Re: simple message under BIOS/CSM/LEGACY

[Pascal]

here is the assembly code used to generate the "special" MBR which includes
the boot strap warning and
the active protective partition :

; nasm bioswarn.asm -f bin -o bioswarn.mbr
; qemu-system-x86_64 bioswarn.mbr

bits 16
org 0x7C00

cli
cld
; <
https://stackoverflow.com/questions/32508919/how-to-produce-a-minimal-bios-hello-world
;xor ax, ax
;mov ax, ds
; end >
; < https://www.apriorit.com/dev-blog/66-develop-boot-loader
mov ax, cs
mov ds, ax
mov es, ax
mov ss, ax
mov bp,0x7C00
mov sp,0x7C00
; end >
; < https://jbwyatt.com/253/emu/8086_bios_and_dos_interrupts.html#int10h_01h
mov ch, 0x20
mov ah, 0x01
int 10h
; end >
mov si, warn
mov ah, 0x0E

loop: lodsb
or al, al
jz halt
int 0x10
jmp loop

halt: hlt
warn: db 0x0A, 0x0A, "BIOS/CSM/LEGACY boot no longer supported : switch to
UEFI boot !", 0

; padding
times 0x01BE - ($ - $$) db 0

; partition #1
db 0x80       ; active
db 0x00       ; Head
db 0x00       ; Sector
db 0x00       ; Cylinder
db 0xEE       ; type GPT
db 0xFF       ; H
db 0xFF       ; S
db 0xFF       ; C
dd 0x00000000 ; LBA start
dd 0xFFFFFFFF ; sectors
; partitions #2, #3 & #4
times 0x30 db 0 ; 16 bytes x 3

# boot signature
dw 0xAA55

it works under qemu : I just have to test it in real life... ;-)

your comments and feedback will be greatly appreciated !

regards, lacsaP.

Le mer. 3 août 2022 à 16:11, Pascal <patatetom@gmail.com> a écrit :

> *this is python biting its tail ;-)*
>
> 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 loader (BSL eg. the code) to display the
> warning.
>
>
>
> *if the BIOS needs nothing but the 0x55AA signature, it loads MBR and runs
> BSL.if the BIOS needs a (table of) partition before loading MBR and handing
> over to BSL, 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 ?
>
> will give feedback...
>
> regards, lacsaP.
>
> Le mar. 2 août 2022 à 23:55, Pascal Hambourg <pascal@plouf.fr.eu.org> a
> écrit :
>
>> Le 02/08/2022 à 12:56, Pascal wrote :
>> >
>> > Gregory(@syslinux) highlights the PBR :
>> > *"""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.
>>
>> That sounds horribly broken. Conversely, some BIOS won't boot if no
>> partition is marked active in the MBR. Granted, that's equally broken.
>>
>>