PlayStation Architecture

A practical analysis by Rodrigo Copetti

Classic edition - Last updated: November 20, 2021

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 eBooks users, legacy internet browsers or readers who use accessibility tools.

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 origins
      1. A bit of history
      2. MIPS and Sony
      3. LSI and the commission
    2. The offering
    3. Taking over the CPU
    4. Complementing the core
      1. System Control Coprocessor
      2. Geometry Transformation Engine
      3. Motion Decoder
    5. Missing units?
    6. Delay galore
  4. Graphics
    1. Organising the content
    2. Drawing the scene
      1. Commands
      2. Visibility approach
      3. Rasterization
      4. Shaders
      5. Textures
    3. Designs
    4. Playing with VRAM
    5. Secrets and Limitations
      1. Distorted models/textures
      2. Contradicting claims
      3. Pre-rendered graphics
    6. Video out
  5. Audio
    1. The streaming era
  6. I/O
    1. CD subsystem
    2. Front ports
  7. Operating System
    1. BIOS/Kernel
    2. Boot process
  8. Games
    1. Development ecosystem
  9. Anti-piracy / Region Lock
    1. Defeat
    2. Retaliation
  10. That’s all folks
  11. Referencing
  12. Sources / Keep Reading
  13. Contributing
  14. Changelog

Supporting imagery

Model

Image
The original PlayStation.
Released on 03/12/1994 in Japan, 09/09/1995 in North America, 29/09/1995 in Europe.
Image
The PS One (slim revision).
Released on 07/07/2000 in Japan, 19/09/2000 in North America, 29/09/2000 in Europe.
This is what you get after cramming so many custom ICs into a single one.

Motherboard

Image
Motherboard
Showing model 'SCPH-1000'.
Remaining chips are fitted on the back.
Later models included SG-RAM instead of VRAM and removed most of the external I/O and video outs.
Image
Motherboard with important parts labelled

Diagram

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Main architecture diagram
The Bus Interface Unit is also connected to special ports of the GPU and SPU.

A quick introduction

Sony knew that 3D hardware can get very messy to develop for. Thus, their debuting console will keep its design simple and practical… Although this may come with a cost!


CPU

This section dissects the ‘Sony CXD8530BQ’, one of the two big chips this console houses. It’s what we would call a ‘System-on-Chip’ in today’s world.

The origins

The main processor is one of those ‘x designed by y, based on z and second-sourced from w’ which is a bit dense to summarise in a few sentences, so why don’t we start with some historical context?

A bit of history

The early nineties were marked by a change in the fates of many popular CPUs. The once-leading 8-bit CPUs, such as the Z80 and 6502, were already out of spotlight and Motorola’s famous 68000, along with other 16-bit designs that enjoyed success in the late 80s, had become candidates for replacement. Even at this time in the PC world, Tanenbaum, in his celebrated debate with Torvalds, gave Intel’s x86 architecture only five more years until its demise from the home market.

At first it may look as if technological development had hit a wall at this point. But in reality there was a new wave of relatively-unknown CPUs making their way into mainstream devices. Many of these designs had originated in academia, and so existed to prove particular sets of design ideas. Examples of next-gen CPUs at this point included:

All of these processors had something in common: They followed the Reduced Instruction Set Computer (RISC) discipline, which radically shifted how these chips had to be designed and programmed. One rule of RISC CPUs dictated that a single instruction can’t mix memory with register operations, allowing hardware designers to simplify the circuitry that executes instructions… and then enhance it with parallelism techniques.

MIPS and Sony

In the late 80s, MIPS CPUs became a topic of conversion after being adopted (and later acquired) by Silicon Graphics Incorporated (SGI) to power their equipment. SGI was an influential innovator in the computer graphics market, especially with the development of hardware-accelerated vertex pipelines, whose task was originally carried out by software (within the CPU).

During the development of the PlayStation, MIPS was offering the R3000A series of processors. These were 32-bit machines and part of their low-end catalogue. Consequently, while the R3000A was not part of the flagship lineup (unlike the R4000, which others would later choose), it was an attractive investment in terms of cost.

Sony designed their audio and graphics chips in-house, but they still needed the leading chip that could drive those two. The selected CPU had to be powerful enough to show off the impressive capabilities of Sony’s chips, but also affordable to keep the console at a competitive price.

LSI and the commission

At the same time, LSI Logic (a semiconductor manufacturer) was providing a ‘build-your-own’ CPU program to businesses. This service was referred to as CoreWare and it enabled clients to build their own CPU packages by choosing a series of building blocks. Part of the CoreWare library included the ‘CW33300’ block, a CPU core derived from the LSI LR33300, an off-the-shelf CPU chip which LSI also commercialised.

Now, where am I going with all this? It turns out LSI’s LR33300 and CW33300 are a binary-compatible version of the MIPS R3000A family. Their architectures slightly differ in some areas but the programming interface (MIPS I ISA) remains the same.

In the end, Sony commissioned LSI to build their CPU package. They chose the CW33000, changed some bits and combined it with other blocks to form the chip you find on the PlayStation’s motherboard.

The offering

The resulting CPU core runs at 33.87 MHz and features:

To do something meaningful, Sony provided 2 MB of RAM for general-purpose use. Curiously enough, they fitted Extended Data Out (EDO) chips on the motherboard. These are slightly more efficient than typical DRAM, obtaining lower latency.

Taking over the CPU

At some point, any subsystem (graphics, audio or CD) will require large chunks of data at a fast rate. However, the CPU will not always be able to keep up with the demand.

For this reason, the CD-ROM Controller, MDEC, GPU, SPU and the Parallel port have access to an exclusive DMA controller whenever they require it. DMA takes control of the main bus and performs a data transfer. The resulting rate is a lot faster than relying on the CPU, though the latter is still needed to set up a DMA transfer.

Also, bear in mind that once DMA kicks in, the CPU can’t access the main bus. This means the CPU will be idling unless it’s got something in Scratchpad to keep it busy!

Complementing the core

Like other MIPS R3000-based CPUs, the CW33000 supports configurations with up to four coprocessors, Sony customised it with three:

System Control Coprocessor

Identified as ‘CP0’, the System Control Coprocessor is a common block found on MIPS CPUs. In R3000-based systems, like this one, the CP0 controls how the cache is implemented. Thus, enabling direct access to data cache (in the form of ‘Scratchpad’) and instruction cache (with ‘cache isolation’). The control coprocessor is also responsible for handling interrupts, exceptions and breakpoints, the latter is useful during debugging.

Wait, shouldn’t co-processors only expand CPU functions? Why is CP0 tightly coupled to the CPU?

Indeed, R3000 cores depend on the system control coprocessor to be able to make use of many components, but whether this should be ‘legal’ or not comes down to the interpretation of the word ‘co-processor’. According to MIPS, a co-processor is not strictly an optional part of the CPU, it may also command the CPU’s surroundings (cache, interrupts, etc). Hence, a co-processor can be an integral part of the system. This is just something to bear in mind while talking about MIPS-related systems.

Later R4000-based systems incorporated a Memory Management Unit (MMU) and a Translation Lookaside Buffer (TLB) into this block, thereby increasing its capabilities and taking up new roles.

Geometry Transformation Engine

The ‘CP2’ or Geometry Transformation Engine (GTE) is a special math processor that accelerates vector and matrix calculations.

While only operating fixed-point types, it still provides useful operations for 3D graphics such as:

You don’t have to remember all these to follow the rest of the article! Just keep in mind that the GTE will be taking care of the first stages of the graphics pipeline, such as 3D projection, lighting and clipping. This will help to generate the required data that will be sent to the GPU for rendering.

Motion Decoder

The Motion Decoder, also called ‘MDEC’ or ‘Macroblock Decoder’, is another processor living next to the CPU. This time, it decompresses ‘Macroblocks’ into a format the GPU can understand. A Macroblock is a data structure that houses a picture with similar encoding to JPEG.

The MDEC decompresses bitmaps made of 8x8 24 bpp pixels at a time. Walker’s programming guide states the MDEC can compute 9,000 macroblocks per second. This would allow to stream a 320x240 px full-motion video (FMV) at 30 frames per second.

DMA is used to send compressed data through CD-ROM → RAM → MDEC. The same path is taken in the opposite direction, though in this case, the destination is VRAM.

While this component is sitting inside the SoC and shares the same data bus, it’s not a MIPS co-processor, so the CPU/DMA access it through the memory map (as opposed to intercepting instructions).

For more info about the MDEC unit, I suggest checking out Sabin’s and Czekański’s resources (see ‘Sources’ section).

Missing units?

So far, we got a ‘CP0’ and a ‘CP2’, but where’s the ‘CP1’? Well, that’s reserved for a Floating Point Unit (FPU) and I’m afraid Sony didn’t provide one. This doesn’t mean the CPU can’t perform arithmetic with decimal numbers, it just won’t be fast enough (software-emulated FPU) or too precise (fixed-point arithmetic instead).

Game logic (involving physics, collision detection, etc) still can get around with fixed-point arithmetic. Fixed-point encoding stores decimal numbers with an immutable number of decimal places. This implies a loss in precision after certain operations, but remember, this is a video-game console, not a professional flight simulator. Hence, the precision-performance trade-off is somewhat feasible.

By the way, sometimes I mix up ‘fixed-point’, ‘floating-point’, ‘decimal’ and ‘integer’ number types (hopefully not anymore!). If you feel the same, I recommend taking a look at Gabriel Ivancescu’s quick summary (see the ‘Sources’ section) to quickly refresh those concepts.

Delay galore

As we’ve seen before, the CW33300 is a pipelined processor, meaning that it queues up multiple instructions and executes them in parallel at different stages. This hugely improves instruction throughput, but if it’s not controlled properly, it can lead to pipeline hazards, resulting in computational errors.

The MIPS I architecture is susceptible to control hazards and data hazards, which means that instructions may get executed when they shouldn’t be; and that instructions may operate with outdated data before it’s been updated.

Image
Instructions from ‘Spyro The Dragon’ visualised on the NO$PSX debugger.
Notice how LW (load word from memory), JAL (jump and link) and BAL (branch on not equal) are followed by a delay slot to prevent hazards.
Marked in red are fillers (useless instructions).
Marked in blue perform meaningful operations.

Consequently, MIPS I CPUs exhibit the following behaviour:

As we can see from the example, some delay slots are filled with meaningful instructions, which perform computations that are not affected by the hazard. Hence, delay slots don’t always imply a waste of cycles.

In most cases, the compiler or assembler will automatically re-arrange instructions to fill in slots, or add useless fillers as a last measure. So, all in all, this phenomenon is a bit of a mixed bag.


Graphics

To recap, a large part of the graphics pipeline is carried out by the GTE. This includes perspective transformation (which projects the 3D space onto a 2D plane using the camera’s perspective) and lighting. The processed data is then sent to Sony’s proprietary GPU for rendering.

Organising the content

The system features 1 MB of VRAM that will be used to store the frame buffer, textures and other resources the GPU will require to render the scene. The CPU can fill this area using DMA.

The type of chip fitted (VRAM) is dual-ported, like the Virtual Boy’s. VRAM uses two 16-bit buses, which enables concurrent access between the CPU/DMA/GPU and the Video encoder.

Image
Memory layout using VRAM
Image
Memory layout using SGRAM

Though in later revisions of this console, Sony switched to SGRAM chips (the single-ported option using an individual 32-bit data bus). Boo!… Well, to be fair, each one comes with its pros and cons. One thing for sure, is that due to the timing differences, later games (such as Jet Moto 3) will display glitched graphics when ran on VRAM-based systems. If you want to know the details, Martin Korth’s ‘Nocash PSX Specifications’ document the different timings and such.

Drawing the scene

If you’ve been reading the Sega Saturn article, let me tell you that the design of this GPU a lot simpler!

Now, to show how a scene is drawn, I’ll mainly use Insomniac’s Spyro: Year of the Dragon as an example. Please bear in mind that the internal resolution of this game is too cramped (292x217 px), preventing me to clearly dissect it at each stage, so I’ve upscaled it a bit for demonstration purposes. Here is a sample at original scale if you are curious.

Commands

Image
Basic GPU pipeline

To start with, the CPU sends geometry data (vertices) to the GPU by filling its internal 64 byte FIFO buffer with commands (up to three). Essentially, a command states how and where to draw one primitive.

Once the geometry is received, clipping is applied to skip operations over unseen polygons (residing outside the camera’s viewport).

The location of the primitive is stated with an X/Y coordinate system pointing to the frame buffer. The PS1’s GPU employs an integer coordinate model, where each coordinate corresponds to the centre point of a pixel (called sampling point). In other words, there are no fractional coordinates.

Visibility approach

Image
This game used pre-ordered geometry to gain performance
Hence its camera only moves backwards or forward
Crash Bandicoot (1996)

Just like the competition, the PS1 doesn’t include any hardware feature that solves the visibility problem. Nonetheless, the GPU handles sorted polygons by providing an ordering table: A dedicated table where each entry is indexed using a depth value (also called ‘Z-value’) and contains the address where the GPU command resides.

The CPU needs to manually sort the polygons first, and then reference them in the correct entry of the table. Finally, the CPU orders DMA to send the table to the GPU. This process will enable the GPU to render the geometry in the correct order.

Multiple DMA functions are also provided to assist both CPU and GPU with the creation and traversal of this table.

Rasterization

Image
Wireframe view of the scene

Once the commands are decoded by the GPU, it’s time to convert the geometry received (vertices) into pixels. This will enable the system to apply texture mapping, effects and ultimately display it on a two-dimensional panel (your TV or monitor). To do this, the GPU allocates a pixel matrix that is used as a working area, this is called a frame buffer. Compared to the more complex Sega Saturn, the GPU only needs a single frame buffer.

The GPU uses triangles as primitives to form 3D models. Being the only available primitive means that backgrounds and foregrounds make no difference in terms of composition (both are made of triangles). 2D games inherit the same nature: They are just two triangles joined to form a quadrangle (though the GPU provides routines for constructing sprites automatically).

The rasterizer is the unit in charge of converting vectors into triangles; and then into pixels. This is done by:

  1. Grabbing each three-point vertex and calculating the edges. This forms a triangle.
  2. Analysing the area of the triangle to identify which pixels of the frame buffer they occupy. Any section of the triangle covering the sampling point is turned into a pixel.

The generated pixels are not written into the frame buffer right away. Instead, they are sent to the next stages of the pipeline for further processing, which we’ll see in the following paragraphs.

Shaders

Image
Gouraud shading in action

In order to apply lighting effects over these polygons, the GPU provides two algorithms:

The reason for having this choice comes down to the fact that flat shading fills ~2.5 times more polygons per second than Gouraud, so it’s important to optimise which polygons need a more realistic shading than others.

Textures

Image
Textures applied (Tada!)

Finally, shaded surfaces are blended with textures (2D bitmaps) to produce the final result.

The GPU performs inverse texture mapping, where the GPU traverses each rasterized pixel and looks for its corresponding pixel in the texture map (called texel). Texels are calculated by linearly interpolating the texture map (found in VRAM) to conform to the shape of the polygon. The routine used for interpolation is called Affine Texture Mapping, this technique only operates using 2D coordinates (X/Y values) while discarding the third coordinate (Z/depth) used for perspective.

Because texture maps rarely have the exact dimension of the rasterized polygon, aliasing (incorrect results) may appear. This is manifested with unwanted distortions, such as missing or enlarged texels. To remedy this, sophisticated GPUs employ texture filtering to smooth out (interpolate) sudden colour changes. Now, The PS1’s GPU doesn’t implement any filter, so it resorts to an algorithm named nearest neighbour to correct scales without smoothing out the results. This is very fast (and cheap) but it also explains why textured models may look ‘blocky’.

The unit also includes the following effects available to use:

It’s worth mentioning that the PS1 happened to excel at those effects!

Once finished, the GPU writes the pixels into the frame buffer area in VRAM, which is in turn picked up by the video encoder and broadcasted to the screen.

Designs

Let’s take a break now from all this theory. Here are some examples of game characters designed from the ground up for the 3D era, they are interactive so I encourage you to check them out!

Image Image Image Interactive model available in the modern edition
Spyro the Dragon (1998)
413 triangles
Image Image Image Interactive model available in the modern edition
Crash Bandicoot (1996)
732 triangles

Playing with VRAM

With the available amount of VRAM (1 whole megabyte), one could allocate a massive frame buffer of 1024×512 pixels with 16-bit colours or a realistic one of 960×512 pixels with 24-bit colours - allowing to draw the best frames any game has ever shown… This sounds pretty impressive, right? Well, it does raise a couple of issues, for instance:

All right, so let’s have a 16 bpp 640x480 buffer instead, which leaves 424 KB of VRAM for materials. So far so good? Again, such resolution may be fine on CRT monitors, but not particularly noticeable on those 90s TVs everyone had at their homes. Then, is there any way to optimise the frame-buffer? Introducing adjustable frame-buffers.

Image
VRAM visualisation on the NO$PSX debugger.
You can spot the dual frame buffers, along with textures (to be translated with a colour-lookup table).

In essence, instead of wasting valuable VRAM by using ‘unappreciated’ resolutions, this console’s GPU allows decreasing the dimensions of the frame buffer to effectively increment the space available for other resources. In ‘Gears Episode 2’ (see ‘Sources’ section), Halkun shows a setup that divides the 640x480 frame-buffer into two 320x480 ones, and then relies on a technique called page-flipping to render multiple scenes at the same time.

Page-flipping consists of switching the location of the frame for display between the two available whenever the game wants it, allowing the game to render one scene while displaying another. Thus, hiding any flickering effect and improving loading times (something that the player will certainly appreciate!).

Overall, Halkun’s layout only consumes 600 KB of VRAM. The rest (424 KB) can be used to store colour lookup tables and textures that, combined with 2 KB of texture cache available, results in a very convenient and efficient setup.

Finally, it is worth mentioning that VRAM can be mapped using multiple colour depths simultaneously, meaning that programmers can allocate a 16 bpp frame buffer next to 24 bpp bitmaps (used by FMV frames, for instance). This is another feature facilitating further optimisation of space.

Secrets and Limitations

Whereas the PS1 had a very simple and suitable architecture, problems ended up arising anyway. Surprisingly, certain issues were tackled with very clever workarounds!

Distorted models/textures

Textures jiggle a bit while they move
Square Soft’s Final Fantasy VIII (1999)

The routines used for handling geometry and applying textures are known to carry some inaccuracies.

Firstly, the rasteriser only handles pixel units: While vertex coordinates are integers, the calculated triangle edges may only occupy a fraction of a pixel. However, the rasterizer will only draw the pixel if the triangle area covers the pixel’s sampling point, and it won’t keep track of the fraction occupied. This brings up some problems:

This is commonly tackled by implementing sub-pixel resolution, from which the rasteriser keeps track of the fractions of pixels occupied by each triangle area. Consequently, anti-aliasing methods can then be added to soften jagged edges or sudden changes in colour.

Moving on, the ordering table puts the burden on the developer/program to show the geometry in the right order. In some cases, the calculations implemented rely on too many approximations to gain performance. This may result in flickering or occluded surfaces that should have been displayed.

Furthermore, setting a low-resolution frame buffer may amplify all these aliasing issues.

Finally, as you know, affine transformations have no sense of depth, which may confuse the user’s perception when the camera is close to the model and is positioned perpendicular to the viewer. The effect is also referred to as texture warping. Hence, some games resorted to tessellation (dividing a big polygon into smaller ones) to reduce distortion, others just swapped textures in favour of solid colours. In general, a GPU ultimately tackles this issue by implementing perspective correction, which interpolates textures using the depth value.

Contradicting claims

If you check other technical channels or forums, you’ll find alternative explanations about the PS1’s wobbling/warping/distorted effects. While some of them will coincide with what I explained before, others will argue differently. So, I’d like to give my opinion on why the following statements are not accurate:

Models/textures wobble due to lack of FPU

An FPU doesn’t dictate whether a computer can operate or not fractional numbers. The PS1, like any other computer without an FPU, can still perform fixed-point arithmetic. Furthermore, using software emulation, floating-point numbers can also be computed (just slower). To sum up, FPUs are just accelerators, do not confuse them with an ALU (the critical portion of a CPU that performs arithmetic) or decimal numbers.

Models/textures wobble due to the GPU’s integer coordinate system

Employing integer coordinates is a common approach to make calculations less expensive in the GPU. It’s the fact of not employing sub-pixel resolution that turns this into a visible problem. To sum up, anti-aliasing could be applied to smooth out sudden changes of colours if the rasterizer kept count of the pixel’s fraction occupied by the triangle.

Textures warp due to lack of mipmapping

GPUs that implement inverse texture mapping such as this one, are subject to a measurement error called ‘undersampling’ (one pixel is mapped to multiple texels). Thus, this creates aliasing (wrong results). The behaviour is noticeable when rendering geometry far away from the camera. To remedy this, modern texture mappers apply ‘trilinear filtering’, which softens texels using the same texture stored at different scales (mipmaps) and does some interpolation in-between. To sum up, mipmapping is an approach to solve aliasing. It’s affine texture mapping (with its lack of perspective correction) that interpolates textures on three-point surfaces without taking into account the depth. Hence, the ‘warping’ effect.

Models/textures flicker due to lack of Z-buffering

A GPU with Z-buffer solves the Visible surfaces determination problem at a hardware level. The PS1 relies on an ordering table, meaning developers are in charge of determining what geometry is in front of others. To sum up, any model exhibiting flickering triangles is caused by the sorting routines (software), so it’s remedied on the same software.

Pre-rendered graphics

Image
Scene with Pre-rendered background
The way models interact and move is essential to trick the user’s perception
Square Soft’s Final Fantasy VII (1997)

Let’s mention a ‘positive’ feature now for a change…

If a game strived for a more realistic scenery than the GPU could provide, one available option was to stack two triangles and use the Motion Decoder to feed pre-rendered cinematic on them. FMV video could weigh a lot of space, luckily the CD-ROM was prepared for this.

Some games relied on it specifically for composing backgrounds and, honestly, it was quite convincing to see them on a CRT TV, obviously modern emulators with upscaling capabilities will tell on very quickly.

Video out

The first revision of this console carries a surprising amount of video signals with the following ports:

Later revisions of the console removed these ports and at the end, only ‘AV Multi Out’ was left.


Audio

Sony’s signature Sound Processing Unit (SPU) takes care of this. This chip supports the enormous amount of 24 channels of 16-bit ADPCM samples (a more efficient version of the well-known PCM sampling) with a sampling rate of 44.1 KHz (Audio CD quality).

This chip also provides the following capabilities:

512 KB of DRAM (called ‘Sound RAM’) are provided as audio buffer. This memory is accessible from the CPU (only through DMA) and the CD controller. Although games only have 508 KB available to store samples, the rest is reserved by the SPU to process Audio CD music. This amount is reduced even further if reverb is activated.

The CD controller is also able to send samples directly to the audio mixer without going through the audio buffer or requiring CPU intervention. Samples can also be compressed using the ‘XA’ encoding, which the SPU can decode on the fly.

The streaming era

Similarly to the Saturn, games are no longer dependant on music sequencing or pre-defined waveforms, and thanks to the amount of storage available on the CD-ROM medium, developers can store fully produced samples and just stream them to the audio chip.


I/O

There are two I/O ports (Serial and Parallel) available for add-ons. However, these were removed in later revisions of the console due to lack of adoption and the fact that they could potentially be used to crack the copy protection system.

CD subsystem

The block controlling the CD drive is an interesting area, you can imagine it as an separate computer living inside the PlayStation.

Image
CD Subsystem layout

This subsystem is composed of:

The subsystem somewhat resembles a typical CD reader everyone had at their home, except with the tweaks Sony implemented in the Sub-CPU program to perform anti-piracy checks.

Front ports

The controller and the Memory Card slots are electrically identical, so the address of each one is hardcoded. Additionally, Sony altered the physical shape of the ports to avoid accidents.

Communication with these devices is accomplished using a serial interface. Commands sent from the console will be delivered to one of the two slots (either ‘mem. card 0’ and ‘controller 0’, or ‘mem. card 1’ and ‘controller 1’). Then, both accessories will answer with their unique identifiers, this will allow the console to focus on a particular type of device (memory card or controller) from now on.


Operating System

The system includes a 512 KB ROM that stores a ‘BIOS’. This program performs many services, including taking care of the startup process, displaying a user shell and finally, exposing a collection of I/O routines.

BIOS/Kernel

The BIOS is a critical dependency for games, as this program bootstraps them from the CD drive. Moreover, the BIOS serves as a ‘middle man’ to interact with the console’s hardware. The latter methodology is similar to what IBM implemented with their IBM PC BIOS, which encouraged developers to make use of a standard interrupt table (containing I/O routines) instead of platform-dependent I/O ports.

Having said that, the PS1 BIOS exposes routines such as:

Since BIOS ROM access is very slow (it’s connected to an 8-bit data bus), the APIs are packaged in the form of a Kernel and copied to main RAM during boot. Thus, 64 KB of main RAM are reserved for said Kernel. By the way, the Kernel is also referred to as PlayStation OS.

Boot process

The CPU’s reset vector is at 0xBFC00000, which points to the BIOS ROM.

Image
Famous splash scene after powering on the console
Image
The shell is displayed if there isn’t any disc inserted

Similarly to the Saturn’s boot process, after receiving power, the PS1 will:

  1. Look for the BIOS ROM and execute routines to initialise the hardware.
  2. Load Playstation OS.
  3. Display the splash screen.
  4. If there is a CD inserted, the CD-ROM controller will check if it’s genuine:
    • It is → The controller will allow to read its content.
      1. The CPU will look for ‘SYSTEM.CNF’ and continue execution from there.
    • It’s not → The CPU will display an error message.
  5. With no CD inserted, the CPU will display the shell. The user is now in control.

The shell is a simple graphical interface that enables the user to copy or delete saves from the memory card; or play an audio CD.


Games

Programs have all the facilities that the CD medium provides: Large storage (640 MB), good audio quality and a ‘not-so-slow’ read speed thanks to the 2x drive.

Development ecosystem

The official SDK provided C libraries which are linked to BIOS routines to access the hardware. If you wonder, this is the main factor that helped to emulate the PS1 on a wide range of platforms.

Along with the SDK, Sony also distributed specialised hardware like the DTL-H2000: a dual-slot ISA card containing the internals and I/O of the PS1, plus extra circuitry for debugging purposes. The board has access to the host’s hard drive and can execute PS1 software without restrictions. Software and drivers used to communicate with the card ran on PCs with Windows 3.1 or 95.


Anti-piracy / Region Lock

Like any other optical media, in order to fetch data from a CD, a laser beam is used to read the pits (zeroes) and lands (ones) from the track of the disc. Now, conventional discs are not 100% flat and they often have tiny fluctuations in their tracks. These defects are completely unnoticeable while reading the data since lasers can automatically calibrate themselves as they read.

This is what Sony based their copy protection on: The Sub-CPU will allow reading discs whose Table of Contents (TOC) are engraved using a defined frequency informally known as Wobble Groove, which is only applied during mastering and cannot be replicated through conventional burners. The TOC is found in the inner section of the CD (called ‘Lead-In’ area) and instructs the laser on how to navigate throughout the disc, it’s repeated many times as a fault-tolerance mechanism.

Within the PS1 game’s TOC, one of the following character strings is embedded:

As you can imagine, the reader applies region-locking using this technique as well.

Defeat

On the other side, this check is only executed once at the start, so manually swapping the disc just after passing the check can defeat this protection… with the risk of damaging the drive. Later on, some games took matters into their own hands and often reinitialised the drive in-game so the check would be executed again, this was done in an effort to prevent users from performing the ‘swap trick’.

Alternatively, tiny boards programmed to mock the wobble signal could be soldered in the console. These boards are known as Modchips and, while legally questionable, they were incredibly popular.

Retaliation

The use of emulators was seen as a threat for publishers as well. As a result, some games included their own checks (mostly checksums) to combat any type of unauthorised use or modification.

One of the checks I was told consisted of deliberately reinitialising the drive and then making it read specific sectors that would not pass the wobble groove check. If this managed to unlock the drive anyway, the game (still residing in RAM) would happily reveal its anti-piracy material. Notice that this approach can also affect modded consoles using genuine games.

Later on, Sony provided a library called Lybcrypt which fortified copy protection with the use of two approaches:


That’s all folks


Contributing

This article is part of the Architecture of Consoles series. If you found it interesting then please consider donating. Your contribution will be used to fund the purchase of tools and resources that will help me to improve the quality of existing articles and upcoming ones.

Donate with PayPal Become a Patreon

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

Interesting hardware to get (ordered by priority)

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


Referencing

This work is licensed under a Creative Commons Attribution 4.0 International License. You may use it for your work at no cost, even for commercial purposes. But I ask that you reference it properly. Please take a look at the following citation guidelines:

Article information

For any referencing style, you can use the following information:

For instance, using the IEEE style:

[1]R. Copetti, “PlayStation Architecture - A Practical Analysis”, Copetti.org, 2019. [Online]. Available: https://classic.copetti.org/writings/consoles/playstation/. [Accessed: day- month- year].

and Harvard style:

Copetti, R., 2019. PlayStation Architecture - A Practical Analysis. [online] Copetti.org. Available at: https://classic.copetti.org/writings/consoles/playstation/ [Accessed day month year].

Special use in multimedia (Youtube, etc)

I only ask that you include at least the author’s name, title of article and URL of article, using any style of choice.

You don’t have to include all the information in the same place if it’s not feasible. For instance, if you use the article’s imagery in a Youtube video, you should state either the author’s name or URL of article at the bottom of the image, and then include the complete reference in the video description.

Appreciated additions

If this article has significantly contributed to your work, I would appreciate it if you could dedicate an acknowledgement section, just like I do with the people and communities that helped me.

This is of course optional and beyond the requirements of the CC license, but I think it’s a nice detail that makes us, the random authors on the net, feel part of something bigger.


Sources / Keep Reading

General

CPU

Graphics

I/O

Operating System

Games

Photography

Other Media


Changelog

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

2021-06-30

2021-06-29

2021-06-18

2021-06-17

2021-06-14

General overhaul with lots of changes:

Thanks dpt, Nicolas Noble and the Emulation Development discord.

2021-02-13

2020-04-24

Thanks simias and /u/impiaaa from HackerNews and /r/programming, respectively.

2020-04-19

2020-04-11

2020-02-28

2020-01-27

2019-10-29

2019-10-09

2019-09-17

2019-08-29

2019-08-09

2019-08-08