Sega Saturn Architecture

A Practical Analysis

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The Sega Saturn, released on 11/05/1995 in America and 08/07/1995 in Europe
The Sega Saturn, released on 22/11/1994 in Japan
Showing 'VA8' revision which includes all components in a single board
Remaining RAM chips are fitted on the back

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 3D, Sega adapted some bits of the hardware to enable polygon drawing as well, hopefully, the result didn’t get out of hand!


The system has not one, but two Hitachi SH-2 CPUs running at ~28.63MHz each. 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 (which can lead to congestion).

These processors are part of the Hitachi SH7600 Series, a series designed for embedded systems featuring:

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

Having two CPUs doesn’t mean that games will work twice as fast, in practice, this requires very complex programming to efficiently manage CPUs that share the same bus! Here is when cache comes very handy.

The console contains an additional coprocessor, the Saturn Control Unit or ‘SCU’ which is composed of two modules:

A divided choice of memory

The system contains a total of 2 MB of RAM for general purpose usage, this is called Work RAM or ‘WRAM’. Now, these two megs are split between two very different blocks. The first one provides 1 MB of SDRAM and due to its fast access rates, this block is also called ‘WRAM-H’. The other block contains the other megabyte, but it’s named ‘WRAM-L’ since it uses DRAM instead, resulting in lower rates. It’s worth mentioning that the SCU can’t access the latter type.


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 32-bit proprietary GPUs, each one serving different purposes while working concurrently:


VDP1 Architecture

The Video Display Processor 1 or ‘VDP1’ is a custom chip specialised in rendering polygons, it is designed to use quadrilaterals as primitives which means that it can only compose models using 4-vertex polygons.

Textures are applied using the following algorithms:

  1. Forward Texture Mapping to map the textures into each quad. It is subject to some aliasing.
  2. Bilinear Approximations to correct unstable textures (noticeable while slowly moving the camera view), this effect is also called texture warping.

Since texture-related operations tend to make intensive use of the memory bus, programmers are provided with 512 KB of VRAM to cache textures and avoid congesting the bus, resulting in better fill-rates.

The chip also provides this selection of effects:

  • Two shading algorithms (Flat and Gouraud) for lighting.
  • Edge anti-aliasing to smooth out jagged edges.
  • Clipping to discard polygons outside the camera’s viewport.
  • Transparency.

Two 256 KB frame-buffers are available to concurrently draw new scenes of the game without breaking the current one being displayed (double-buffering). When the secondary buffer is finished being drawn, it is then copied to the primary one during special events (like V-Blank) so the user doesn’t notice this operation.


VDP2 Architecture

The Video Display Processor 2 or ‘VDP2’ specialises in rendering large (4096×4096 pixels) planes with the ability to apply transformations (rotation, scale and translation) on them. It can either draw up to four 2D planes and one 3D plane; or two 3D planes.

Its features were technically advanced at the time, algorithms used to accomplish these include:

  • The use of classic tile-maps.
  • Perspective transformations to project 3D objects onto a 2D space and map textures.
  • Multi-texturing to map more than one texture per polygon.
  • Bump-mapping to simulate bumps on the plane’s surface without spending extra polygons.

This chip also contains 4 KB of CRAM enabling it to display up to 16.7 millions of colours, which the VDP1 can also access. The VDP2’s frame is generated by first mixing the VDP1’s frame-buffer with its own ones.

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 3D graphics by software.

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.


VDP1/Sprites plane
Mega Man X4 (1997)

In this case, the VDP1 is tasked to draw plain individual quadrilaterals that are filled with textures (one per polygon), this is how sprites are achieved.


VDP2/Background planes
Mega Man X4 (1997)

The VDP2 is then required to draw multiple background planes that are finally mixed forming a fully coloured scene.

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


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

Not much mystery here, the VDP2 is tasked with the last step of combining all frame-buffers and letting the video encoder take it from there.

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 their unique solution, the possible permutations were almost infinite!


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

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

In a nutshell, the CPU is tasked with formulating a 3D world, while both VDPs will be commanded to project it, apply textures and effects on it and finally display it in a 2D space.


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

Either VDP can draw this new 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 (off-loading the amount of geometry fed into the VDP1). The challenge was to design an efficient engine that could display impressive graphics while keeping an acceptable frame-rate.

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. 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.

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

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 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, it 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 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!

3D model
Sonic in Sonic R (1997)
298 triangles
3D model
Tails in Sonic R (1997)
425 triangles

The transparency issue

The Sega Saturn is capable of drawing half-transparent graphics, in other words, mixing overlapping layers of colours 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 spread around the two VDPs at the same time.

As a workaround, games could activate the ‘mesh’ property on a texture. With ‘meshed’ textures, the VDP sets the odd X/Y texture coordinates as ‘transparent’. 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 halt-transparency.

As you may suspect, this just wasn’t viable for some games, so at the end, these had no option but to ditch half-transparency all-together.
Although… some 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. The VDP2 draws instead the landscape image far away and the stats in front of the 3D models. With this layout, VP1 models with half-transparency won't refract the VDP2's landscape as the VDP1 is not aware of the VDP2's frame-buffers.

Apart from 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.


The sound subsystem consists in several components:


The console starts by booting from the IPL (initial program loading) ROM which initialises the hardware and displays the splash screen. Then the game is loaded from the 2x CD-ROM reader, its medium (CD) has a capacity of 680 MB of data.


At first, Sega didn’t provide complete software libraries and development tools (even the 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.


Peripherals are handled by the SMPC (System Management & Peripheral Control), a micro-controller that also provides a real-time clock and allows the SH-2 to program them.


The cartridge slot is used to provide storage (save data) or extra RAM. Another expansion slot is found near the CD Reader, this one expects a ‘Video CD Card’ that, as the name suggests, enables to play Video CD.

Anti-Piracy & Homebrew

Copy protection on CDs is applied by burning special data out of reach from conventional burners, the Saturn CD reader refuses to read the disc if the out-of-reach data is not found. The disc reader also contains a custom SH-1 processor that interfaces with the rest of the system using encrypted communication. It’s worth mentioning that Saturn CDs don’t have any reading protection, so you can actually access its content from a PC.

A popular method of disabling the copy protection was by installing mod-chips that could trick the CD reader when a burned disc is inserted.

A more sophisticated method for running unauthorised code was published in 2016 (almost 20 years later) by exploiting the fact that the Video CD add-on can inject unencrypted code to the CD subsystem (bypassing the CD reader altogether), this finally allowed load custom code without depending on the ageing drive.

Sources / Keep Reading




Copy protection




This article is part of the Architecture of Consoles series. If you found it interesting please consider donating, your contribution will be used to get more tools and resources that will help to improve the quality of current articles and upcoming ones.

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A list of desirable tools and latest acquisitions for this article are tracked in here:

## Interesting hardware to get (ordered by priority)

- A PAL/NTSC/JAP Saturn console with a controller (£50 - ?)
- An optical drive emulator (only if found at a reasonable price)

Alternatively, you can help out by suggesting changes and/or adding translations.


Always nice to keep a record of changes.

## 2020-04-10

- New sub-section explaining the visibility problem

## 2020-04-08

- New memory section, thanks /u/EmeraldNovaGames.
- Added more content to the CPU section, thanks Ponut64 from Sega Xtreme.

## 2020-04-07

- Small corrections, thanks /r/SegaSaturn.

## 2020-02-18

- Improved some explanations.

## 2019-10-30

- Added 3d models.

## 2019-09-17

- Better wording.

## 2019-09-17

- Added a quick introduction.

## 2019-08-27

- Corrected some explanations.

## 2019-08-09

- Improved wording.

## 2019-08-03

- Ready for publication.

Rodrigo Copetti

Rodrigo Copetti

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