: The Mega-Drive was actually compatible with the Master-System.
: You just needed an adaptor because of the different cartridge slot.

your image here is broken

: This was possible because the hardware was fundamentally very similar. In
: some cases, they had some of the same chips. The Master-System CPU and
: Sound chip were both in the Mega-Drive, as co-processors, making the
: 16-bit console fundamentally compatible with the 8-bit one.
: http://en.wikipedia.org/wiki/Mega_Drive#Master_System_compatibility .

: Most consoles are just incompatible on a hardware level as the technology has
: moved too far forward to maintain it.

: The reason the Wii is compatible with Gamecube games is because on a hardware
: level, the Wii is just a slightly beefed up Gamecube. Just like the
: Mega-Drive was a beefed up Master System.

: The PS2 is backwards compatible with the Playstation because it actually has
: the Playstation CPU inside it, the MIPS R3000A.

: Likewise the early NTSC PS3's were backwards compatible with the PS2 because
: they included the the PS2's CPU and GPU chips.
: By the time the PS3 reached PAL regions 5 months after the NTSC release, Sony
: had already stripped out the PS2's CPU to reduce costs, but left the GPU,
: so you had a hybrid of emulated and hardware compatibility.
: More PS2 games would run on these PS3's compared to Xbox games on Xbox 360,
: with widely variable levels of quality.
: 4 months later, NTSC consoles were also stripped of the Emotion Engine CPU.

: When MGS4 arrived in PAL regions in late 2007, the PS2's GPU was also deleted
: and from that point on, all future revisions of PS3 consoles had zero
: compatability with PS2 games.

: PSOne games are entirely emulated on the PS3, but that's really rather easy
: to do with the hardware.

nice history lesson

: Emulation requires a huge amount of grunt compared to being compatible on a
: hardware level.

I'd just like to add: http://www.thocp.net/software/games/reference/compatibility_backward.htm