A quick introduction
The internal design of the Game Boy Advance is quite impressive for a portable console that runs on two AA batteries.
This console will carry on using Nintendo’s signature GPU. Additionally, it will introduce a relatively new CPU from a UK company that will surge in popularity in the years to come.
Most of the components are combined into a single package called CPU AGB. This package contains two completely different CPUs:
- A Sharp SM83 running at either 8.4 or 4.2 MHz: If it isn’t the same CPU found on the Game Boy! It’s effectively used to run Game Boy (DMG) and Game Boy Color (CGB) games. Here’s my previous article if you want to know more about it.
- An ARM7TDMI running at 16.78 MHz: This is the new processor we’ll focus on, it most certainly runs Game Boy Advance games.
Note that both CPUs will never run at the same time or do any fancy co-processing. The only reason for including the very old Sharp is for backwards compatibility.
Before ARM Holdings (currently “Arm”) became incredibly popular in the smartphone world, they licensed their CPU designs to power Acorn’s computers, Apple’s Newton, Nokia’s phones and the Panasonic 3DO. Nintendo’s chosen CPU, the ARM7TDMI, is based on the earlier ARM710 design and includes:
- ARM v4 ISA: The 4th version of the 32-bit ARM instruction set.
- Three-stage pipeline: Execution of instructions are divided into three steps or stages. The CPU will fetch, decode and execute up to three instructions concurrently. This enables maximum use of the CPU’s resources (which reduces idle silicon) while also increasing the number of instructions executed per unit of time.
- 32-bit ALU: Can operate 32-bit numbers without consuming extra cycles.
Moreover, this core contains some extensions referenced in its name (TDMI):
- T → Thumb: A subset of the ARM instruction set whose instructions are encoded into 16-bit words.
- Being 16-bit, Thumb instructions require half the bus width and occupy half the memory. However, since Thumb instructions offer only a functional subset of ARM you may have to write more instructions to achieve the same effect.
- Thumb only offers conditional execution on branches, its data processing ops use a two-address format, rather than three-address, and it only has access to the bottom half of the register file.
- In practice Thumb uses 70% of the space of ARM code. For 16-bit wide memory Thumb runs faster than ARM.
- If required, ARM and Thumb instructions can be mixed in the same program (called interworking) so developers can choose when and where to use each mode.
- D → Debug Extensions: Provide JTAG debugging.
- M → Enhanced Multiplier: Previous ARM cores required multiple cycles to compute full 32-bit multiplications, this enhancement reduces it to just a few.
- I → EmbeddedICE macrocell: Enables hardware breakpoints, watchpoints and allows the system to be halted while debugging.
The inclusion of Thumb in particular had a strong influence on the final design of this console. Nintendo mixed 16-bit and 32-bit buses between its different modules to reduce costs while providing programmers with the necessary resources to optimise their code.
Usable memory is distributed across the following locations (ordered from fastest to slowest):
- IWRAM (Internal WRAM) → 32-bit with 32 KB: Useful for storing ARM instructions.
- VRAM (Video RAM) → 16-bit with 96 KB: While this block is dedicated to graphics data (explained in the next section of this article), it’s still found in the CPU’s memory map, so programmers can store other data if IWRAM is not enough.
- EWRAM (External WRAM) → 16-bit with 256 KB: A separate chip next to CPU AGB. It’s optimal for storing Thumb-only instructions and data in small chunks. On the other side, the chip can be up to six times slower to access compared to IWRAM.
- Game PAK ROM → 16-bit with variable size: This is the place where the cartridge ROM is accessed. It has a very slow access rate so it’s mirrored in the memory map to manage different access speeds. Additionally, Nintendo fitted a Prefetch Buffer that interfaces the cartridge to alleviate excessive stalling. This component independently caches continuous addresses when the CPU is not accessing the cartridge.
- In practice, however, the CPU will rarely let the Prefetch Buffer do its job. Since by default it will keep fetching instructions from the cartridge to continue execution (hence why IWRAM and EWRAM are so critical).
- Game PAK RAM → 8-bit with variable size: This is the place where the cartridge RAM (SRAM or Flash Memory) is accessed.
- This is strictly an 8-bit bus (the CPU will see ‘garbage’ in the unused bits) and for this reason, Nintendo states that it can only be operated through their libraries.
Although this console was marketed as a 32-bit system, the majority of its memory is only accessible through a 16-bit bus, meaning games will mostly use the Thumb instruction set to avoid spending two cycles per instruction fetch. Only in very exceptional circumstances (i.e. need to use instructions not found on Thumb while storing them in IWRAM), programmers will benefit from the ARM instruction set.
Becoming a Game Boy Color
Apart from the inclusion of GBC hardware (Sharp SM83, original BIOS, audio and video modes, compatible cartridge slot and so forth), there are two extra functions required to make backwards compatibility work.
From the hardware side, the console relies on switches to detect if a Game Boy or Game Boy Color cartridge is inserted. A shape detector in the cartridge slot effectively identifies the type of cartridge and allows the CPU to read its state. It is assumed that some component of CPU AGB reads that value and automatically powers off the hardware not needed in GBC mode.
From the software side, there is a special 16-bit register called
REG_DISPCNT which can alter many properties of the display, but one of its bits sets the console to ‘GBC mode’. At first, I struggled to understand exactly when the GBA tries to update this register. Luckily, some developers helped to clarify this:
I think what happens during GBC boot is that it checks the switch (readable at REG_WAITCNT 0x4000204), does the fade (a very fast fade, hard to notice), then finally switches to GBC mode (BIOS writes to REG_DISPCNT 0x4000000), stopping the ARM7.
The only missing piece of the puzzle is what would happen if you were to remove a portion of the GBC cartridge shell so the switch isn’t pressed anymore, then did a software mode-switch to GBC mode. Multi-boot mode could help here. I’m not sure if the switch needs to be pressed down for the GBC cartridge bus to work properly, or if it just works. I’m willing to guess that the switch is necessary for the bus to function, but that’s just a guess.
– Dan Weiss (aka Dwedit, current maintainer of PocketNES and Goomba Color)
Before we begin, you’ll find the system a mix between the SNES and the Game Boy, the graphics core is still the well-known 2D engine called PPU. I recommend reading those articles before continuing since I’ll be revisiting lots of previously-explained concepts.
Compared to previous Game Boys we now have a colour LCD screen that can display up to 32,768 colours (15-bit). It has a resolution of 240x160 pixels and a refresh rate of ~60Hz.
Organising the content
We have the following regions of memory in which to distribute our graphics:
- 96 KB 16-bit VRAM (Video RAM): Where 64 KB store background graphics and 32 KB store sprite graphics.
- 1 KB 32-bit OAM (Object Attribute Memory): Stores up to 128 sprite entries (not the graphics, just the indices and attributes). Its bus is optimised for fast rendering.
- 1 KB 16-bit PAL RAM (Palette RAM): Stores two palettes, one for backgrounds and the other for sprites. Each palette contains 256 entries of 15-bit colours each, colour
Constructing the frame
If you’ve read the previous articles you’ll find the GBA familiar, although there is additional functionality that may surprise you, and don’t forget that this console runs on two AA batteries.
I’m going to borrow the graphics of Sega’s Sonic Advance 3 to show how a frame is composed.
GBA’s tiles are strictly 8x8 pixel bitmaps, they can use 16 colours (4bpp) or 256 colours (8bpp). 4bpp tiles consume 32 bytes, while 8bpp ones take 64 bytes.
Tiles can be stored anywhere in VRAM, however, the PPU wants them grouped into charblocks: A region of 16 KB. Each block is reserved for a specific type of layer (background and sprites) and programmers decide where each charblock starts. This can result in some overlapping which, as a consequence, enables two charblocks to share the same tiles.
Due to the size of a charblock, up to 256 8bpp tiles or 512 4bpp tiles can be stored per block. Up to six charblocks are allowed, which combined require 96 KB of memory: The exact amount of VRAM this console has.
Only four charblocks can be used for backgrounds and two can be used for sprites.
The background layer of this system has improved significantly since the Game Boy Color. It finally includes some features found in the Super Nintendo (remember the affine transformations?).
The PPU can draw up to four background layers. The capabilities of each one will depend on the selected mode of operation:
- Mode 0: Provides four static layers.
- Mode 1: Only three layers are available, although one of them is affine (can be rotated and/or scaled).
- Mode 2: Supplies two affine layers.
Each layer has a dimension of up to 512x512 pixels. If it’s an affine one then it will be up to 1024x1024 pixels.
The piece of data that defines the background layer is called Tile Map. To implement this in a way that the PPU understands it, programmers use screenblocks, a structure that defines portions of the background layer (32x32 tiles). A screenblock occupies just 2 KB, but more than one will be needed to construct the whole layer. Programmers may place them anywhere inside the background charblocks, this means that not all tiles entries will contain graphics!
The size of a sprite can be up to 64x64 pixels wide, yet for having such a small screen they will end up occupying a big part of it.
If that wasn’t enough, the PPU can now apply affine transformations to sprites!
Sprite entries are 32-bit wide and their values can be divided into two groups:
- Attributes: Contains x/y position, h/v flipping, size, shape (square or rectangle), sprite type (affine or regular) and location of the first tile.
- Affine data: Only used if the sprite is affine, specify scaling and rotation.
As always, the PPU will combine all layers automatically, but it’s not over yet! The system has a couple of effects available to apply over these layers:
- Mosaic: Makes tiles look more blocky.
- Alpha blending: Combines colours of two overlapping layers resulting in transparency effects.
- Windowing: Divides the screen into two different windows where each one can have its own separate graphics and effects, the outer zone of both windows can also be rendered with tiles.
On the other side, to update the frame there are multiple options available:
- Command the CPU: The processor now has full access to VRAM whenever it wants. However, it can produce unwanted artefacts if it alters some data mid-frame, so waiting for VBlank/HBlank (traditional way) remains the safest option in most cases.
- Use the DMA Controller: DMA provides transfer rates ~10x faster and can be scheduled during VBlank and HBlank. This console provides 4 DMA channels (two reserved for sound, one for critical operations and the other for general purpose). Bear in mind that the controller will halt the CPU during the operation (although it may hardly notice it!).
Sometimes we may want to compose a background from which the tile engine won’t be able to draw all required graphics. Now, modern consoles addressed this by implementing a frame-buffer architecture but this is not possible when there’s very little RAM… Well, the GBA happens to have 96 KB of VRAM which is enough to allocate a bitmap with the dimensions of our LCD screen.
The good news is that the PPU actually implemented this functionality by including three extra modes, these are called bitmap modes:
- Mode 3: Allocates a single fully-coloured (8bpp) frame.
- Mode 4: Provides two frames with half the colours (4bpp) each.
- Mode 5: There’re two fully-coloured frames with half the size each (160x128 pixels).
The reason for having two bitmaps is to enable page-flipping: Drawing over a displayed bitmap can expose some weird artefacts during the process. If we instead manipulate another one then none of the glitches will be shown to the user. Once the second bitmap is finished the PPU can be updated to point to the second one, effectively swapping the displayed frame.
Overall it sounds like a cutting-the-edge feature, however, most games held on to the tile engine. Why? Because in practice it costs a lot of CPU resources.
You see, while using a tile engine the CPU can delegate most of the computations to the graphics chip. By contrast, the frame-buffer system that the PPU provides is limited to only displaying that segment of memory as a single background layer, which means no more individual affine transformations, layering or effects unless the CPU computes them. Also, the frame-buffer occupies 80 KB of memory, so only 16 KB (half) are available to store sprite tiles.
For this reason, these modes are used exceptionally, such as for playing motion video (Game Boy Advance Video completely relied on this) or rendering 3D geometry with the CPU.
The GBA features a 2-channel sample player which works in combination with the legacy Game Boy sound system.
Here is a breakdown of each audio component using Sonic Advance 2 as an example:
The new sound system can now play PCM samples, it provides two channels called Direct Sound where it receives samples using a FIFO queue (implemented as a 16-byte buffer).
Samples are 8-bit and signed (encoded in values from -128 to 127). The default sampling rate is 32 kHz, although this depends on each game: since a higher rate means a larger size and more CPU cycles, not every game will spend the same amount of resources to feed the audio chip.
DMA is essential to avoid clogging CPU cycles. Timers are also available to keep in-sync with the queue.
While the Game Boy subsystem won’t share its CPU, it does give out access to its PSG. For compatibility reasons, this is the same design found on the original Game Boy. I’ve previously written this article that goes into detail about each channel in particular.
The majority of GBA games used it for accompaniment or effects. Later ones will optimise their music for PCM and leave the PSG unused.
Finally, everything is automatically mixed together and output through the speaker/headphone jack.
Even though the GBA has just two PCM channels, some games can magically play more than two concurrent samples. How is this possible? Well, while only having two channels may seem a bit weak on paper, the main CPU can use some of its cycles to provide both audio sequencing and mixing (that should give you an idea of how powerful the ARM7 is!). Furthermore, in the ‘Operating System’ section, you’ll find out that the BIOS ROM included an audio sequencer!
Best of both worlds
Some games took the PCM-PSG duality further and ‘alternated’ the leading chip depending on the context.
In this game (Mother 3), the player can enter two different rooms, one relatively normal and the other with a nostalgic setting. Depending on the room the character is in, the same score will sound modern-ish or 8bit-ish.
ARM7’s reset vector is at
0x00000000, which points to a 16 KB BIOS ROM. That means the Game Boy Advance first boots from the BIOS, which in turn shows the iconic splash screen and then decides whether to load the game or not.
That ROM also stores software routines that games may call to simplify certain operations and reduce cartridge size. These include:
- Arithmetic functions: Routines to carry out Division, Square Root and Arc Tangent.
- Affine matrix calculation: Given a ‘zoom’ value and angle, it calculates the affine matrix that will be input to the PPU in order to scale/rotate a background or sprite.
- There are two functions, one for sprites and the other for backgrounds. Their parameters are slightly different but the idea is the same.
- Decompression functions: Implements decompression algorithms including Run-Length, LZ77 and Huffman. It also provides bit unpacking and sequential difference.
- Memory copy: Two functions that move memory around. The first one copies 32-byte blocks using a specialised opcode for this type of transfer (
LDMIAto load and
SDMIAto store) only once. The second one copies 2-byte or 4-byte blocks using repeated
LDMIA/STMIAopcodes, respectively. Thus, the second function is more flexible but not as fast.
- Sound: Implements a complete MIDI sequencer! It includes many functions to control it.
- Power interface: Shortcuts for resetting, clearing most of the RAM, halting the CPU until a certain event hits (V-blank or custom one) or switching to ‘low-power mode’.
- Multi-boot: Uploads a program to another GBA and kickstarts it. More details are in the ‘Game’ section.
The BIOS is connected through a 32-bit bus and it’s implemented using a combination of Arm and Thumb instructions, though the latter is the most prominent.
Also, remember that all of this will only run on the ARM7. In other words, there isn’t any hardware acceleration available to speed up these operations. Hence, Nintendo provided all of this functionality through software.
Programming for the GBA was similar to the SNES with the addition of all the advantages of developing games in the early 2000s: Standardised high-level languages, better compilers, faster RISC CPUs, non-proprietary computers for development, comparatively better documentation and… Internet access!
Programs are mostly written in C with performance-critical sections in assembly (ARM and Thumb) to save cycles. Nintendo provided an SDK with libraries and compilers.
Games are distributed in a new proprietary cartridge called Game Pak.
Accessing cartridge data
While the ARM7 has a 32-bit address bus, there are only 24 address lines connected to the cartridge. This should mean that up to 16 MB can be accessed on the cartridge without needing a mapper, however, the official docs state that 32 MB of cartridge data are mapped in memory. So what’s happening here? The truth is, the Gamepak uses 25-bit addresses (which explains that 32 MB block) but its bottommost bit is fixed at zero, so the only 24 remaining bits are set. This is how Gamepak addressing works.
Now, does this mean that data located at odd addresses (with its least significant bit at ‘1’) will be inaccessible? No, because the data bus is 16-bit: For every transfer, the CPU/DMA will fetch the located byte plus the next one, allowing to read both even and odd addresses. As you can see, this is just another work of engineering that makes full use of hardware capabilities while reducing costs.
Cartridge RAM space
To hold saves, Game Paks could either include:
- SRAM: These need a battery to keep their content and can size up to 64 KB (although commercial games did not exceed 32 KB). It’s accessed through the GBA’s memory map.
- Flash ROM: Similar to SRAM without the need for a battery, can size up to 128 KB.
- EEPROM: These require a serial connection and can theoretically size up to anything (often found up to 8 KB).
The famous Game Boy Link Cable provided multi-playing capabilities. Additionally, the cable has a special feature internally known as Multi-boot: Another console (either GBA or GameCube) can send a functional game to the receiver’s EWRAM, then the latter would boot from there (instead of needing a cartridge).
Anti-Piracy & Homebrew
In general terms, the usage of proprietary cartridges was a big barrier compared to the constant cat-and-mouse game that other console manufacturers had to battle while using the CD-ROM.
To combat against bootleg cartridges (unauthorised reproductions), the GBA’s BIOS incorporated the same boot process found in the original Game Boy.
As solid-state storage became more affordable, a new type of cartridge appeared on the market. Flashcarts looked like ordinary Game Paks but had the addition of a re-writable memory or a card slot that enabled to run game ROMs. The concept is not new actually, developers have internally used similar tools to test their games on a real console (and manufacturers provided the hardware to enable this).
Earlier solutions included a burnable NOR Flash memory (not exceeding the 32 MB) and some battery-backed SRAM. To upload binaries to the cartridge, the cart came with a Link-to-USB cable that was used with a GBA and a PC running Windows XP. With the use of a proprietary flasher software and drivers, the computer uploaded a multi-boot program to the GBA, which in turn was used to transfer a game binary from the PC to the Flashcart inserted in the GBA. Overall, the whole task of uploading a game was deemed sluggish. Later Flashcarts (like the ‘EZ-Flash’) offered larger storage and the ability to be programmed without requiring the GBA as an intermediate. The last ones relied on removable storage (SD, MiniSD, MicroSD or whatever).
Commercial availability of these cards proved to be a grey area: Nintendo condemned its usage due to enabling piracy whereas some users defended that it was the only method for running Homebrew (programs made outside game studios and consequently without the approval of Nintendo). Nintendo’s argument was backed by the fact flashers like the EZ-Writer assisted users to patch game ROMs so they can run in EZ-Flash carts without problems. After Nintendo’s legal attempts, these cartridges were banned in some countries (like in the UK). Nonetheless, they persisted worldwide.