In preparation for an upcoming video, I decided to get some experience with 6502 assembler and NES development. I built a ROM that will take a 24 character password like in Metroid or Kid Icarus and change it to either use Space or - characters where possible. It kinda simulates how spaces were a valid character in Metroid passwords whose bit value of %111111 was the same as -, but had the additional behavior of setting the prior character’s lower bits to on. Download the ROM here and try it out in you own emulator!

In the example ROM, I use the exclamation point ! character instead of the - to make the change more visually interesting and because exclamation points are cool. ! in Kid Icarus has the same bit value as - in Metroid.

Process

• I used 8bitworkshop’s IDE to write the code and build the ROM.
• I learned the basics of 6502 assembler from Easy 6502.
• I used these two 6502 opcode references to know how to make the CPU do things:
  • http://www.6502.org/tutorials/6502opcodes.html
  • http://www.obelisk.me.uk/6502/reference.html
• I used the PPU reference on the NESDev Wiki to understand the PPU enough to make the demo.

Code

• https://github.com/TheIndustriousRabbit/nes-password-rom

References

• http://games.technoplaza.net/mpg/password.txt

Changelog

• 2021-07-15: Initial post.
• 2021-07-16: ROM updated to better support Nestopia. It didn’t like the palette definition for some reason. Thanks Tyrel!