Re: Custom boot loader stub

[Martin Husemann <martin%duskware.de@localhost>]

On Thu, Oct 16, 2025 at 12:22:36PM +0100, Sad Clouds wrote:
> The layout of blocks on disk should look like this:
> 
>         1                 2 - 8           => 512-byte sector/block numbers
> +----------------+---------------------+
> | Sun Disk Label | bootblk flat binary |
> +----------------+---------------------+
>  0            511 512              8191   => LBA offsets in bytes
> 
> 
> The first sector (1) is occupied by Sun disk label, which is padded to
> 512 bytes. This must have the correct magic value, etc, otherwise
> OpenBoot reports errors.
> 
> Sectors 2 - 8 contain bootblk binary, which can be maximum 7.5 KiB in
> size. Although I'm not sure how OpenBoot figures out its size so it can
> correctly map text and data segments in memory for execution.

That "flat binary" in our case is /usr/mdec/bootblk

-r-xr-xr-x  1 builds  builds    7550 Apr 25 09:58 bootblk*

which is a tokenized FCode object file (created by $TOOLDIR/bin/nbfgen
from the source I referenced earlier, src/sys/arch/sparc/stand/bootblk/bootblk.fth
with a bit of constants generated automatically)

> hexdump -C bootblk | head
00000000  fd 08 b4 9e 00 00 1d 7e  cc a5 b6 0b 62 6f 6f 74  |.......~....boot|
00000010  2d 64 65 62 75 67 3f 08  00 b8 b6 02 4b 42 08 01  |-debug?.....KB..|
00000020  b7 10 00 00 04 00 20 c2  12 07 2f 63 68 6f 73 65  |...... .../chose|
00000030  6e 02 04 34 14 00 1a 12  13 43 61 6e 6e 6f 74 20  |n..4.....Cannot |
00000040  66 69 6e 64 20 2f 63 68  6f 73 65 6e 90 a5 b2 b6  |find /chosen....|
00000050  0e 63 68 6f 73 65 6e 2d  70 68 61 6e 64 6c 65 08  |.chosen-phandle.|
00000060  02 ba 12 19 2f 6f 70 65  6e 70 72 6f 6d 2f 63 6c  |..../openprom/cl|
00000070  69 65 6e 74 2d 73 65 72  76 69 63 65 73 02 04 34  |ient-services..4|
00000080  14 00 24 12 1b 43 61 6e  6e 6f 74 20 66 69 6e 64  |..$..Cannot find|
00000090  20 63 6c 69 65 6e 74 2d  73 65 72 76 69 63 65 73  | client-services|

and as far as I can tell it just finds the /ofwboot binary file and loads
it into memory, then evaluates it. I can't see any ELF or a.out specific
handling in there, so I guess OF must do binary format interpretation.

Uwe certainly can tell us this in more details and w/o guessing.

Martin