Wide Boy mapper

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Fiskbit
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Wide Boy mapper

Post by Fiskbit »

I've been interested in how the Wide Boy mapper's non-Game Boy components work. Thanks to Chris Covell, we have the firmware and a hacked firmware for the Wide Boy 2, and recently, a Wide Boy-3(A) sold on eBay with some pictures, including a high quality image of the parts side. It appears that the Wide Boy 3 has its own firmware with a border image, much like Chris' modified firmware, but I don't think a dump of it is available. There is also a Wide Boy-3(B), though I don't know how it differs.

The Wide Boy has 8 KB of dual port RAM, presumably used for CHR to hold the Game Boy screen image. For the Game Boy region of the NES screen, the firmware sets up nametables that use sequential tile IDs from left to right, top to bottom, starting with $00. The last tile for the top half of the screen is $B3, and then the tile IDs start again at $00 for the lower half. It's not clear to me how the Wide Boy knows whether the top or bottom half is being drawn nor where it's actually pulling the graphics from in its 8 KB of RAM. The hacked firmware's border graphics are only written into $0BF0-0FFF, not touching the upper 4 KB of CHR-RAM, but it's still able to use those graphics across the entire height of the screen, so whatever the Wide Boy is doing to handle the different halves, the effect doesn't seem to be universal.

Is there any chance someone can shed some light on this mapper?
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Wide Boy-3(A).jpg
Wide Boy-3(A) solder.jpg
Wide Boy-3(A) border.jpg
Fiskbit
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Joined: Sat Nov 18, 2017 9:15 pm

Re: Wide Boy mapper

Post by Fiskbit »

Firmware from a Wide Boy-3 (letterless) was dumped. It has the same border we saw on the Wide Boy-3(A). It loads the entire 8 KB ROM into PPU $0000-1FFF to start with, which I suspect ends up having no effect, and then loads the border graphics into the $0B40-0FDF region, like what Chris Covell did. Notably, it's not using anything in the $0000-0B3F range, which likely gets filled with Game Boy screen data. I'm still not sure how it handles the swap between the two screen halves.

It looks like the Wide Boy-3, unlike the 2, doesn't allow for changing the screen palette. Both firmwares also seem to be looking for a controller with a 24-bit report, though I don't think anything is done with the latter 16 bits. Edit: Also of interest, both firmwares do MMC1-style writes at the start. I don't know if there's just a lot of boilerplate here from some other source or if this is actually meaningful for this hardware.
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Ben Boldt
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Re: Wide Boy mapper

Post by Ben Boldt »

This thing uses an MMC5 with CHR-RAM, which is quite interesting. I looked at this some time ago, and I don’t remember what revision it was. But it does use extended attribute mode. The center portion gets drawn from 3 separate CHR banks, which are presumably from the dual-port RAM.

We discussed it and I don’t think anyone knew if/how the dual-port CHR-RAM was synchronized between the NES PPU and the gameboy. If it wasn’t buffered or synchronized, this would show up as tearing across the screen when scrolling horizontally. I looked at the ROM and didn’t find any unexpected registers written to. It would still be highly interesting to see a schematic of one of these things. Actually it would be pretty neat to make one. I remember playing with these at Toys R Us.
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Quietust
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Re: Wide Boy mapper

Post by Quietust »

Ben Boldt wrote: Tue Apr 05, 2022 7:04 pm This thing uses an MMC5 with CHR-RAM, which is quite interesting. I looked at this some time ago, and I don’t remember what revision it was.
I think you're confusing the Wide Boy 2/3 with the Demo Vision which used an MMC5.
Quietust, QMT Productions
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Ben Boldt
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Re: Wide Boy mapper

Post by Ben Boldt »

Yes, I believe you are correct.
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