Sega Saturn Architecture

A practical analysis by Rodrigo Copetti

Classic edition - Last updated: August 23, 2022

Languages available: 🇬🇧 - English, 👋 - Add translation


About this edition

The ‘classic’ edition is an alternative version to the ‘modern’ counterpart. It doesn‘t require Javascript, state-of-the-art CSS or convoluted HTML to work, which makes it ideal for readers who use accessibility tools or legacy internet browsers. On the other hand, eBook users can now check the eBook edition.

This edition is identical content-wise. However, interactive widgets have been simplified to work with pure HTML, though these will offer an link to the original article in case the reader wants to try the ‘full version’.

As always, this article is available on Github to enable readers to report mistakes or propose changes. There‘s also a supporting reading list available to help understand the series. The author also accepts donations to help improve the quality of current articles and upcoming ones.


Table of Contents

  1. Supporting imagery
  2. A quick introduction
  3. CPU
    1. The final product
    2. A divided choice of memory
    3. Another processor
  4. Graphics
    1. Sega’s offering
      1. VDP1
      2. VDP2
    2. Defining the problem
    3. As a powerful 2D console
      1. Sprites
      2. Backgrounds
      3. Result
    4. As a challenging 3D console
      1. 3D modelling
      2. Pixel processing
    5. The new designs
    6. An introduction to the visibility problem
    7. The transparency issue
  5. Audio
    1. The opportunity
  6. Operating System
    1. Interactive shell
    2. No BIOS?
  7. Games
    1. Development
    2. I/O
    3. Expansion methods
  8. Anti-Piracy & Homebrew
    1. Defeat
  9. That’s all folks
  10. Referencing
  11. Sources / Keep Reading
  12. Contributing
  13. Changelog

Supporting imagery

Model

Image
The Sega Saturn, released on 22/11/1994 in Japan
Image
The Sega Saturn, released on 11/05/1995 in America and 08/07/1995 in Europe

Motherboard

Image
Motherboard
Showing 'VA8' revision which includes all components in a single board.
Remaining RAM chips are fitted on the back
Image
Motherboard with important parts labelled

Diagram

Image
Main architecture diagram

A quick introduction

Welcome to the 3D era! Well… sorta. Sega enjoyed quite a success with the Megadrive so there’s no reason to force developers to write 3D games right now.

Just in case developers want the extra dimension, Sega adapted some bits of the hardware to enable polygon drawing as well. Hopefully, the result didn’t get out of hand!


CPU

Just like its close competitors drowned with options during the RISC fever, Sega had to go through all the conundrums of choosing a vendor that could bring up the next generation of games (including those with ‘3D’ capabilities). In the end, the company chose a fresh CPU whose creator was desperately looking for an adopter, the Hitachi SuperH or ‘SH’.

Hitachi’s new creation implemented modern arts such as a RISC instruction set and a pipelined data path, however, Sega wasn’t satisfied by the final product, especially due to the small 16-bit multiplier. Thus, Hitachi synthesised a second revision with a larger multiplier unit and other requirements on Sega’s checklist [1], leading to a new CPU called SH-2.

Even so, Sega couldn’t stand still after hearing what choice of CPU its competitors went for. So, they asked Hitachi to step up the clock frequency of the SH-2 - an impossible task once the chip is already out for manufacturing. Luckily, Hitachi had another trick up in their sleeve, multiprocessing. During the research phase of the SH, the team added minimal circuitry to allow the SH to work with other SHs within the same system at the same time. Upon hearing that, Sega decided on a two-chip configuration for the Sega Saturn. And the rest is history.

The final product

Having explained the origins, let’s take a look at the shipped product.

This console has not one but two Hitachi SH-2 CPUs running at ~28.63 MHz each [2]. While both physically identical, they are placed in a master-slave state, where the first one may send commands to the second one. This can achieve some degree of parallelism, albeit both sharing the same external bus [3] (which can lead to congestion).

These processors are part of the Hitachi SH7600 brand, a series designed for embedded systems featuring [4]:

The specific CPU model selected for this console, the ‘SH7604’ or just ‘SH-2’, contain the following additions:

Please note that having two CPUs doesn’t mean that games will work twice as fast! In practice, however, this requires very complex programming to efficiently manage CPUs that share the same bus. Cache also plays a critical part in this occasion.

A divided choice of memory

The Sega Saturn contains a total of 2 MB of RAM for general purpose usage called Work RAM (WRAM). Now, these two megs are split between two very different blocks:

Another processor

If the two SH-2 CPUs weren’t enough, the console contains an additional coprocessor, the Saturn Control Unit or ‘SCU’ which is composed of two modules [5]:

On the bright side, the SCU comes with 32 KB of SRAM for local use. On the bad side, the SCU can’t access WRAM-L.


Graphics

Since the Saturn is the first ‘3D console’ reviewed for this series, let us first go over the fundamental design changes that made way to the new generation of 3D graphics:

Sega’s offering

This console includes two proprietary GPUs, each one serving different purposes while working concurrently. Some may argue that the new GPUs are an evolution of the classic VDP, while others may say it’s a complete redesign… I think it’s a bit of both.

Having said that, let’s take a look at the two chips.

VDP1

Image
VDP1 Architecture.

The Video Display Processor 1 (VDP1) is a chip that draws sprites with geometric transformations [6]. The results are written onto a frame buffer, which is in turn streamed to the VDP2 for display.

This chip is programmed by sending ‘drawing commands’ to it. So, programmers are provided with 512 KB of dedicated RAM to store these drawing commands and the required materials (textures/tiles, colour lookup tables, etc).

Consequently, the VDP1 is designed to use quadrilaterals as primitives, which means that it can only compose models using 4-vertex polygons (sprites). The chip applies Forward Texture Mapping to connect texture points onto the quadrilateral, in that direction. It doesn’t come with any filtering/interpolation technique, so the calculations are subject to aliasing.

The VDP1 also provides this selection of effects:

Two 256 KB frame buffer chips are available to concurrently draw new scenes of the game without breaking the current one being displayed. When the secondary buffer is finished being drawn, the VDP1 starts broadcasting the latter instead (page-flipping), and the cycle continues.

VDP2

Image
VDP2 Architecture.

The Video Display Processor 2 (VDP2) specialises in rendering large (4096×4096 pixels) planes with transformations (rotation, scale and translation) applied on them [7].

More importantly, the VDP2’s renders on-the-fly (without a frame-buffer) like previous tile-based engines. It can display up to 16.7 million colours (24-bit). This chip is also responsible for displaying the VDP1’s buffer, which can also be transformed and/or mixed with the VDP2’s layers. The VDP2’s ‘frame’ is composed of up to four 2D planes and one 3D plane; or two 3D planes.

This chip relies on tile-maps to compose planes and performs perspective correction for 3D texture mapping, this is a more sophisticated approach which takes into account the depth value to compute rotations.

Effects available include multi-texturing (mapping more than one texture per polygon) and shadowing. With the latter, after the VDP2 receives the sprites generated by the VDP1, it can reduce their brightness and blend them with half-transparency. Nonetheless, the VDP2 only receives a sprite stream from the VDP1 (in pace with the CRT beam) so this function tends to be tricky to encode and operate.

This chip also houses 4 KB of Colour RAM (CRAM) which is used to translate VDP1’s custom colour values (index colours) into 24-bit RGB colours.

Finally, even though the VDP2 is limited to two 3D planes, nothing prevents the CPU from using its VRAM as frame-buffer area to draw additional 2D or 3D graphics in software.

I recommend checking out the sources (at the end of the article) if this section got your attention, since the VDPs have a lot more quirks that are beyond the scope of this article.

Defining the problem

As you can see, the architecture of the graphics sub-system is quite complex, so it’s interpreted differently depending on the needs:

As a powerful 2D console

The capabilities of the Saturn for drawing 2D scenes were huge compared to the MegaDrive or SNES, although they weren’t the main selling point of this console.

Sprites

Image
Mega Man X4 (1997).
VDP1’s Sprites plane.

In this case, the VDP1 is tasked with plotting traditional sprites without any 3D distortion applied.

The CPU sets up the VDP1 by writing over its registers and filling its VRAM with commands and tiles. The process can also be accelerated thanks to the DMA controller.

Backgrounds

Image
2D plane 1.
Image
2D plane 2.
Image
2D plane 3.
Mega Man X4 (1997). VDP2’s Background planes.

The VDP2 is then instructed to draw background planes. These, along with the sprite layer, are automatically mixed to form a fully coloured scene.

The commanding part is fundamentally similar to the VDP1: Programmers got registers and VRAM to set up accordingly.

Some functions from the VDP2 can be exploited to create more realistic scenery, such as scaling to simulate a heatwave (see ‘2D plane 2’).

Result

Image
Mega Man X4 (1997). Mixed planes (Tada!).

Not much mystery here, the VDP2 is responsible for the last step of sending the processed signal to the video encoder.

The VDP2 operates in sync with the CRT beam, meaning that its computations correspond to the pixels that will be displayed on the next scan line.

As a challenging 3D console

Here’s where the Saturn shined and struggled at the same time. While this console had eight processors to take advantage of, it all came down to:

For this reason, most games ended up dramatically ranging in quality since each studio came up with a unique solution.

3D modelling

Image
Virtua Fighter Remix (1995).
3D models of characters without textures or background. Notice the primitives used to build the models.

So far we’ve been using individual regular quadrilaterals to form sprites and/or background layers. But what if we group multiple irregular primitives and arrange them to form a more complex figure? This is how 3D models come to fruition.

To put it in simple terms, classic 2D consoles like the Super Nintendo arranges their graphics (backgrounds and sprites) in quasi-rectangular areas. In some cases, such as with Mode 7, programmers can supply a rotation matrix to apply transformations over some of these areas. The Saturn, by contrast, allows defining 4-point quadrilaterals with arbitrary angles between their edges (Sega calls them ‘distorted sprites’). Then, the VDPs’ texture mapping capabilities paint the quadrilateral’s area with a texture, the latter is scaled to conform to the polygon’s shape.

In terms of operations needed with a 3D game, the CPUs and SCU are assigned to formulating a 3D world and project it in a 2D space. Then, both VDPs are commanded to render it, apply effects and finally broadcast it on TV.

Pixel processing

Image
Virtua Fighter Remix (1995).
Rendered scene with 3D models and backgrounds.

Either VDP can draw this new (projected) 3D space and stamp textures and effects. Now, which chip is ‘in charge’ varies between each game.

Some prioritised the VDP1 to draw the closest polygons and left the VDP2 to process distant scenery, others found interesting workarounds to task the VDP2 to draw closer polygons (thereby off-loading the amount of geometry fed into the VDP1). The challenge consists in designing an efficient engine that could display impressive graphics while keeping an acceptable frame rate.

The new designs

These are some examples of characters that were re-designed for this console, the models are interactive so do try to fiddle with them!

Image Image Image Interactive model available in the modern edition
Sonic in Sonic R (1997).
185 quadrilaterals.
Image Image Image Interactive model available in the modern edition
Tails in Sonic R (1997).
254 quadrilaterals.

While the Saturn is only able to draw quadrangles, you’ll soon notice that these models exhibit two triangles instead of a single quadrangle in ‘Wireframe’ mode. This is because the format used to encode this model (glTF, an open standard for modern 3D modelling), so your modern device can render it, doesn’t support quadrangles at the time of this writing. So, I recommend switching to ‘Surface’ mode to observe the quads.

In some way, this tells you how the current graphics technology can struggle to reproduce their ~30-year-old predecessors!

An introduction to the visibility problem

When 3D polygons are projected onto a 2D space, it is crucial to determine which polygons are visible from the camera’s position and which are hidden behind [8]. Otherwise, models are not drawn correctly, effects like transparency appear ‘broken’ and ultimately, hardware resources are wasted. This process is widely known as Visible surface determination or ‘VSD’ and it’s a fundamental problem in the world of computer graphics. There are multiple papers published that describe algorithms that tackle this at different stages of the graphics pipeline. Some of them give very accurate results, while others trade precision for better performance.

Now, unlike academic/professional equipment, consumer hardware is incredibly limited, so the choice of algorithm is narrowed down to just a few… or none whatsoever.

Image
Project Z-Treme (2019, Homebrew) [9].
This engine ditched Z-sort in favour of a binary space partitioning (BSP) approach, fixing the glitches.

The Sega Saturn approach is what I consider a ‘semi-solved’ case. The VDP1 doesn’t implement any VSD function: You either feed the geometry in the correct order or you get a mess. However, Sega provided a graphics library called ‘SGL’ that implemented a solution called Z-sort or Painter’s algorithm [10] which performs polygon sorting by software.

Essentially, SGL allocates a buffer to sort the polygons based on the distance from the camera (from furthest to nearest), then, issues the display commands to the VDP1 in that order.

One of the issues of Z-sort with 3D spaces is that its distance value (Z-order) is approximated, so graphic glitches may still appear. For this, programmers can skip SGL in favour of implementing their own algorithm.

In later articles, you will see alternative approaches. Some still rely on software, while others are accelerated by hardware.

The transparency issue

The Sega Saturn is capable of drawing half-transparent graphics, in other words, mixing overlapping layers of colours (blending) to give the illusion we can see through them. Unfortunately, both VDPs aren’t as coordinated as one would expect, so this effect will not work properly when these layers are found in different VDPs.

As a workaround, games can activate the ‘mesh’ property on a texture. With ‘meshed’ textures, the VDP1 sets the odd X/Y texture coordinates as ‘transparent’ (empty). Making it possible to blend other layers using the transparent pixels. Curiously enough, the mesh would appear blurred if the console was connected to the TV using the composite video signal (which was pretty much the standard back then, aside from RF) resulting in an accidental but effective way to accomplish half-transparency [11].

As you may suspect, this just wasn’t viable for some games, so in the end, these had no option but to ditch half-transparency altogether… Although some studios found ingenious fixes, take a look at these two cases:

Sega’s Daytona (1993).
Traveller’s Tales’ Sonic R (1997).

Both games command the VDP1 to draw foreground objects and background scenery. In turn, the VDP2 renders the landscape image far away and the stats in front of the 3D models. Consequently, VDP1 models with half-transparency won’t refract the VDP2’s landscape as the VDP1 is not aware of the VDP2’s frame buffers.

During my terrible gameplay, you’ll notice that the background of the first game pops out of nowhere (no half-transparency) whereas the second game not only accomplished half-transparency but also a fading effect: Traveller’s Tales found a workaround by changing the ‘mix ratio’ registers of the VDP2 (used for defining the texture’s alpha) combined with switching the lighting levels as the character gets closer [12].


Audio

The sound subsystem consists of several parts [13]:

The opportunity

The new audio capabilities mean that studios can finally record/produce soundtracks in-house and then bundle them in the game without having to re-arrange it (as it happened with limited sequencers or chips with strict synthesis methods).

This has been possible thanks to a combination of many factors:


Operating System

Once the user powers on the console, the first component that starts up is the System Management & Peripheral Control (SMPC), a 4-bit microcontroller that takes care of initialising the neighbouring chips (such as switching on two SH-2s and setting them in a ‘master-slave’ configuration) [14].

Image
Japanese version.
Image
European and American versions.
Logo displayed after splash animation finishes.

Afterwards, the master SH-2’s reset vector is set to 0x00000000 [15], which points to an internal ROM containing the Initial Program Loader (IPL). This program performs the following functions [16]:

  1. Finish initialising the hardware.
  2. If there’s a cartridge inserted and it includes a program, continue booting from there.
  3. If the ‘Video CD’ card is inserted, boot it.
  4. If there’s a disc inserted, check that it’s genuine.
    • While at it, it displays the splash screen animation.
  5. If the disc is genuine, boot the game.
  6. If the disc is not genuine or there’s no disc inserted, run the shell.

Interactive shell

Alternatively to playing games, the Saturn included a music player called ‘Multiplayer’, from which a save manager can be opened.

Image
Interactive shell called ‘Multiplayer’ or ‘Audio CD Control Panel’.
Image
Memory Manager. Moves saves between the cartridge and internal memory.

If a Video CD card is inserted, the player can reproduce MPEG video decoded from the card itself.

No BIOS?

Unlike the PlayStation whose ROM chip bundled a BIOS, which in turn exposed APIs for programmers to use. The Saturn’s ROM is often called ‘IPL’ presumably since its main job is to bootstrap the game and run the shell. However, the latter still stores some routines (called services) to manipulate the hardware (such as managing save data and power control). It even implements a ‘semaphore’! (used to synchronise operations that involve multiple processors at the same time). Hence, that part of the ROM is called System program.


Games

Official Sega Saturn games are loaded from the 2x CD-ROM reader. Its medium, the compact disc (CD), has a capacity of 680 MB. Sega Saturn games follow the ISO9660 for storing data in the CD [17]. Many games store audio tracks next to the data tracks for streaming uncompressed audio while executing the game.

Development

At first, Sega didn’t provide complete software libraries and development tools (in fact, the initial documentation was inaccurate) so the only way to achieve good performance was through harsh assembly.

Later on, Sega released complete SDKs, hardware kits and some libraries to ease I/O and graphics operations. Overall, games are written in a mix of C and various assemblies targeting individual components.

I/O

Peripheral management and real-time clock are also provided by the aforementioned System Management & Peripheral Control (SMPC). The SMPC is controlled with commands sent by the SH-2s.

Expansion methods

This console bundles a considerable number of external connectors and interfaces that only received a handful of uses, at most.


Anti-Piracy & Homebrew

In response to the easiness of cloning a CD, Saturn added a copy protection system (along with region locking) to control the distribution of games.

Copy protection on CDs is applied by burning special data (called ‘system area’) out of reach from conventional burners, the Saturn refuses to boot the disc as a ‘game disc’ if the out-of-reach data is not found or it’s invalid. The disc reader also contains a custom SH-1 processor that interfaces with the rest of the console using obscured protocols.

It’s worth mentioning that since Saturn CDs follow the ISO9660 (a standard file system for CD data), PCs can read the game disc without problems (but, of course, they can’t execute the game unless they use an emulator).

Defeat

First of all, the classic method used for disabling the copy protection consisted in installing a mod-chip that could trick the CD reader when a burned disc is inserted. There was also a ‘swap trick’ that consisted in hot-swapping a genuine disc with a burned one just after the protection checks passed… with the risk of damaging the drive!

After the turn of the century, alternative but more sophisticated methods used for running unauthorised code were discovered, for instance:


That’s all folks

Image
A Japanese Saturn I acquired to get more material for this article. While the games look fine, it was thanks to the Saturn’s enormous Homebrew library that I was able to understand the real capabilities of this console.

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Acquired tools used

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@misc{copetti-saturn,
    url = {https://classic.copetti.org/writings/consoles/sega-saturn/},
    title = {Sega Saturn Architecture - A Practical Analysis},
    author = {{Rodrigo Copetti}},
    year = {2019}
}

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[1]R. Copetti, "Sega Saturn Architecture - A Practical Analysis", Copetti.org, 2019. [Online]. Available: https://classic.copetti.org/writings/consoles/sega-saturn/. [Accessed: day- month- year].

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Sources / Keep Reading

Anti-Piracy

Audio

CPU

Games

Graphics

Operating System

Photography


Changelog

It's always nice to keep a record of changes.

2022-01-26

2021-05-04

2020-04-10

2020-04-08

2020-04-07

2020-02-18

2019-10-30

2019-09-17

2019-09-17

2019-08-27

2019-08-09

2019-08-03