Homebrew Mega-CD import mod

[Skullmonkey]

Hope this isn't too silly a question, but I have the decided to go ahead with my homebrew switchmod for my PAL Mega-CD, but I just need some help with which of the two chips would work the best:

1) 27C4001 512K x 8 bit 100ns EPROM (4Mbit)
2) 27C4002 256K x 16 bit 100ns EPROM (4Mbit)

I know that I only need 3 bios banks, so the 4th position will just be a double of one of the other 3 zones: ie 1xJAP, 1xPAL 2xUS or similar.  Was wanting to utilise this wiring setup found here: http://arakon.dyndns.org/tutorial/bios.html.

He seems to just be using the - Address 16 and 17 inputs to divide a 4Mbit chip into 4 parts.

Maybe its just that its really late atm and I'm missing something overly obvious, but will stacking 4 bios onto one of the above have better/worse results than the other. 4 x 128K = 512K = 4Mbit.

Will either work just as well, or will I need to change the wiring.  I was leaning towards the 27C4002, but will it change the wiring?

I guess I don't quite get why the switching creates the 4 banks, and also will one being 8bit and the other 16bit change how it all works, not to mention?

Will be greatfull for any light some of the more knowledgeable people around here could shed on this for me.

Cheers

[Skullmonkey]

Hope this isn't too silly a question, but I have the decided to go ahead with my homebrew switchmod for my PAL Mega-CD, but I just need some help with which of the two chips would work the best:

1) 27C4001 512K x 8 bit 100ns EPROM (4Mbit)
2) 27C4002 256K x 16 bit 100ns EPROM (4Mbit)

Doh, partially answered my own question, in that the choice of chip is obvious, its the 4002, as the 4001 is only 32DIP, but I need 40DIP so the 4002 it is.  This is why its a good reason to go to bed before it hits 3am, instead of trying to soldier on and finish the problem   .

Still not sure about WHY the switches work to divide the chip into 4 equal memory banks, but I will either figure that out with further checking of the datasheet, or just buy and flash the chip, install as per the wiring diagram and see what hapens.

Will keep you all up to date as things change or are achieved.  But if anyone knows about how the wiring and banking works, please feel free to speak up and let me know.

Cheers