Amiga 500 OCS/ECS vs Mega Drive VDP

Question

Superficially the Amiga 500 and Mega Drive are similar. Both run a Motorola 68000 CPU at similar clocks and both have dedicated hardware for sprite and other graphics operations.

Comparing the arcade ports of Mortal Kombat (which were both handled by Probe), the Mega Drive version is visually improved in the following ways:

• Has parallax scrolling
• Has background animation
• Has higher frame rate

This finding also seems to apply more generally when comparing Mega Drive and Amiga ports.

I accept that these differences can often be best explained by software factors like developer skill and budget.

Putting that aside, what extra hardware capabilities does the Mega Drive have (if any) that could explain the graphical improvements?

Answer 1

The [pre-AGA] Amiga:

• uses a planar frame buffer, stored in memory that is shared with the CPU (and which therefore reduces the CPU's speed);
• provides dual-playfield hardware scrolling, but the total bit fetch total per output byte doesn't stretch beyond six, so you end up with e.g. a 7 colour foreground and an eight colour background;
• provides 8 hardware sprites, but with no DMA support for positions and image pointers the programmer is responsible for sorting and finding appropriate transition points if multiplexing is desired; but
• has the Copper for no-CPU cost raster racing (including sprite multiplexing); and
• has a blitter for quickly populating frame buffer contents.

Not all memory has to be shared with the CPU but Commodore being the company it was, the mainstream stock Amigas shipped with only shared memory.

Philosophically the Mega Drive is from a completely different school of thought:

• the VDP has 64kb of its own memory, which the CPU can access very slowly or else it can request a DMA transfer and yield the bus;
• from that memory the Mega Drive draws two tile-based play fields of 4bpp each, for which the programmer is permitted directly to supply per-line scrolling offsets;
• one of those play fields may optionally also display a non-scrolling window;
• it also supports a sprite layer with another 4bpp, for which it parses an 80-entry table to draw up to 20 16px-across sprites per line; and
• it has a Z80 entirely dedicated to audio, making that one less thing to do.

So unlike the Amiga, the Mega Drive is heavily optimised for 2d games of the era, providing native tile-based graphics and a large number of sprites that it can automatically multiplex.

Tiles and sprites are also great for forcing the programmer's hand: either the hardware can display what you want, and it'll work at the native refresh rate, or it can't. Unlike the blitter on the Amiga you can't decide you're going to run at 25fps rather than 50 in order to draw more.

There's also the advantage of the dedicated VDP RAM and the greater bandwidth the Mega Drive therefore enjoys, giving it two background layers of 4bpp graphics compared to the Amiga's 6bpp total — even before you start counting the 320px of further per-line video data provided by the Mega Drive's comparatively large sprite total.

Of course, anything that can't be rationalised down to sprites and tiles isn't a good fit. You can see it in the relative frame rate of the few 3d titles that are on both platforms, such as Hard Drivin', and productivity software would likely have been very tiresome had there ever been a reason to write any. The Amiga is leagues ahead for anything that most naturally maps to a frame buffer.

Answer 2

The Megadrive comes from an arcade machine lineage, it's cut-down Sega arcade machine hardware. The Amiga comes from a computer lineage, it's an evolution of the Atari 800, same designer, and many of the same features, just expanded.

Particularly, the Amiga, like most computers, deals with pixels. Better for things like line drawing, proportional text, and er anything you might want pixels for!

The Megadrive deals in tiles. A tile is an 8x8 block of pixels. They are laid out in a grid of 40x30, as the background. Actually there are 2 background layers, 2 grids, for better effects. Parts of each layer can go on top of, or behind things, according to the game code.

The Master System and NES only had one background layer, so weren't as impressive. You can see that the Megadrive has two, in many games.

Just one access to the tile table in graphics RAM lets you change any tile in the 40x30 grid. 64 pixels, 8x8, can be swapped out in an instant. The Amiga had to do that one at a time, though admittedly it had custom chips to help some things.

For 2D arcade games tiles are a great thing. It's most of the reason why 80s and early 90s arcade machines had such an advantage over home computers, despite using processors that weren't much faster. It's the reason the SNES does so well, even though it's CPU has an 8-bit bus and runs at half the clock rate of the Megadrive's. The graphics chip does basically all the lifting, the CPU just sends it arrangements! By splitting the display into tiles, a processor can throw around graphics at an amazing rate.

So the CPU tells the graphics chip what pixels to put in each tile, often as the level is loading. Then it says which tiles to put in each location of the 2 background layers. Once each tile has been defined, to switch the tiles about is very quick. That's where the power is!

Sprites go on top of the background. Or sometimes behind it, depending on priority! Sprites are made up of a grid of tiles. Each sprite has a table in RAM telling the graphics chip which tiles to use. A sprite can be moved around the screen, rather than staying fixed like the background does.

The Megadrive's multiple layers of screen make parallax effects easy. If you want true parallax, ie per-scanline, that's also a hardware function, a program can add a horizontal offset, a different amount per scanline. Scroll each scanline at a different speed and you have parallax. All for free, done by the graphics chip.

There are lots of interesting differences, but the important one is "tile mapped".

You can do per-pixel access on a Megadrive, by storing your tiles in RAM, and altering a pixel of the relevant tile. If you lay your tiles on the screen in a fairly straightforward order, the formula to calculate what address your pixel is stored at isn't complicated. Still generally slower than a computer though. So there were few polygon games on 16-bit consoles.

https://megacatstudios.com/blogs/press/sega-genesis-mega-drive-vdp-graphics-guide-v1-2a-03-14-17

is a very good guide to this, with lots of example screens from games with parts of each layer showing or removed, so you can see how it all adds up. Should tell you everything, and more, you need to know! And as a bonus you'll also understand the NES, SNES, Master System, MSX, and most old arcade boards too!

[I've edited this to make it a bit clearer, hope it helps!]

Answer 3

TL;DR:

For all of this it might be helpful and take a step back from bits and bytes and take a step back to look at the whole process of Game development. Doing so will show that quality is much more dependant on the budget available than the hardware to be used. Budget can buy time to create quality and pay for creativity.

The market for Mega Drive games was, depending on the region 5-10 times as large as for Amiga developments - and so were the budgets available to be invested in game quality. For example Sonic 2 (*1) alone sold with 6 million more copies for the Mega Drive, than there have been Amigas ever produced (<5M).

We, as computer nerds, tend to over emphase on bits, bytes and hardware in general, even when they don't really matter for the product the customer sees (and buys).

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The Long Read

What extra hardware capabilities did the Mega Drive have that could explain these differences?

It doesn't need to be any hardware differences. As soon as machines reach a certain level it's all about game programming (and design). Usually ports are not done the same team or company across various system. Even less are they done with the same dedication. So quality may differ quite a lot according to selected contractor and budget. (*2)

Remember Xenon2? An awesome scroller, with several layers scrolling in different speed and direction, iced with animation was extreme at its time. Noteworthy the original Atari ST version looked and played best, while the Amiga version, despite the more capable hardware, played still great, but less perfect - and the Mega Drive version simply sucked. Not only was gameplay horrible and graphics less what other games of the same time offered, they even cut down the originally quite good sound track so much it became extreme boring. Having a dedicated sound processor didn't help here either.

So while hardware can play a role, at the time of 16 bit systems of the mid to late 1980s it became less of a defining issue while budget became the ruling issue. Money defined the quality of programmers/companies to be hired and how much time they got to make the game great.

And that's where scale of a market comes into play. The market for Mega Drive games was at least a magnitude (10 times) larger than for Amiga games (*3). As a result game development fr the Mega Drive was based on way better funding and larger margins than anything done for the Amiga.

Base line: Within the 16 bit era the hardware played a minor role. Good game development (portage as well) needs good funding which in turn needs good market expectations. Here the Mega Drive played in a different league than the Amiga (or any other home computer).

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*1 - Sonic 1 sold more than 15 million copies, but it was in many markets bundled with the console, so not really useful as measurement.

*2 - In fact, the backport of Sonic 1 to the Master System is an equal great example that it's not so much about the hardware available but dedication and support for programmers doing a port. The Master System version has been noted as on par to the Mega Drive version, despite the fact that Sonics movement was considered top line development in 1991, it was possible to move it quite good over to the way less capable system.

*3 - The Mega Drive sold in the US during its first year about 500k units (plus about 400k in Japan), while the Amiga sold less than 100k worldwide. Over the 10 year lifetime of either machine the Amiga sold less than 5M units (worldwide, all models combines) and about 700k in the US. Sega alone sold more than 30 million units during the first run - 4 M units alone in the US during the first 4 years.

Answer 4

Let's assume your assumption "MegaDrive games are generally 'better' than Amiga games" was true - Which is a statement hard to prove and very probaby only backed by opinion:

Games Console vs. Home Computer - Marketing goals

The main (rather: only) selling point for consoles is the quality of games available for the platform. There is absolutely no other reason people buy them. A console manufacturer needs to put a lot more focus on availability of killer games for the platform than a company selling home computers. It is part of the marketing machine of a games console manufacturer that they need to

• Support game developers as far as they can (documentation, (cross) development systems, training, tech tips, ...) to enable them to create technically "good" games. Most console manufacturers even had in-house game development divisions that could work closely with hardware guys.
• Quality Control: Games for consoles are typically marketed through the manufacturer's channels rather than independent publishers. That gives the manufacturer a choice of accepting a game into their sales channel or not. Crap games are filtered out early and don't even reach the market.
• Price control: Traditionally, there never was a budget games market for consoles (which definitlely existed for home computers). The manufacturer of the console fixes a price level for the game, and can make sure the game's quality justifies that price level. A console manufacturer could simply not accept low quality games, because that would have depreciated the console's value.

So, generally, the amount of control the manufacturer had (and had to have) on the software available for their machine was much higher on the console market than in the home computer market. There's a reason the Megadrive had special circuitry that would only allow Sega-licenced games to be run on the console...

Home computers, on the other hand, are entirely different. Even if some home computer manufacturers had own software labels to distribute their own games, traditionally there has always been an independent market for software in general (not only games) - That typically also not had the rigid quality control that console manufacturers had. Support for developers was much more basic, beyond documentation of various quality and depth, there wasn't much. Developers traditionally had to explore the innards and capabilities of the hardware themselves.

The Amiga is yet again a special case: Commodore's initial aim was to sell the Amiga as a "Multimedia Machine", rather than a home computer, or, heaven forbid, a games console - Games were, at least in the early years, not the main focus of Amiga marketing (on the contrary: Initially, Commodore went to a great extent to market the Amiga as a Multimedia machine and tried to avoid at all cost the Amiga to be categorized as "games machine only". Games were seen as software that tended to erode the market price of the machine. That also influenced the choice of developers they supported and software they pushed. Only as late as in the early nineties Commodore marketing was actually starting to accept the Amiga as a games machine and started to change their marketing strategy)

Hardware Differences

Comparing a games console and a home computer is, typically, comparing apples to oranges. They're built with an entirely different focus, and, obviously, with very different architecture. Games console architecture is mainly driven by no-compromise video and sound performance, while a home computer needs to achieve a much higher amount of versatility and flexibility.

The main difference between a Megadrive and an Amiga is (apart from the Megadrive's architecture and custom chips being at least 5 years newer) the video subsystem: The Megadrive is architectured for raw 2D-graphics performance. VRAM is isolated from the CPU bus and only mapped to the VDP. The CPU "only" tells the graphic subsystem to shove tiles and sprites around, and can then concentrate on what it does best: computing. The CPU is not able to adress single pixels on the screen (and doesn't have to).

Aiming at versatility, the Amiga graphics chips have to share the bus and the VRAM with the CPU - That makes the graphics subsystem more flexible, but at a cost: Graphics is slower and more limited (number and size of sprites and tiles that can be managed) than with dedicated VRAM.

Ports

Ported games between platforms are yet another story. I actually can't remember a single game that was ported from one platform to another (the direction traditionally was Arcade->Console->Home computer) where the port was considered "better" than the original. The simple reason: Obtaining the legal rights to a game that already exists consumes a large upfront amount of development budget. The original game's mechanics might also have been tailored to the original hardware, so ports are specifically hard to do. You could also assume that ports often simply relied on the fame of the original rather than technical perfection of the actual implementation. Considering that, it is relatively unfair to compare platforms based on games ported between them.

Answer 5

It really comes down to getting good performance on the Amiga requiring a lot more time and skill than on the Megadrive. The Sega machine makes it very easy to do things like parallax scrolling and large sprites. The Amiga is capable, but requires a lot more effort to tease the performance out.

There are other fighting games on the Amiga, like Shadow Fighter and Elf Mania, that do feature multi-layer parallax and other graphical effects. They require careful planning to execute, making the best possible use of the Amiga hardware. That somewhat dictates what the stages and characters can look like, so it helps if the game is designed for the Amiga, rather than a port. However, with some effort it is possible, e.g. this effort to bring SF2 to the Amiga 1200: https://youtu.be/33kH9DdNznA

Often there is limited time available to make these ports. At the time the Megadrive market was also a lot bigger than the Amiga one. As such a comparison is a little unfair, and arguably the Amiga version being inferior boils down to "market forces".