Initially, the I/O of the PS2 wasn't particularly complicated. Yet, subsequent revisions of this console completely disrupted both the internal and external designs. So, overall, this console exhibits many forms of I/O distributed across different revisions.
The special CPU
To start with, there's a dedicated processor that arbitrates the communication between different components, this CPU is no other than the original MIPS R3000-based core found in the PlayStation 1. This time, it's called I/O Processor (IOP) and runs at 37.5 MHz attached to a 32-bit bus .
Main diagram of the PlayStation 2's architecture. Notice the I/O Processor exhibiting exclusive access to most of the I/O.
The IOP communicates with the Emotion Engine using a specialised I/O interface called System Interface or 'SIF', both endpoints use their DMA units to transfer data between each other. The IOP also comes with dedicated memory - 2 MB of EDO RAM (just like the PS1) - used as a buffer.
All in all, this processor gives access to the front ports, DVD controller, SPU2, the BIOS ROM and the PC card slot.
The special upgrade
Be as it may, one year after the 'Slim' revision arrived in 2005, the MIPS core was swapped for a very unusual SoC that instead houses :
- A PowerPC 440x5 CPU. This chip is part of the streamlined PowerPC 4xx series led by IBM and tailored for microcontroller applications.
- An Auxiliary Processing Unit (APU), a package composed of partial MIPS R3000A circuitry (the decoder and ALU), the good-old Geometry Transformation Engine and extra registers for inter-process communication.
- An Ethernet transceiver (previously sold as an external accessory).
There's also 4 MB of SDRAM sitting next to the chip (2 MB more than before and 'SDRAM' this time).
The new package is referred to as PPC-IOP and runs at a lightning 440 MHz instead. Curiously enough, the PPC CPU will still be tasked with running MIPS code . This is done with the help of a MIPS emulator called DECKARD (stored in the BIOS ROM ), which makes use of the additional SDRAM and the APU to accelerate this.
As the inner workings and operating speed of PPC-IOP are considerably different from the original IOP, there's an additional database of patches with entries for each affected game . Furthermore, DECKARD tries to approximate timings using an event handler. However, performance degrades during certain instructions . Additionally, almost 24% of SDRAM is left unused for some reason .
Considering all this, I wonder why Sony redesigned the I/O ecosystem in a way that not only it degrades performance but it also wastes all its perks. Perhaps, it was only done to reduce manufacturing costs while keeping backwards compatibility (I explain more in the next paragraphs). It's worth mentioning that, around that time, Sony had recently struck a deal with IBM to build the Cell processor as well.
Inherited compatibility
For those models bundling the predecessor's CPU, one can suspect PS1 compatibility would be part of the package. Conveniently enough, Sony did bundle a PS1 emulator (called PS1DRV) that loads whenever a PS1 disc is inserted. When this happens, the IOP is under-clocked to run at PS1 speed, the EE is 'repurposed' to emulate the old GPU and the SPU2 is remapped to behave like the original SPU.
With PowerPC-based models, PS1DRV runs on top of DECKARD.
Available interfaces
This console kept the previous front ports that were included in the original PlayStation, it also features a couple of 'experimental' interfaces that appeared very promising at first.
Front of the PS2 showing common ports including Controllers and Memory Cards. Plus, the new USBs and i.Link ports .
The most popular additions are the two USB 1.1 ports. Its theoretical speed is 12 Mbps but that's highly dependent on the IOP's bandwidth (which tends to be considerably slower). Nevertheless, it was widely adopted by third-party accessories.
There were also some 'experiments' that didn't last very long. For instance, the front i.Link port (also known as IEEE 1394 - or 'FireWire' in the Apple world). This port was used to connect two PS2s for local multiplayer, but it was removed after the third revision (presumably replaced by the 'Network card', more details below).
The unusual Ethernet + HDD combo
On the back of the console we also had a slot for PC cards. For this, you could buy the 'Network Adaptor card' from Sony that provides two extra interfaces:
- An Ethernet port, for online multiplayer.
- A proprietary and external Hard Disk Drive Unit port: This was sold by Sony and packaged a typical 3.5" ATA hard drive with 40 GB of space. It enabled games to store temporary data (or permanently install themselves there) for faster load times. Just a few games used this feature, though.
In later revisions, the PCMCIA port was replaced by an Expansion Bay where the 3.5" HDD could now be fitted inside the console. You had to buy first a Network adaptor which not only provided Modem and/or Ethernet ports (depending on the model) but also included the sockets to plug an ATA-66 hard disk.
Back of PS2 showing the Expansion Bay (with the cover removed) .
Behind the scenes, the data in the HDD is structured using a file system called 'PFS' . Strangely, the layout doesn't contain a partition table, just a very primitive catalogue called 'Aligned Partition Allocation' (APA). This may be because Sony only shipped 40 GB drives. Hence, scalability wasn't on their list of priorities.
Network adaptor as seen from the front . This particular model provided modem and Ethernet ports.
Network adaptor as seen from the back , with a hard drive fitted.
The Ethernet transceiver within the adaptor supports transfer rates of up to 100 Mbps (12.5 MB/s). However, the observed rate is notoriously lower (down to 2 MB/s in some cases). The explanation for that is relatively simple: To achieve usable network communication, one is required to implement all the layers of the standard 'OSI Model'; and the transceiver is just one piece of the puzzle. The rest is often delegated to the IOP (thus, done in software) but due to the IOP's limited performance , this results in bottlenecks.
Slimming down
The Slim revision revised the whole Ethernet + HDD model: there's no more Expansion Bay, but an Ethernet port is permanently installed on the back (particular models also included a Modem).
Back of the slim model, showing a fixed Ethernet port.
In addition to that, the new revision added a new infrared sensor, to be used with a Sony-branded remote controller (sold separately).
Interactive accessories
The new version of their controller, the DualShock 2, is a slightly improved version of DualShock.
The DualShock 2 controller .
An official Memory Card (8 MB model) .
During the days of the original PlayStation, multiple revisions of the original controller were released featuring different features (and also bringing fragmentation to the market). Now, for the benefit of developers, there was a single controller that unified all the previous properties.
Compared to the original DualShock, the new version features a slight redesign, it includes two analogue sticks and two vibration motors for richer input and feedback, respectively.
Next to the controller slot is the Memory Card one, now compatible with PS1 and PS2 cards. The latter cards embed extra circuitry for security purposes referred to as MagicGate, which enables games to restrict data transfers between different memory cards. The IOP takes care of encrypting and decrypting the content, and it does so with the help of the MagicGate chipset (found inside the Memory Card) and the DVD drive, the latter bundles the encryption keys.
Some third-party Memory Cards didn't support MagicGate, however.