
	pushw	$1	/* Sector count */
	pushw	$16	/* Size of packet */
	movw	%sp, %si

	/* This chunk is skipped if we have ebios */
	/* Do not clobber %eax before this chunk! */
	/* This also relies on %bx and %edx as set up above. */
read_sector_cbios:
	divl	(secpercyl)
	shlb	$6, %ah
	movb	%ah, %cl
	movb	%al, %ch
	xchgw	%dx, %ax
	divb	(sectors)
	movb	%al, %dh
	orb	%ah, %cl
	incw	%cx	/* Sectors are 1-based */
	movw	$0x0201, %ax

read_common:
	movb	(driveno), %dl
	int	$0x13
	leaw	16(%si), %sp	/* Drop DAPA */
	popal
	ret

disk_error:
	call	print_string
	.ascii	"Operating system load error.\r\n"
	jmp	boot_failure


	/*
	* Prints char in al.
	* Destroys AH. Possibly BP.
	*/
wrchr:
	push	%bx
	movb	$0x0e, %ah
	movb	(BIOS_page), %bh
	movb	$0x07, %bl
	int	$0x10		/* May destroy %bp */
	pop	%bx
	ret

	/*
	* Prints byte in AL as hex (e. g. 255 prints FF).
	* Destroys AX.
	*/
print_al:
	push	%ax
	shr	$4, %al
	call	print_nibble
	pop	%ax
print_nibble:
	and	$0x0F, %al
	cmp	$9, %al
	jbe	.wc
	add	$('A'-'0'-10), %al
.wc:
	add	$'0', %al
	call	wrchr
	ret

/* 
 * Print message. This is invoked with "call", with the message at the
 * return address.
 * Destroys SI.
 */
print_string:
	pop	%si
	push	%bp
	push	%ax
	push	%bx
2:
	lodsb
	call	wrchr
	cmpb	$10, %al	/* Newline? */
	jne	2b

	pop	%bx
	pop	%ax
	pop	%bp
	push	%si
	ret

end_of_code:
	/* Address of pointer to isolinux.bin */
lba_offset = _start+432