[syslinux] simple message under BIOS/CSM/LEGACY

[Pascal]

here is what I plan to test :
leave the protective partition in place (1), mark it as active (2) and
change its first sector to zero (3).

1) its absence seems to be a problem (at least with qemu/ovmf),
2) gdisk does not activate it by default,
3) the MBR and the first sector of the active partition will be confused
and will both have the boot strap code (BSC eg. the code) to display the
warning.



*if the BIOS needs nothing but the 0x55AA signature, it loads MBR and runs
BSC.if the BIOS needs a (table of) partition before loading MBR and handing
over to BSC, it has it.if the BIOS needs an active partition, it has it and
if it loads its PBR instead of MBR, it has it too.*

the only point that seems contentious to me is whether a partition can have
its first sector set to zero ?
does the BIOS check this kind of thing ?

regards, lacsaP.

Le mar. 2 août 2022 à 11:55, Pascal <patatetom at gmail.com> a écrit :

> I use gdisk which adds protective MBR which could be problematic in this
> case.
> this false partition is not marked as active but its presence could be
> annoying.
> thanks for this return.
>
> Le mar. 2 août 2022 à 08:40, Gregory Bartholomew <
> gregory.lee.bartholomew at gmail.com> a écrit :
>
>> Well, I'm still just guessing, but some BIOS systems might execute a PBR
>> (partition boot record) instead of the MBR if one exists and the partition
>> is marked active. You might need to make sure that there is no DOS
>> partition table on the system (or at least that none of the partitions are
>> marked active).
>>
>> On Tue, Aug 2, 2022 at 1:25 AM Pascal <patatetom at gmail.com> wrote:
>>
>>> yes, my live (Linux USB) system is UEFI-only, but this tiny MBR boot
>>> code is intended to be run/executed on a pc that boots under BIOS and not
>>> under UEFI :
>>> *"hey, your pc boots in BIOS : if you want to play with me, boot in UEFI
>>> ;-)"*
>>> it works well under some BIOS and not under others where it displays
>>> nothing or just a character that looks like a smiley.
>>>
>>> Le lun. 1 août 2022 à 23:35, Gregory Bartholomew <
>>> gregory.lee.bartholomew at gmail.com> a écrit :
>>>
>>>> On Mon, Aug 1, 2022 at 2:54 AM Pascal via Syslinux <
>>>> syslinux at syslinux.org> wrote:
>>>>
>>>>> hi,
>>>>>
>>>>> I have a live system with only UEFI boot supported.
>>>>>
>>>>> to indicate this to users who have not yet switched to UEFI, I have
>>>>> below
>>>>> this simple little piece of code (partly found on the internet) that I
>>>>> compile with nasm and I push onto the MBR.
>>>>>
>>>>> it runs perfectly on some PCs and in virtual machine with Qemu, but
>>>>> refuses
>>>>> to run on other PCs by displaying either a kind of smiley logo or a
>>>>> simple
>>>>> blinking cursor.
>>>>>
>>>>> does anyone have any idea or clue about this?
>>>>>
>>>>>
>>>> I guess a truly UEFI-only system won't execute a MBR first-stage
>>>> bootloader. You'd probably have to compile your program as an EFI
>>>> executable and put it on the ESP on such systems. :)
>>>>
>>>