Some kind chap over at Neo-Arcadia has taken the trouble to dump the 2009 Mod BIOS and share. ^_^
Prerequisites.
G-Net Motherboard
PCMCIA Compact Flash Adapter
Compact Flash card
Windows PC capable of writing to the Compact Flash card
1 x Blank 27C800 EPROM
EPROM burner compatible with 27C800 EPROMs
G-Net Mod BIOS
G-Net Flashing software (this is to put the games on to the CF card)
G-Net CHDs (zip archive containing all chd files)
Update the G-Net BIOS.
Download this BIOS file and burn to an blank M27C800
Remove any PCMCIA cards from the G-Net system
Bridge Jumper JP1
Insert the M27C800
Power Up
There will be three progress bars across the screen and then a message in Japanese telling you the flash is complete. Power off the system and remove the jumper. The system has now been updated.
Putting the games on the cards.
Download and extract the G-Net Flashing software and the chd pack
Open up the flasher, select the chd file and write it to the card. Once complete insert in to the G-Net and power up.
Non Windows users should use chdman to extract the image file from the chd. (You may need to compile/build this from the MAME surce code)
chdman usage;
chdman -extract game.chd game.img
Once extracted write the game.img image to the CF card using the dd command or and image writer
If you get 000001 SYSTEM ERROR It is likely that the CF card you are using is not adequate. Sandisk (64MB - 2GB are preferred) If the message 000777 SYSTEM ERROR is displayed then the mod BIOS hadn't been correctly flashed.
If anything goes wrong or you want to return to stock flash the original V2.0 BIOS. This and the flashing guide can be found here.
Thanks to aje_fr at the Neo-Arcadia forums for dumping the BIOS.
Update.
Here is a link to the 2011 version if the netload link dies. Thanks. :)
G-Net Mod BIOS - 2011 version
