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Thread: So SNES is Pin compatable with NES and the CPU disabled legacy mode for NES?! WTF

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    SNES So SNES is Pin compatable with NES and the CPU disabled legacy mode for NES?! WTF


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    Mega Driver Hedgehog-in-TrainingMaster of Shinobi Gryson's Avatar
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    The SFC was first unveiled in November 1988, less than a month after the Mega Drive was released. Nintendo announced a release date of July 1989.

    There's a lot of speculation about what happened next, but the SFC ended up releasing on November 21, 1990.

    So yeah, things didn't exactly go according to plan.

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    Nintendo even said at one point (in an old issue of Nintendo Power) they had considered making the SNES backwards compatible but removed it for cost reasons. Considering by the time the SNES launched almost everyone who wanted a NES already had one I think they made the right decision. But if you look at some of the programming resources for the SNES you can run the system in 8-bit mode, some of the games like Super Mario All Stars even use code that's directly from the 6502. I think Dr. Mario and Tetris do the same but I have not seen the disassemblies.

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    Raging in the Streets Sik's Avatar
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    Yeah, they clearly planned for backwards compatibility and then had to keep lowering their expectations. It had gotten to the point where it got so low that they considering offering "backwards compatibility" by… letting you connect a Famicom through a Super Famicom (it's as laughable as it sounds). And even that didn't pan out.

    A large part of the problem is that the NES/Famicom games are extremely timing sensitive (as well as several of their mappers), in large part because several advanced effects require you to do things at an exact point. One thing is slightly off and suddenly a lot of games can be thrown out of whack. Now imagine keeping everything exact while also adding new features. Yeah, that sounds like a headache… no wonder they had to drop the idea in the end. I can already imagine Nintendo getting tired of tracking down bugs and just giving up.

    The one thing that seemed to have worked out is making it relatively easy to port over code from the NES to the SNES. The LoROM mapping is essentially the same as the NES (in terms of address ranges in use), with a part of RAM mirrored at $0000-$1FFF (though this time 8KB instead of 2KB) and ROM at $8000-$FFFF. You have to ditch the original mapper in favor of the 65816's bank switching mechanism, but changing that across games wasn't uncommon anyway. When you consider that many Japanese programmers had developed for the Famicom, this probably was very helpful in the early days.

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    Quote Originally Posted by Sik View Post
    Yeah, they clearly planned for backwards compatibility and then had to keep lowering their expectations. It had gotten to the point where it got so low that they considering offering "backwards compatibility" by… letting you connect a Famicom through a Super Famicom (it's as laughable as it sounds). And even that didn't pan out.

    A large part of the problem is that the NES/Famicom games are extremely timing sensitive (as well as several of their mappers), in large part because several advanced effects require you to do things at an exact point. One thing is slightly off and suddenly a lot of games can be thrown out of whack. Now imagine keeping everything exact while also adding new features. Yeah, that sounds like a headache… no wonder they had to drop the idea in the end. I can already imagine Nintendo getting tired of tracking down bugs and just giving up.

    The one thing that seemed to have worked out is making it relatively easy to port over code from the NES to the SNES. The LoROM mapping is essentially the same as the NES (in terms of address ranges in use), with a part of RAM mirrored at $0000-$1FFF (though this time 8KB instead of 2KB) and ROM at $8000-$FFFF. You have to ditch the original mapper in favor of the 65816's bank switching mechanism, but changing that across games wasn't uncommon anyway. When you consider that many Japanese programmers had developed for the Famicom, this probably was very helpful in the early days.
    That's good to know, I wasn't aware of that. I wonder if a Super Gameboy-like adapter could have been possible? I want to say it couldn't work because it has to update the entire screen unlike the Gameboy but I don't know enough about the SNES capabilities.

    Edit: If I'm understanding it right the SNES can DMA 6k of data to the PPU every vblank, which is just enough for the original Gameboy's 2 bpp display (160*144*2/8=5760). So any NES game either needs to be running on the SNES CPU or have its own video output like the Super 8. For the NES it would 256*224*6/8=43k, meaning more than 7x as much data to be transferred.
    Last edited by axel; 01-31-2020 at 12:03 AM.

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    Nintendo at the very least could have sold an adapter. The race was that close and they could have sold something to play games on SNES from the best selling console of all time (at the time).

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    Smith's Minister of War Hero of Algol Kamahl's Avatar
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    I wish Sega had given up on backwards compatibility too, the console needs an adapter anyway so what's the point. More space on the board for other features, and less bugs (like vertical scrolling on the first column).
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    Raging in the Streets Sik's Avatar
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    How long before somebody says that they should have ditched the Z80, and also that Sega is stupid for not having predicted their future and not being willing to risk throwing away their whole userbase in case it flopped (or that said userbase was non-existent, as if the UK and Brazil had never existed).

    Quote Originally Posted by SegataS View Post
    Nintendo at the very least could have sold an adapter. The race was that close and they could have sold something to play games on SNES from the best selling console of all time (at the time).
    The problem is that such an adapter would include practically the whole NES and at that point just buy the whole console. It's not like with the Super Game Boy where there's an obvious benefit even without the enhancements (outputting to a large screen, something the original hardware can't do).

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    I don't think Sega was stupid for including backwards compatibility, but they were stupid to have it depend on an optional adapter. At that point, they might as well just put a Z80 in the adapter and call it a day, and have a proper PCM chip instead in the console.

    Why not include an adapter with the console if you're making the console compatible with the previous gen? Was there any other console that needed an adapter for backwards compatibility that was just for the cart shape/pins?
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    Mega Driver Hedgehog-in-TrainingMaster of Shinobi Gryson's Avatar
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    Sega didn't announce the Power Base Converter until after the Mega Drive had launched, so it's not really clear what their intentions were.

    Beep: Moving on to the main topic of the Mega Drive’s add-ons, we received many more requests for the Mega Adapter than we had predicted…

    Sega: We were also surprised by the results of our own survey, which showed the same demand. Due to the memory shortage that we just mentioned, we’ve only released two Mega Drive games so far, and there are also an unexpected number of new users who don’t already have a Master System or Mark III. It’s only natural that they want to try out the previous games. At present, we’ve made it our number one priority to get the adapter ready, and here is a mockup of it. You insert it into the cartridge slot like this and secure it with a screw on the back. It also has a separate Sega Card slot.

    Beep: Somehow, the Mega Drive looks even cooler with the adapter.

    Sega: We put some effort into that…

    Beep: Do you have any information on the release date and the cost?

    Sega: It will be released between December 10 and 15 and will cost about 4000 yen.

    Beep: Together with the Mega Drive that comes to 25,000 yen. That’s quite nice.

    Sega: We don’t intend to make a profit on this. It’s like a complimentary gift.
    I'm glad that they didn't build that cost into the Mega Drive.

    https://mdshock.com/2018/07/15/the-m...-has-launched/

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    Did Game Gear recycle MS tech or was it just different hardware that was on par? I hear conflicting things. If it did re-use it then it shows back then and to this day they got a lot out of it. More than Nintendo did with Gamecube chips.

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    Quote Originally Posted by SegataS View Post
    Did Game Gear recycle MS tech or was it just different hardware that was on par? I hear conflicting things. If it did re-use it then it shows back then and to this day they got a lot out of it. More than Nintendo did with Gamecube chips.
    Game Gear is basically a portable SMS with a larger master color palette and a lower resolution screen mode.

    There's a reason why the Game Gear can play SMS games with a simple adapter, and why so many Game Gear games were also released on the Master System in Europe and Brazil:

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    Raging in the Streets Sik's Avatar
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    There's slightly more to it (e.g. a proper Start button and the PSG is now stereo) but yep that's pretty much the gist of it. Oh, and they fixed the MAG bit so it's actually reliable on Game Gear (Virtua Fighter Mini puts it to great use).

    There's also System E (which has two Master System VDPs), also both Mega Play and Mega Tech have a Master System VDP for the information stuff (with the Mega Drive VDP being used for the game proper). Oh yeah, you can link together the two VDPs… (you should find that more questionable than including backwards compatibility, if you ask me)

    For the record, a lot of the Master System hardware is reused in Mega Drive mode: the Z80 is given the role of sound CPU, the PSG is still available (as a complement to the YM2612), the controller ports have the same shape (and Mega Drive controller is backwards compatible)… really the only thing that truly goes to waste in Mega Drive games is mode 4 (the Master System video mode), and even then mode 4 and mode 5 seem to share a good bunch of hardware (definitely the tilemap hardware, and the suspicion is that MAG is gone because mode 4 is likely going through the whole new sprite system that mode 5 uses). The VDP die doesn't show any clear demarcation between Mega Drive and Master System hardware, though we haven't done much research into it yet.

    EDIT: also, we think they may have been able to squeeze the second set of palettes they wanted if they had enough time to tidy it up a bit more :​P (but no guarantees, this is just speculation). So you can blame deadlines too.

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    Quote Originally Posted by Kamahl View Post
    I don't think Sega was stupid for including backwards compatibility, but they were stupid to have it depend on an optional adapter. At that point, they might as well just put a Z80 in the adapter and call it a day, and have a proper PCM chip instead in the console.

    Why not include an adapter with the console if you're making the console compatible with the previous gen? Was there any other console that needed an adapter for backwards compatibility that was just for the cart shape/pins?
    Isn't the Z80 as a sub-CPU also very common in arcade machines, including Sega's System 16? So, the configuration makes sense, I guess.

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    Quote Originally Posted by Sik View Post
    There's slightly more to it (e.g. a proper Start button and the PSG is now stereo) but yep that's pretty much the gist of it. Oh, and they fixed the MAG bit so it's actually reliable on Game Gear (Virtua Fighter Mini puts it to great use).

    There's also System E (which has two Master System VDPs), also both Mega Play and Mega Tech have a Master System VDP for the information stuff (with the Mega Drive VDP being used for the game proper). Oh yeah, you can link together the two VDPs… (you should find that more questionable than including backwards compatibility, if you ask me)

    For the record, a lot of the Master System hardware is reused in Mega Drive mode: the Z80 is given the role of sound CPU, the PSG is still available (as a complement to the YM2612), the controller ports have the same shape (and Mega Drive controller is backwards compatible)… really the only thing that truly goes to waste in Mega Drive games is mode 4 (the Master System video mode), and even then mode 4 and mode 5 seem to share a good bunch of hardware (definitely the tilemap hardware, and the suspicion is that MAG is gone because mode 4 is likely going through the whole new sprite system that mode 5 uses). The VDP die doesn't show any clear demarcation between Mega Drive and Master System hardware, though we haven't done much research into it yet.

    EDIT: also, we think they may have been able to squeeze the second set of palettes they wanted if they had enough time to tidy it up a bit more :​P (but no guarantees, this is just speculation). So you can blame deadlines too.
    Very good explanation.

    Don't you think without Master System compatibility there would be probably no PSG sound chip inside the Mega Drive? The PSG complements the FM chip very well. A lot of my favorite songs uses the PSG extensively. The PSG is also important to reproduce sound effects, without sacrificing the FM channels allocated to the music.

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