Boot N64 with boot chip but no ROM chip.

Hello, I am fairly new to modifying anything and I decided I would mod my CD64 plus backup unit.
Without going into too much convoluted details of what I am trying do, the jist of it is that I want to modify an old Winback cartridge w/ 6102 CIC boot to have no save chip or readable rom chip. I can remove the save chip just fine, but as soon as I remove the ROM chip the thing will no longer boot. I am assuming that the rom chip and boot chip must communicate with eachother for some reason. Is there a chip I can use to replace the rom game chip? Thanks!

 

There isn't any single chip you can use to replace a N64 mask ROM - the cartridge bus is a mutiplexed address/data bus with burst mode read support - so basically you need to decode the bus phase, capture the address when it's sent and then return the data during the read cycle. Subsequent reads return the contents of the next location. It's not terribly complicated hardware, but there isn't any off-the shelf chip that implements it.

When you turn the console on, the first thing that happens is that the PIF chip inside the console tries to communicate with the CIC chip in the cartridge - assuming this succeeds, then the PIF asserts the NMI line and the processor starts running the boot code (which is also stored in the PIF) - this code then reads the boot code off the cartridge and executes it. There is a little bit of additional security there, which basically means that you have to use the correct boot code for the CIC (or, in other words, that the CIC type has to match the ROM image).

 

Intesersting, thanks. Are you aware of how the everdrive 64 gets around this limitation? Do they use their own custom PIF chip?

 

The ED64 and older backup units I think use a "boot emulator" or something to that effect to startup software in its expected state. There are some RARE developed games that have more security in their CICs that the game can interact with. Some of those games have cracks available since they were needed for older backup units.

 

No, the PIF has to stay the same, since it's built into the console. Since a flash cart has to be able to emulate those special N64 ROMs anyway, there is no problem with including logic to access an additional ROM with the boot code in it - this can either be boot code specific to the CIC in use, or more commonly is a specially written version that doesn't care what the CIC is - try searching for "LaC's universal boot emulator".

The same approach was used in the import adapters like the Passport 64 - in fact, when cross booting a NTSC game on a PAL console or vice versa the boot code will always be wrong, since the PAL and NTSC versions use a different series of CICs.

As Mottzilla mentioned above, there were also some additional checks in certain games - but these didn't affect the boot process and were carried out by the game code after the game had booted.

 

I suspect this has to do with the cd64 its self, It checks the cart name from the mask rom when booting, Its then displayed on the left hand side of the screen (under card name). If theres no mask rom then these checks fail, thus displays nothing. Like TriMesh says you can't replace the rom with anything else other than another mask rom from another n64 cart. For example swap a mask rom from a Mario 64 with one from golden eye. Provided the cic are the same n the save chip is the same there should be no reason that shouldn't work.

 

That's a good point, actually - it's probably checking the ROM to determine what sort of CIC is installed. I suppose one way to test that would be to get a cart and swap out the CIC with a different one, then see if the menu boots. If so, then check if you can load a game and run it.

1) Menu doesn't boot - the copier is using the boot code from the cart for it's own boot process
2) Menu boots, game doesnt - the copier is using universal boot code, but is using the ROM type as a hint for the CIC type
3) Both menu and game boots - this would be confusing...

 

I suppose I should explain what I am trying to do you sound like some very knowledgeable people. The CD64 had an accessory called the "deprotect cart" it has two slots the top... the top slot is for a 6102 boot cartridge and the bottom is for any cartridge with a different boot chip that you wanted to load into the CD64 to back up the games. I am tying to modify he cd64 to do the same thing but without the clunky deprotect cart. (jumpering the contacts of a 6102 carts from inside the CD64 so it can boot up without a cartridge, but also be able to load the rom from any other cartridge I put into the external slot. The problem I am having is I obviously can't have to CD64 reading two Roms at once. . I should also note I swapped the 6102 chip with the ultracic 2 so I can finally play banjo Tooie on the dam thing lol.