yea. well the reason i am asking questions like this, because in my spare time, i have a 512k system here, i was going to start modding it. keep it strictly for a mod machine.
my goal is to make a SCSI system like they did back then with 128K ROM. but i am going to code an Atmega128 to emulate the 5380 SCSI controller and the SCSI drive interface, so i could use a USB flash drive, or SD memory, or for simpler reasons, ATA CF card. or basically like a SCSI --> USB mass storage
Heck i could even do an ATmega256 and put the ROM in the flash with everything else to minimize chip count down to a mere 1. so everything is all on 1 chip. problem with that though is the number of pins to address full ROM to the 68K CPU. but i doubt it'll be an issue. Also need to figure out the timing diagram to the original apple ROMs so i can emulate that, basically figure out when a valid address is given when it should return valid data and upon what state the CS/OE is in. and probably put the CS/MAD logic in the CPU code itself as a state machine, so it knows when its talking with ROM area or SCSI area. run the controller at a full 16mhz should be plenty fast enough for everything to happen. nice thing with the 5XXXXX area being open, i can do lots more than just SCSI, but my issue is drivers. not a very good driver writer, still need to learn the architecture of MacOS and how its drivers are implemented, etc etc. probably have to read the inside macintosh manuals from the time.
my goal is to make a SCSI system like they did back then with 128K ROM. but i am going to code an Atmega128 to emulate the 5380 SCSI controller and the SCSI drive interface, so i could use a USB flash drive, or SD memory, or for simpler reasons, ATA CF card. or basically like a SCSI --> USB mass storage
Heck i could even do an ATmega256 and put the ROM in the flash with everything else to minimize chip count down to a mere 1. so everything is all on 1 chip. problem with that though is the number of pins to address full ROM to the 68K CPU. but i doubt it'll be an issue. Also need to figure out the timing diagram to the original apple ROMs so i can emulate that, basically figure out when a valid address is given when it should return valid data and upon what state the CS/OE is in. and probably put the CS/MAD logic in the CPU code itself as a state machine, so it knows when its talking with ROM area or SCSI area. run the controller at a full 16mhz should be plenty fast enough for everything to happen. nice thing with the 5XXXXX area being open, i can do lots more than just SCSI, but my issue is drivers. not a very good driver writer, still need to learn the architecture of MacOS and how its drivers are implemented, etc etc. probably have to read the inside macintosh manuals from the time.