Sega Mega Drive / Genesis Hardware Notes | Hacker News Reader

Sega Mega Drive / Genesis Hardware Notes

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163 pointsAudiophilip21 comments

I'm one of the coders who worked on the Overdrive 2 demo (although not on the reverse engineering that resulted in this document). We're going to release more follow-ups about the development of the demo.

I'm currently working on a write-up of the polygon to tile drawing routine used for the fullscreen 3D scenes.

I'm not sure about the time frame yet, but they will be posted at http://www.pouet.net/prod.php?which=69648 and/or http://www.pouet.net/topic.php?which=11035.

That's really exciting to hear! The fullscreen 3D bits were some of the ones that really caught my attention.

I know with X on the Game Boy [1], lookup tables were used to speed up the arithmetic. I'd be curious to see if the Genesis used similar techniques, or if specific things were done with the hardware.

[1] https://www.youtube.com/watch?v=pbtmBQIKTh8

Context from ShonumiGBE+ at /r/emulation :

Might help to explain some context for people who don't know what Overdrive 2 is, or why it's important.

TiTAN is a pretty prominent group in the demo scene. They do a lot of crazy demos, basically coding consoles to their limit while making cool audio-visual presentations. Overdrive 2 is the follow up to the original Overdrive demo[1], and once again TiTAN have pulled off a bunch of insane effects on the Mega Drive.

Overdrive 2 is relevant to /r/emulation because it can't be emulated (yet). From what I understand, it's doing a bunch of tricks (described in the write-up on Google Docs) that emulators currently can't handle. That's right, even in 2017, Genesis/MD emulation has a ways to go, which is kind of exciting to me. I'd love to see emulators like Blastem, Exodus, and higan step up to the plate. Then maybe the demo scene can come up with even more torture tests ;)

Anyway, you guys can catch Overdrive 2 over on YouTube.[2] This stuff is sick.

[1] http://www.pouet.net/prod.php?which=61724

[2] https://www.youtube.com/watch?v=OeGdJk5zb6c

I was surprised to discover that MAME can emulate the Genesis, because a small number of arcade games actually used Genesis hardware internally.

It's my understanding (which could easily be wrong) that MAME uses near-circuit-level emulation of hardware in order to be as accurate as possible. This is why it is so resource intensive.

Could the MAME Genesis emulator run this demo?

MAME nowadays is a general machine emulator, so everything from arcades, calculators, consoles, fruit machines, computers, etc. are within the scope of the project.

This happened when MAME and MESS merged a few years ago.

What a fantastic demo! Thanks for sharing your in-depth notes about reverse-engineering the hardware's behavior.

I admit, previously I didn't understand why some emulators promoted so heavily the fact that they did more hardware-accurate emulation.

My relationship with old consoles is one of casual nostalgia, and a crash-free experience was all I asked for, one devoid of glaring visual glitches or inopportune jerkiness as you're being attacked by a boss.

That's a decent goal too, but just like the demoscene attempts to push the hardware and software to the ultimate edge of the envelope, I now better appreciate the developers who have taken extra care to mimic the exact operating quirks of the original. Resources like these help build the body of knowledge for new and existing developers, and help raise the bar of what the community desires as well.

Love it! I've written a hopefully accurate Genesis emulator (https://github.com/rasky/genemu) and Overdrive is one of the holy grails of perfect emulation :)

I've found a video of Overdrive 2 demo running on an emulator [1]. It is curious/interesting how different/broken is in the emulation vs the real hardware.

[1] https://www.youtube.com/watch?v=WJ3AsZp-yTA

For reference, here's a capture from an actual Genesis: https://www.youtube.com/watch?v=gWVmPtr9O0g

And a live capture from the demoparty it premiered at: https://www.twitch.tv/videos/135854774?t=03h45m11s It's interesting to hear the crowd's live reactions.

Thank you for the links. Simply amazing.

I would suggest anyone with a history of seizures consider not viewing this link. For anyone else, it is fascinating.

The authors of the demo actually mention the emulation... difficulties. It's visible at 03:53:30 in the Twitch video:

    Grief Counselling for emulator
    developers is now available at

    irc.efnet.org
    #titandemo

This is a phenomenal demo.

Do the 3D bits implement the additional SVP chip used in (AFAIK only) Virtua Racing?

I used to do some interesting stuff with the MD / Genesis back in the day (like when I was 10-12 years old) - first with ROM hacking and moving into small bits of 68k ASM before getting into GBA and DS homebrew development, which was loads of fun (especially with an R4! screw commercial ROMs, this was a dream come true for me for homebrew! :D)

I was really involved in the Sonic Homebrew and fangaming scene and ended up sticking around on Sonic Retro (formerly SWS2B) since 2001 but haven't been active in ages...this makes me miss it! (I mostly do VR stuff these days :3)

No. Apart from the mapper chip used to address 8MB of ROM this demo requires no additional chips on the cartridge. Everything is done by the 68k CPU (running the effects) and the z80 CPU (running the sound engine).

I was hoping it would go into detail about the Blast processing tricks.

Was the blast processing marketting only in America? Since I had never heard of blast processing until I got on the web years later.

It was coined by Sega of America's marketing team, according to this wikia: https://segaretro.org/Blast_processing

I guess I'll have to get that everdrive flash cart after all!

Not all everdrive flash carts support the mapper/rom size our demo uses (the SSF2 mapper with an 8MB rom). The included nfo file lists some supported flash carts.

Thanks for showing what's the mega drive was capable of ! :-)

signed : a forever mega drive fan, snes sucks ! :p

I didn't know that. Thanks for info. I had set up a sweet broadcast CRT and Mega Drive with RGB, along with a 50/60Hz switch on it. Looking forward to enjoying demo on real hardware. I'm looking into Mega EverDrive X7.

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