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I know that the Megadrive contains the Z80 CPU to help control the soundchips for 16-bit games, and that it provides backwards compatibility for the CPU processing of Master System software, but beyond that how are the sound processing and graphics processing of Master System games handled when playing on a Megadrive?🧐🤔

  • What role do the Yamaha & Texas instruments chips play in allowing the Megadrive to output sound from Master System games?

  • What is the role of the video display processor in outputting Master System graphics?

  • What role does the 68000 processor play in all of this?

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  • I guess you mean the Sega System
    – Cesar
    Apr 12, 2021 at 10:26
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    @Cesar It was called the Master System in the UK. I don't know what it was called in the US or Japan. I remember we used "Megadrive" and "Mega Drive" pretty much interchangeably. Also, I'm not sure whether to edit the question to clarify that the Power Base Convertor was needed for this.
    – AJM
    Apr 12, 2021 at 15:39
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    The name of the console was "Sega System" worldwide, "Master" was just the name of the bundle that came with the pistol, in the same way that the "Sega Super System" was the bundle that came with the 3D glasses.
    – Cesar
    Apr 13, 2021 at 11:39
  • @Cesar I have never heard it referred to as the "Sega System", and the Wikipedia article has an extensive discussion of the origins of the "Master System" name. You appear to be right that "Super System" was the name for a boxed edition, but this unboxing video of one clearly shows the console inside is labelled "Master System Power Base", not "Sega System" (or "Super System"). The only references I find for "Sega System" are for various arcade systems with generic names like "Sega System 18".
    – IMSoP
    Oct 24, 2021 at 21:01
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    @Cesar Fascinatingly, that Golden Axe manual refers to the console twice (per language): on page 2, it mentions "your SEGA SYSTEM manual", but on page 36 "intended exclusively for the SEGA MASTER SYSTEM". It seems like the early branding just wasn't very consistent, but at least some people at Sega were referring to the whole platform as the "Master System" quite early on. If you look at later releases like Sonic the Hedgehog, the brand was quite clearly established. Calling it "Sega System" now is like calling Google "BackRub", or Facebook "The Face Book" - that name is long gone.
    – IMSoP
    Dec 13, 2021 at 22:17

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The 68000 does nothing; it is entirely disabled. Ditto the Yamaha OPN.

The Mega Drive's VDP is almost entirely backwards compatible with that of the Master System — it implements the graphics mode used by all but one of the Master System's games natively*. So there's no emulation or translation, it's just a different graphics mode.

Similarly the SN76489 is exactly the sound chip used in the Master System, so it's just presented as is. With a potential caveat: the physical chip might be absent; Sega rolled a reimplementation** into the VDP for the Master System 2 and Game Gear so I dare imagine its functionality got subsumed in the Mega Drive too.

In short: the relevant components are hardware-compatible with those in the Master System [almost] as far as it matters; the others are disabled when a Master System cartridge is detected.

If you wanted to produce your own at-home alternative to the official adaptor for Master System games all you'd need to do is:

  • ground the Mega Drive's M3 cartridge slot pin, to signal that the Mega Drive should run in Master System mode;
  • connect all Master System cartridge port pins one-to-one with the Mega Drive equivalents; and
  • add a physical button to trigger NMI, that being what the Master System's 'pause' button does.

* The Master System is in-turn backwards compatible with the SG1000, reimplementing its graphics modes (which as those of the TMS9918a, also seen in the ColecoVision, MSX, TI-99/4a and others). The Mega Drive omits those. One early Master System title, F-16 Fighting Falcon, uses one of those modes rather than the Master System's native mode.

** Sega's version has slightly different noise-channel output, as it uses a different LFSR polynomial. But it's similar and the exact noise waveform isn't that important to Master System software.

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    Good answer, but it seems relevant to also discuss how the Mega Drive "detects" (or "accepts"?) a Master System cartridge in order to give control to the Z80.
    – Brian H
    Apr 12, 2021 at 15:06

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