nesdev.com
http://forums.nesdev.com/

Megadrive game with 8bit ROM
http://forums.nesdev.com/viewtopic.php?f=23&t=17320
Page 1 of 1

Author:  Fisher [ Thu May 03, 2018 7:25 am ]
Post subject:  Megadrive game with 8bit ROM

Hello.
Recently I've found in this page a schematic of a MegaDrive game wich uses an 8bit ROM and a circuit to "convert" it to 16 bit.
Here's the schematic:
Attachment:
File comment: 8 to 16
conversão 8bits para 16 bits.jpg
conversão 8bits para 16 bits.jpg [ 145.58 KiB | Viewed 2810 times ]

My doubt is: would this simply work out of the box or the pirate game was somehow altered to make this possible?
I know that would be simple to just use 2 8bit ROMs with the odd/even bytes, but this circuit seems interesting, specially to me who only have a single 8bit ROM to try.

Thanks in advance.

Author:  tepples [ Thu May 03, 2018 7:44 am ]
Post subject:  Re: Megadrive game with 8bit ROM

Latching the upper and lower bytes on each access and assembling the 16-bit words before they get to the CPU isn't conceptually that different from the N64 RAM being only 9 bits wide and then the RCP memory controller reassembling words before the R4300i sees them. But you'd need a fairly fast ROM for this, and I remember reading somewhere that some early Genesis games couldn't DMA from ROM to VRAM because they were manufactured on slow ROM. Instead, they had to PIO-copy data to VRAM, as if it were a filthy Master System, or copy it to work RAM and then DMA it from there.

But I don't see much point, particularly given that the larger 5.0 V EPROMs (27C160 and 27C322) are 16-bit anyway, to the point where people end up using multiplexer circuits to get a 27C322 working with the Super NES's 8-bit data bus. Or were 16-bit EPROMs not made in smaller capacities?

Author:  Fisher [ Thu May 03, 2018 8:14 am ]
Post subject:  Re: Megadrive game with 8bit ROM

tepples wrote:
isn't conceptually that different from the N64 RAM being only 9 bits wide

That's pretty interesting!
Also I didn't know about the DMA, wich is interesting too.
tepples wrote:
But I don't see much point...

Sure! I agree with that too.
But that's not a matter of getting new parts, but to try to (re)use the ones I have.

Author:  getafixx [ Thu May 03, 2018 8:16 am ]
Post subject:  Re: Megadrive game with 8bit ROM

tepples wrote:
Or were 16-bit EPROMs not made in smaller capacities?


There were lots of them. The 27C400 and 27C800 were both 16-bit chips with 4Mbit and 8Mbit sizes, respectively.

Author:  TmEE [ Thu May 03, 2018 10:15 am ]
Post subject:  Re: Megadrive game with 8bit ROM

http://www.tmeeco.eu/BitShit/8bitROMonMD.jpg

This is what is done in Turrican cartridge (seems to be same as the schematic earlier). It works but only on real original hardware that has no overclock or TMSS bypass mod done to it. It will not work on any clones except the superclones that use 1:1 copied parts.

If you have to use 8bit ROMs use pair of them, it is guaranteed to work and allows you to save money on bulk ordering of the memory chips.

Author:  lidnariq [ Thu May 03, 2018 10:54 am ]
Post subject:  Re: Megadrive game with 8bit ROM

The pictured schematic uses B21 / "/CAS2", which is "asserted for reads and writes within 000000-7FFFFF". I'm vaguely surprised that it's a useful amount of phase delay relative to the other signals to use as a demultiplexing clock.

getafixx wrote:
There were lots of them. The 27C400 and 27C800 were both 16-bit chips with 4Mbit and 8Mbit sizes, respectively.
Were being the key word. There are no new 5V 16-bit ROMs, other than Atmel's expensive OTP offerings.

TmEE wrote:
If you have to use 8bit ROMs use pair of them, it is guaranteed to work and allows you to save money on bulk ordering of the memory chips.
The volume discount doesn't hit 50% though.

Author:  TmEE [ Thu May 03, 2018 11:33 am ]
Post subject:  Re: Megadrive game with 8bit ROM

From my experience pair of 4Mbit chips are only tiny bit more expensive than single 8Mbit chip, you'll be reaching the first discount tier pretty quickly where things even out and pretty quickly after that a point where they are cheaper. Of course it only matters when you start doing tens of carts rather than one or two at a time...

The circuit works only because of the slight timing difference between various signals, there's some tens of ns between the edges.

Author:  Fisher [ Fri May 04, 2018 4:04 am ]
Post subject:  Re: Megadrive game with 8bit ROM

That's great!
Thanks for the info!!

Page 1 of 1 All times are UTC - 7 hours
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/