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...t; isohdpfc pushes the CX value to the stack which it gets from INT 13 AH 41. > Quite surely bit 0 of that CX is not set. But bit 2 "Enhanced Disk Drive" > could be set. > https://en.wikipedia.org/wiki/INT_13H > > I understand on David's BIOS after > > andw $1,%cx /* Bit 0 = fixed disk subset */ > jz 1f > > jz jumps because CX is 0. This gets pushed to the stack. > (So the code around the int 13 assumes that either int 13 fails and returns > CX == 0, or the reply is good enough to reach "andw 1,%cx"....

Hi, i am looking now at the code which i assume loads the rest of isolinux.bin. The entry point for program execution from the MBR is obviously at http://git.zytor.com/syslinux/syslinux.git/tree/core/isolinux.asm#n186 (Do i get it right that this is the Intel syntax ? (Gronfff)) If POP yields the victim of the most recent not yet popped PUSH, then this does not look correctly coordinated

...value to the stack which it gets from INT 13 AH 41. >> Quite surely bit 0 of that CX is not set. But bit 2 "Enhanced Disk Drive" >> could be set. >> https://en.wikipedia.org/wiki/INT_13H >> >> I understand on David's BIOS after >> >> andw $1,%cx /* Bit 0 = fixed disk subset */ >> jz 1f >> >> jz jumps because CX is 0. This gets pushed to the stack. >> (So the code around the int 13 assumes that either int 13 fails and returns >> CX == 0, or the reply is good enough to reach &quo...

...isolinux.bin on our virtual BIOSes: isohdpfc pushes the CX value to the stack which it gets from INT 13 AH 41. Quite surely bit 0 of that CX is not set. But bit 2 "Enhanced Disk Drive" could be set. https://en.wikipedia.org/wiki/INT_13H I understand on David's BIOS after andw $1,%cx /* Bit 0 = fixed disk subset */ jz 1f jz jumps because CX is 0. This gets pushed to the stack. (So the code around the int 13 assumes that either int 13 fails and returns CX == 0, or the reply is good enough to reach "andw 1,%cx". Ewww ...) Now in isolin...

...2, %ah ; jmp read_common */ movl $0xeb42b4+((read_common-read_sector_cbios-4) << 24), (read_sector_cbios) - jmp 1f + jmp 2f 1: + xor %cx, %cx /* Clear EBIOS flag. */ +2: popw %dx pushw %cx /* EBIOS flag */ /* Get (C)HS geometry */ movb $0x08, %ah int $0x13 - 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 */ + pushw %ax /* -16: Save heads on...

...00110000 00110000 00110000 980af9d0 0x0033ee00: 9806d679 980af9d0 0033ee3c 98077c22 0x0033ee10: 0011001c 00000001 00000000 00000000 0200: sel=1007 base=981bc000 limit=00001f97 32-bit rw- Backtrace: file_set_error: Bad address =>1 0x0040664d (0x0033f1dc) file_set_error: Bad address 0x0040664d: andw $0,0x0(%esi) Wine-dbg>bt Backtrace: file_set_error: Bad address =>1 0x0040664d (0x0033f1dc) file_set_error: Bad address Wine-dbg>q WineDbg terminated on pid 0xc ----------------- Running random other Setup.exe's hosted on that CDROM produced roughly the same results: the fault...

...gmail.com> --- mbr/isohdpfx.S | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/mbr/isohdpfx.S b/mbr/isohdpfx.S index 17e1efe..f9e9691 100644 --- a/mbr/isohdpfx.S +++ b/mbr/isohdpfx.S @@ -175,12 +175,12 @@ next: /* Get (C)HS geometry */ movb $0x08, %ah int $0x13 - 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 */ + pushw %ax /* -16: Save heads on the stack */ + andw $0x3f...

...uot;, "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...

...en wrote: > No carry. > 3000 > AA55 > 0004 > 0080 > I die now Hah ! CX announces no LBA addressing but Enhanced Disk Drive Support. https://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D41h:_Check_Extensions_Present So isohdpfc pushed CX = 4 onto the stack, because it skipped andw 1,%xc So isolinux.bin read in "EHDD" mode by LBA and succeeded although it did not announce this capability. isohdpfd reached the "andw" instruction. So CX was set to (1 & 4) = 0 before it got pushed. So isolinux.bin read in "CHDD" mode by C/H/S and thus ran into...

...hich is invariant under swapping, there remains as suspect in read_sector_cbios only this division: divb (sectors) There is a suspicious discrepancy in the code: heads = (stack-16) sectors = (stack-18) ... /* 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 */ Does &...

It is good to see this ----- Forwarded message from syslinux-bot for Martin Str|mberg <ams at ludd.ltu.se> ----- Date: Sat, 25 Mar 2017 05:54:09 -0700 From: syslinux-bot for Martin Str|mberg <ams at ludd.ltu.se> To: Syslinux commits mailing list <syslinux-commits at zytor.com> Cc: ams at ludd.ltu.se Subject: [syslinux:master] mbr/isohdpfx.S: correct pointer for heads/sectors

...scii " =$PnP\0" scn_pnp: call crlf /* $PnP Scan */ movw $0xf000, %dx pushw %dx popw %es movw $0, %si movw $0x1000, %cx /* 0x506E5024 */ movw $0x5024, %dx movw $0x506E, %bx ch_pnp: es lodsw cmpw %dx, %ax jne ch_pnp_l es lodsw cmpw %bx, %ax je pr_pnp ch_pnp_l: addw $14, %si andw $0xFFF0, %si loopw ch_pnp jmp end pr_pnp: pushw %si call wrstr .ascii "$PnP-\0" movw %es, %dx call wrhexw movb $':, %al call wrchr popw %dx andw $0xFFF0, %dx call wrhexw #endif /* DEBUG_PNP */ call crlf end: jmp bootfail ASCII_MARKER1 "wc" wrchr: movb $0x0...

...leaq 24(%rsp), %r10 1800a9f75: 4c 2b d0 subq %rax, %r10 1800a9f78: 4d 0f 42 d3 cmovbq %r11, %r10 1800a9f7c: 65 4c 8b 1c 25 10 00 00 00 movq %gs:16, %r11 1800a9f85: 4d 3b d3 cmpq %r11, %r10 1800a9f88: 73 15 jae 21 <__chkstk+0x3F> 1800a9f8a: 66 41 81 e2 00 f0 andw $61440, %r10w 1800a9f90: 4d 8d 9b 00 f0 ff ff leaq -4096(%r11), %r11 1800a9f97: 45 84 1b testb (%r11), %r11b 1800a9f9a: 4d 3b d3 cmpq %r11, %r10 1800a9f9d: 75 f1 jne -15 <__chkstk+0x30> 1800a9f9f: 4c 8b 14 24 movq (%rsp), %r10 1800a9fa3: 4c 8b 5c 24 08 movq...

On Wed, Mar 22, 2017 at 12:06:12PM +0100, Thomas Schmitt via Syslinux wrote: > There is a suspicious discrepancy in the code: > > heads = (stack-16) > sectors = (stack-18) > > ... > /* 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 -> s...

Hi, Martin Str|mberg wrote: > Given David's last report the part: > - jmp 1f > + jmp 2f > 1: > + xor %cx, %cx /* Clear EBIOS flag. */ > +2: > > can be ignored. His BIOS isn't corrputing CX (which I > thought). However that part doesn't hurt if it remains. I think it still has good reason to exist. The test cmpw

...n Sat, Mar 25, 2017 at 09:28:16PM +0100, Thomas Schmitt via Syslinux wrote: > I think it still has good reason to exist. The test > > cmpw $0xaa55, %bx > jne 1f > > could cause a jump which could push a non-zero CX to the stack. Yes. But that can be fixed by putting the "andw $1, %cx" test before the "cmpw $0xaa55, %bx" test. But then if CX is filled with rubbish and carry is set (the first test) could lead to 0 != CX... I have no idea if this can happen. As you say, as is is the most safe. -- MartinS

Hi, Geert Stappers wrote: > Contact David off-list, he did unsubscribe. Can you tell when he unsubscribed (i.e. which of the messages he got as last one) ? > Martin posted elsewhere in this thread "Thomas, you found the bug" > and provided a patch. Open question is whether the fix helps with David's BIOS. The newest answer from Martin indicates that a failure to read

...try length between the MBR and GPT entries. That's four more bytes. So 16+4=20. I think si points at the fake MBR entry at the insruction you ask about. But isohdpfx.S is buggy: /* 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...

...; +2: + call print_string + .ascii "Key?\r\n" + pop %si + xor %ax, %ax + int $0x16 +#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 */ @@ -223,9 +257,15 @@ next: ljmpw $0, $isolinux_start_hybrid bad_signature: - call error + call print_string .ascii "isolinux.bin missing or corrupt.\r\n" +boot_failure: +...

...+ */ + movb $0x48, %ah + movw %sp, %si + jmp 2f /* this can be replaced by a .byte 0xbf to gain + * another byte hidding next instruction */ +1: /* Get (C)HS geometry */ movb $0x08, %ah +2: int $0x13 + + /* here we computer CHS values of just do some dummy computation for EBIOS */ andw $0x3f, %cx /* Sector count */ - movw %sp, %bp /* %bp -> frame pointer: LEAVE UNCHANGED */ - pushw %cx /* -2(%bp) Save sectors on the stack */ + pushw %cx /* -32(%bp) Save sectors on the stack */ movzbw %dh, %ax /* dh = max head */ incw %ax /* From 0-based max to count */ mulw %cx /* He...