Converting a 16bit eprom to run as 8bit

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Drakon
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Converting a 16bit eprom to run as 8bit

Post by Drakon » Wed Mar 20, 2013 3:39 pm

I stumbled across this:

http://www.jammaplus.co.uk/forum/forum_ ... =starfox-2

Could someone please explain how on earth you convert a 16 bit eprom to 8 bit? And is it simple?

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Re: Converting a 16bit eprom to run as 8bit

Post by 3gengames » Wed Mar 20, 2013 3:44 pm


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blargg
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Re: Converting a 16bit eprom to run as 8bit

Post by blargg » Wed Mar 20, 2013 6:11 pm

I'm assuming that you just ignore the upper 8 bits of each word (leave unconnected), and adjust the image you send to the programmer.

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getafixx
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Re: Converting a 16bit eprom to run as 8bit

Post by getafixx » Wed Mar 20, 2013 7:57 pm

Didn't magno perfect using the 27C322 chips before? I know he has eluded to it a number of times that he has a working design on how to wire them to a cart. Drakon, if you search through this forum I believe there is some info about it.

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mikejmoffitt
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Re: Converting a 16bit eprom to run as 8bit

Post by mikejmoffitt » Wed Mar 20, 2013 10:26 pm

I'm a little confused - this example with Star Fox 2 isn't supposed to involve what you are talking about, is it? I see a 16-bit EPROM on his SF2 PCB, unless you are talking about his other board.

He probably just has the upper 8 lines ignored. If it's for use in an NES/FC cartridge, you could put your CHR data in the upper 8 bits and your PRG data in the lower 8 bits to make a merged 16-bit file to put on a single EPROM, though be sure to mirror your data to fill extra space (I.E. if you have an 8MBit (1MByte) 16-bit ROM, 512KByte PRG and 128KByte CHR, CHR must be copied four times to make 512KByte).

If it is for an SFC / SNES cartridge, what would you be doing with an 8-bit ROM?

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Re: Converting a 16bit eprom to run as 8bit

Post by blargg » Wed Mar 20, 2013 11:25 pm

mikejmoffitt wrote:If it's for use in an NES/FC cartridge, you could put your CHR data in the upper 8 bits and your PRG data in the lower 8 bits to make a merged 16-bit file to put on a single EPROM.
This doesn't make sense. Why would you put CHR data into the upper bytes?
If it is for an SFC / SNES cartridge, what would you be doing with an 8-bit ROM?
You're asking why you would use an 8-bit ROM in a SNES cartridge? Because it has an 8-bit bus?

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Re: Converting a 16bit eprom to run as 8bit

Post by tepples » Wed Mar 20, 2013 11:38 pm

blargg wrote:You're asking why you would use an 8-bit ROM in a SNES cartridge? Because it has an 8-bit bus?
Its competitor has a 16-bit bus. Genesis does what Nintendon't.

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Re: Converting a 16bit eprom to run as 8bit

Post by 3gengames » Thu Mar 21, 2013 12:15 am

There's a byte pin to select bit size, guys.

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Re: Converting a 16bit eprom to run as 8bit

Post by TmEE » Thu Mar 21, 2013 6:23 am

But not on a 27C322.

On other chips there is, and it turns D15 into an additional address pin, usually called A-1. D8 to D14 will do nothing in that mode.

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Re: Converting a 16bit eprom to run as 8bit

Post by mikejmoffitt » Thu Mar 21, 2013 7:34 am

blargg wrote:
mikejmoffitt wrote:If it's for use in an NES/FC cartridge, you could put your CHR data in the upper 8 bits and your PRG data in the lower 8 bits to make a merged 16-bit file to put on a single EPROM.
This doesn't make sense. Why would you put CHR data into the upper bytes?
Christ, I was tired and that makes no sense, disregard that. That would only work if you were for some reason reading from PRG the exact same address as CHR at all times.

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Re: Converting a 16bit eprom to run as 8bit

Post by blargg » Thu Mar 21, 2013 8:21 am

3gengames wrote:There's a byte pin to select bit size, guys.
So this whole thread is for nothing? :)

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Re: Converting a 16bit eprom to run as 8bit

Post by nintendo2600 » Thu Mar 28, 2013 8:45 pm

TmEE wrote:But not on a 27C322.

On other chips there is, and it turns D15 into an additional address pin, usually called A-1. D8 to D14 will do nothing in that mode.
Ya D15 becomes A0 and you shift the rest of the addy lines +1.

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nintendo2600
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Re: Converting a 16bit eprom to run as 8bit

Post by nintendo2600 » Thu Mar 28, 2013 8:51 pm

Also does anyone here know "AndrewC" aka "Pootle" on other forums?
I really wanna get in touch with him about buying some of his 27c160 and 27c322
adaptor boards for the SNES. Hell if I could even just get the egale files I'd be
a happy man and I would just get a batch done myself.

I can design the one for use with the 27c160 in 8bit mode but I'm not ninja
enough to design the one with the 27c322 and the 245's and 139. That one
I would love to buy\and or get some made.

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