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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 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-Reinstate-Monica Apr 12 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 at 11:39
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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 at 15:06

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