PC Engine / TurboGrafx-16 Architecture

Supporting imagery

Model

Motherboard

Diagram

A quick introduction

Hudson and NEC joined forces to kickstart the 4th generation of consoles. Unfortunately, their efforts will be eventually eclipsed once the competition arrives. Nonetheless, their console will stay on the top as one of the most compact designs on the market.

Models and variants

Just like the Master system, NEC shipped a lot of revisions and strange variations which can be a bit troubling to follow at first. So, for future reference, here are the most important models:

• PC Engine: The first console featuring this architecture, but only released in Japan. It looks like a small white box with minimal ports.
• TurboGrafx-16: Redesigned PC Engine for the American market. It has the same features with a different external appearance. Reminds me of the NES/Famicom adaptation.
• Supergrafx: Enhanced version of the PC Engine with a completely different case and new hardware. Its exclusive features are out of the scope of this article.
• PC Engine Duo/TurboDuo: A combination of the PC Engine/TurboGrafx and the aftermarket CD-ROM² expansion.

This article will focus on the PC Engine/TurboGrafx-16, but will also discuss the expansions that led to the release of the PC Engine Duo/TurboDuo.

CPU

Inside this console, we find the HuC6280, a chip made by Hudson Soft that houses two components. One of them is the CPU, which can operate at two speeds: ~1.79 MHz and ~7.16 MHz.

The HuC6280 is not an off-the-shelf component, like the Z80, but instead a proprietary CPU designed by NEC. After checking out the official docs, it does seem to replicate a lot of behaviour from the famous MOS 6502 and WDC’s 65C02. That being said, what does all of this mean for the programmer?

Before we start, we’ve already covered some parts of the MOS 6502 in the NES article and also described a 16-bit variant made by WDC and Ricoh in the SNES one, in case you want to check them out first.

The 65C02 is another modified version of the MOS 6502 made by Western Design Center, it’s a lot more efficient due to its manufacturing approach (CMOS). From the software side, the 65C02 adds a couple of new instructions and modifies the behaviour of some existing ones ^[1].

We could talk a lot more about the 65C02, but I think it’s better to focus on the features added by Hudson (complementing the 65C02) because they are critical to efficiently operate this console. Compared to the 65C02, the HuC6280 has ^[2]:

• More opcodes, some targeting the components surrounding the CPU.
• An 8-bit I/O port used to interface external components.
• An interrupt controller to receive interrupts from other devices.
• A timer. It works by counting down from a specified value and when it finishes, it dispatches an interrupt to the CPU.

Finally, there are 8 KB of RAM available for general purpose.

Memory access

One thing I didn’t mention yet is that NEC also added a Memory Management Unit or ‘MMU’ next to the CPU, allowing it to handle 21-bit addresses (remember the original 6502 only has 16-bit addresses) ^[3]. Thus, the amount of memory that can be accessed raises from 64 KB to 2 MB.

This MMU is very different from any modern-day MMU, I would say the former is closer to a mapper. Having said that, the MMU found in the PC Engine is composed of eight 8-bit registers (called Mapping Register or ‘MPR’) which are combined with the CPU’s 16 address lines to form a 21-bit address bus.

This works as follows:

1. The CPU can read from and/or write to any MPR using the special TAM and TMA instructions, respectively.
2. When physical memory is accessed, the MMU reserves the address lines A13-A15 from the CPU to select one of the eight MPRs.
3. The final address is derived by combining the CPU lines (placed at A0-A12) with the MMU lines (placed at A13-A20, where the output is the 8-bit value of the selected MPR), resulting in a 21-bit address.

Consequently, this MMU groups physical memory in 8 KB pages (13 CPU lines = 8 KB pages), which is the amount of memory the CPU can access without swapping out the MPR or altering its value.

Anyway, don’t overwhelm yourself if you have trouble understanding it (it’s just an unconventional modus operandi some may find interesting).

Graphics

This is taken care of by the Hudson Soft HuC6270, a separate chip also referred to as the Video Display Controller or ‘VDC’. The HuC6270 will draw everything that the player will see on the screen and its functionality is very similar to Sega’s counterpart, so please check out that article beforehand since I’ll focus on what’s different with Hudson’s offering.

Organising the content

First things first, the VDC is a tile engine (pretty much the standard until the 5th generation showed up) but notice how the PC Engine includes 64 KB of VRAM which is a significant amount compared to the competition. This may lead to a new type of content, which we’ll check later on.

The way graphics data is arranged is a bit confusing: Both CPU and VDC use 16-bit addresses, but while the CPU can only handle 8-bit words, the HuC6270 stores 16-bit words in VRAM ^[4]. This means that a single address in RAM contains a byte while an address in VRAM stores two bytes, so developers had to watch out for that discrepancy when transferring data to VRAM.

The reason for this comes down to the way Hudson organised the circuitry: The VDC has a 16-bit address bus but only the first 15 lines are controlled (the last one is always set to ‘0’), so odd addresses are fetched from the second byte. I don’t know why Hudson went down this path, but I do know that all of this would make better sense if the system had instead 128 KB of VRAM (since a 16-bit address bus can only access up to 64 KB, so by somehow doubling the data bus, up to 128 KB can be retrieved). Maybe that was the original plan for Hudson/NEC?

Constructing the frame

Aside from the aforementioned granularity oddity, functionality-wise the VDP is very simple. The subsystem has three main components: The VDC and VRAM, which we already discussed - and the video encoder (also called ‘VEC’, we’ll see more about it in due time).

The system can use multiple resolutions, this is because the game can alter a set of registers that act as parameters for controlling the display timings, which in turn alter when the CRT starts beaming the frame (as opposed to overscan). The minimum resolution is 256 × 224 pixels ^[5], while some homebrew has proved this system can go as high as 512 × 240 pixels.

Now let’s see how a frame is drawn step by step, for this, I’ll borrow Bonk’s Adventure’s assets.

Tiles

As a quick reminder, tiles are just 8x8 pixel bitmaps that the renderer fetches to draw portions of the screen. With the VDC, the frame is composed of two planes: The background layer and the sprite layer.

Inside VRAM there’s an area called Character generator where tiles exclusive to the background layer are defined. Each pixel of a tile occupies four bits, so it can use up to 16 colours. In theory, up to 4096 background tiles can be defined, but this is less in practice due to VRAM being much smaller.

Sprites, on the other hand, are drawn using tiles from a separate memory location in VRAM, this is called Sprite Generator and differs from the previous Character generator as tiles here are 16x16 pixels wide.

The video encoder is a separate chip that stores 32 colour palettes (16 for the background and 16 for sprites) ^[6], each palette stores 16 colours and each colour is 9-bit wide (3 bits for Red + 3 bits for Green + 3 bits for Blue).

Storing tiles

(This section is written for those interested in how Hudson took advantage of 64 KB of VRAM with that 16-bit granularity, but you don’t have to understand it completely to be able to follow the rest of the article).

So far we’ve discussed that each pixel of a tile is stored using 4 bits (or a half byte, also called a nibble). Now, Hudson dictates that tiles are composed of four 8x8 bitmaps (called ‘CH0’, ‘CH1’, ‘CH2’ and ‘CH3’, respectively). Each map is 1-bit wide, but once the four are combined, they form the final tile with 4-bit pixels.

Also, remember the 16-bit alignment on VRAM? Each 16-bit word stores a single row of two 1-bit bitmaps (8 rows + 8 rows). So, after writing eight entries, two maps will be stored (instead of just one). Please have a look at the diagrams to help understand this better.

The same happens with Sprite tiles, but since they are 16x16 bitmaps, each bitmap occupies 16 words. To put it another way, to store a single sprite tile, 64 words are taken (equivalent to 8 bytes in VRAM).

Background Layer

The background layer is constructed by filling the Background Attribute Table with entries in VRAM, the position of each entry defines X/Y coordinate of the tile on the screen. Each entry contains the tile index from the Character Generator and the colour palette.

The maximum dimension of this layer is 1024 x 512 pixels (128 x 64 tiles), but programmers can set up a 256 x 256 pixels (32 x 32 tiles) layer as the minimum.

As always, this layer is scrollable by changing the value of some registers in the VDC.

Sprite Layer

The VDC contains an internal memory called Sprite Attribute Table Buffer where up to 64 sprites can be defined. Each entry of the table stores the independent X/Y position, colour palette, tile index and H/V flip. Furthermore, there’s an attribute allowing to combine a sprite with another one.

Each entry is 8 bytes long, although some space is wasted due to the 16-bit granularity.

To top it all off, the CPU can’t access this table, so it needs to be completed in VRAM and then activate a DMA channel to copy it to the VDC (so the latter can use it).

Regarding limitations, there can only be up to 16 sprites per scan-line. On the other side, interrupts can be set so the game can be notified when there’s been a case of sprite overflow or collision.

Result

So far we’ve seen how the VDC does all the heavy work, but the last task is actually delegated to the Video Encoder or ‘VCE’. Hudson named this chip HuC6260 and its basic function is to receive 9-bit data streams from the VDC, apply the colour palettes, and send the result to the TV (in the form of an analogue signal).

If you read previous articles you may be familiar with the importance of timing. Well, here is no exception: To avoid unwanted artefacts (such as ‘snow’), the VCE can only be updated during a vertical interrupt.

Video Output

The Video encoder outputs RGB (along with Sync) and YPbPr, this is ideal for use with a SCART cable or component cable, respectively. This looks good so far!…

… Unfortunately, Hudson decided to fit an RF modulator as the only way to get video out of the box, so it’s not that great after all. But then again, the PC Engine was designed in the 80s, so this approach guaranteed compatibility with all kinds of TVs (assuming they are from the same region of the console).

On the bright side, the Expansion Port contains pins that carry RGB video and multiple sync types, but they will need an external accessory to be able to advantage of them.

Audio

The PC Engine contains a Programmable Sound Generator or ‘PSG’ like many other third-generation consoles analysed on this website. On the other side, this PSG particularly relies on the waveform memory (also called ‘waveform buffer’) approach to synthesise sound, as opposed to using a pre-defined set of waveforms (i.e. pulse, triangle, etc). Waveform memory enables programmers to define their own waveforms, which gives more flexibility for soundtrack arrangements.

There are six audio channels in this system ^[8], each one is set up by writing to a set of registers. The latter stores the attributes of the channels, which include:

• Waveform shape: The waveform cycle is generated using thirty-two 5-bit values, each one corresponds to the amplitude of the wave at a particular time.
  • Some games like Fire Pro Wrestling 2 and Bloody Wolf even altered the values during playback to obtain special sounds (at the expense of audible clicks during transfers).
• Frequency control: Two 8-bit registers alter the frequency of the channel to produce different musical notes with the same waveform.
• Amplitude level: A single 8-bit register stores two 4-bits values that specify how loud the channel will sound. The two values correspond to the ‘Left’ and ‘Right’ panning control, a pioneering feature compared to what the competition offered.

Extra functions

Some groups of channels have other modes of operation. For instance, the last two channels are connected to a noise generator. Moreover, the second channel can act as a Low-frequency oscillator to modulate the first channel (I initially assumed this to be a synonym of FM synthesis, but LFOs are more a ‘subset’ that achieve a vibrato effect).

Speaking of modes, there’s another one available called Direct D/A (DDA) which enables the CPU to write over the audio buffer directly (bypassing the PSG). Consequently, the PC Engine can play PCM samples. These are still 5-bit and tightly dependent on CPU cycles. Although, two channels can be combined to reproduce 10-bit samples! I strongly suggest you check out the ‘Sources’ section to listen to examples ^[9].

All in all, this makes the PC Engine’s PSG an incredibly flexible piece of hardware.

Final sound

The PSG will mix everything and output a stereo signal. But again, unless you find a way to bypass the RF port, you’ll only hear mono from your telly.

I/O

This console had a massive amount of accessories, some of which completely enhanced the console internally (by providing more RAM, for instance) and externally (by adding more ports and/or the possibility to read other storage mediums).

This is greatly attributed to the inclusion of the Expansion Port on the back of the console, which gave access to the following components ^[10]:

• +5 Volts line: To power the accessory.
• A Voltage input line: To bypass the original PSU and power the console through the accessory.
• Audio Input and Output: Which can receive the stereo audio output and/or send audio from an input line.
• CPU data and address lines: Enables the CPU to access the accessory and vice versa.
• VDC data lines (those that go to the Video encoder): Allows the accessory to tap those lines, I’m not sure what for though.
• CPU control lines: Reads the HuCard and CD detection flag, speed mode used and also has access to interrupt lines.
• Analogue video out: Including RGB + Vertical Sync and Horizontal Sync + Composite sync.

I’m curious to know to which extent NEC or Hudson envisioned this console, were they trying to build some sort of ‘modular console’ that could be upgraded to step into the next generation?

The rest

I still haven’t mentioned the controller (or Joypad, as some places call it). It’s very similar to the other offerings. The console only provides one controller port and games check a single address in memory to get the key presses. That address returns a 4-bit value ^[11].

Operating System

There’s no internal ROM or ‘BIOS’ or anything else that runs before the game, thus we don’t have an Operating System here. The reset vector is found at addresses $FFFE and $FFFF (remember that the internal registers are 8-bit but memory addresses are 16-bit, hence we need two words to initialise the program counter) and those addresses point to the game card ^[12].

This also means that it’s up to the programmer to take care of ‘housekeeping’ (i.e. initialising memory, setting up the MMU and so on…).

Games

Programs are written in 6502 assembly, spiced with extra 65C02 opcodes and the ones introduced by Hudson.

Standard Medium

Instead of relying on those chunky and boring cartridges everyone else seems to like, NEC/Hudson devised yet another medium, this time with the size of a credit card, called HuCard. They derive from an older medium called ‘Bee Card’, used by some MSX games.

Curiously enough, they are very similar to the Sega Card but they contain 38 pins instead of 35 ^[13]. Internally, the differences are more significant:

• The CPU memory map can access up to 1 MB of HuCard memory without a mapper, hence the majority of games stuck to 1 MB ROMs. Meanwhile, games like ‘Street Fighter II’ resided in a 2.5 MB ROM! ^[14]
• There’s an Audio In pin which seems to allow the card to feed the console an extra audio channel.

CD-ROM Expansion

That expansion port opened the door to such a massive amount of accessories and expansion units that they eventually turned a simple PC Engine into something completely different. As if wasn’t enough, the HuCard slot complemented the possibilities of expansion. What I mean to say is that for this article, I’ll have to focus on the notable upgrades (some of which were included in future revisions of the console).

Let’s take a look at the CD-ROM² expansion, which consists of a CD-ROM reader and a special HuCard called System Card, the latter acts as a BIOS to bootstrap the game and provide some routines to interface the reader. Internally, the reader included 64 KB of RAM for general purpose, another 64 KB of RAM to stream ADPCM samples and finally, 2 KB for save data. As you can guess, this enabled game developers to take the advantage of extra storage and CD audio, while giving publishers the economic relief to distribute their games in a non-proprietary medium.

Years later, NEC released another CD module called Super CD-ROM² and while the reader’s hardware remained mostly the same, its general-purpose RAM was incremented to 256 KB. Customers that already owned a CD-ROM² unit were able to ‘update it’ by purchasing the Super System Card, which included the extra RAM (and one more I/O routine!). On top of all that, NEC later shipped new BIOS updates called Arcade Card, which in this case contained 2 MB of RAM.

CD-based games strictly depended on the BIOS card they were developed for, although newer cards were backwards compatible (with some exceptions). Thus, the Arcade Card is the preferable choice for users that want to play almost all CD-ROM-based games. I mentioned ‘almost’ since third-party companies also released their own BIOS cards (i.e. ‘Games Express CD Card’), and these were required to play games specifically from that studio.

With the Super CD-ROM², NEC/Hudson also shipped a new variant of the PC-Engine/TurboGrafx-16 called Turbo Duo that bundled the console, reader and BIOS card in a single package.

Other expansions

If you are curious about other expansions released, check out the online catalogue ^[18].

Anti-Piracy and Homebrew

HuCards are region-locked, meaning that a PC-Engine game won’t work on a TurboGrafx-16 (or vice versa) out-of-the-box. This is because the order of the data lines is inverted on the American version. As you can guess, this isn’t something a cheap adapter can’t tackle. However, games may include routines to check the system’s region.

On the other side, CD-ROM games are neither region-locked nor copy-protected, but bear in mind that they still need a System Card to bootstrap them (which is region-locked).

That’s all folks

This is it! You’ve just read the last article of the year.

2020 has been an ‘interesting’ year for the website (setting aside all those negative events that have affected all of us). The year started with the Wii article and then carried on with the PS2, Xbox, Nintendo DS, Master System and finally, this one.

A lot has changed in the number of visitors too. It had a slow start at first, but then during the summer, it caught an amazingly high number of visits. I want to thank Hacker News users, OSNews, Reddit, Gigazine, YouTubers, Twitter users and lots of other forums and people for sharing the articles. And of course, a big shout-out to the donors whose generosity helped keep the site afloat!

Have a nice Christmas, happy New Year 2021 and until next time!
Rodrigo