Is there any information on how this is done?
The loader gets the disc type. If the inserted disc is an audio CD, it tries to load an ELF/KELF from supported devices.
In the above video, it loads
Code:
hdd0:__system:pfs:/osd100/hosdsys.elf
with params
Code:
hdd0:__system:pfs:/osd100/hosdsys.elf
and
If the files cannot be found, it goes to the XMB.
The HDDOSD resource files were copied to the HDD (osd100 folder in the system partition) as well.
Some files you may want to toy with.
osd100/hosdsys.elf is the original HDDOSD ELF, re-signed to decrypt on both the PS2 and the PSX.
MBR.ELF is the original MBR ELF in its decrypted form. Could be, for example, run from FMCB or signed then run from the XMB.
HDDOSD.ELF is the HDDOSD ELF, with hardcoded params and a patch so it runs with the US languages set, VMODE and button mapping when executed in a Japanese system. Running it in a PSX2 will glitch the XMB, since the system software v2.xx partly handles language variables according to the non-volatile memory language setting. At this point it's better to use this shit in a PSX1 lol.