This is a fork / modification of Just4Fun's excellent firmware for their MBC2 project:

https://hackaday.io/project/159973-z80-mbc2-4ics-homemade-z80-computer

Changes:

• now targetted to an Atmega644 (from an Atmega32A)
  • The '644 has more flash for more storage (64k over 32k)
  • I had one on hand
  • This may also add support for other similar 40-pin Atmegas
• added new IO opcodes that allow EEPROM access
  • a new write opcode that sets the "EEPROM cursor"
  • two more opcodes that read from or write to the current cursor position
  • not the most sophisticated method, but it got things working quickly
  • this scratchpad EEPROM area is 256 bytes long, out of 512 total

Future Changes:

• bulk read and write to EEPROM opcodes
• boot option to copy EEPROM scratch pad to RAM at start-up and then jump execution there
  • this will allow user-definable boot loaders
• I2C connected character devices
  • 40x4 character LCD
  • ps2 keyboard (or 8x8 scanned keyboard)
• a different FORTH that isn't so ornery