NROM running CNROM code without a 74HC161 ?

nintendo2600 · Post by **nintendo2600** » Sat Jul 04, 2009 6:40 pm

Hey,

Is it possible to rewire an NROM board without adding a 161 to make it be able to run CNROM code? If so anyone care to share the info?

Thanks In Advance!

Post by **tepples** » Sat Jul 04, 2009 8:01 pm

I know of one CNROM game that appears in several multicarts along with NROM games: Tetяis by Tengen.[1] The game ordinarily has 16 KiB of CHR, but in the pirate version, any screen that uses the "other" bank was reprogrammed to use the "playfield" bank.

[1] This is commonly listed as "Tetris 2" on menus because it came out after BPS's version. It is unrelated to Bombliss and Tetris Flash, the games that were licensed by Elorg to use the name "Tetris 2".

MottZilla · Post by **MottZilla** » Sat Jul 04, 2009 8:16 pm

Some CNROM games, maybe all of them, will check the contents of CHR-ROM and hang if it doesn't read the value it wants. I noticed this during emulator development.

Post by **tepples** » Sun Jul 05, 2009 6:07 am

MottZilla wrote:Some CNROM games, maybe all of them, will check the contents of CHR-ROM and hang if it doesn't read the value it wants. I noticed this during emulator development.

This has two causes:

Some CNROM games have 8 KiB or 16 KiB of unique actual CHR data. They use diodes and such to set up the enable lines in such a way that only some banks actually get decoded, and the game checks this for copy authentication.
Several CNROM games, such as Milon's Secret Castle, read data back from CHR ROM much like Super Mario Bros. does for its title screen data.

The version of Tetяis that made it onto pirate multis appears to use neither technique.