2018-06-17

Professor Sinister And His Marauding Mechanicals

I’ve given up on Chuckie Egg for the GBA. Not because it got too complicated; the opposite in fact. I wanted to play with things the GBA will allow me to try out, like parallax scrolling, chiptunes and pixel art, and remaking a four color Dragon 32 game just wasn’t cutting it. I still adore the game though. I’m keeping the engine but I’m going to do a full reskin with new graphics, music, sound, and possibly new level layouts.

The remake was drifting further and further from the original anyway. The first change I made was to eliminate the flicker. Then I added scrolling for the GBA, and then I replaced the music with XM versions. Why not just modernize the whole thing?

The new game is called “Professor Sinister and his Marauding Mechanicals”. You play Professor Sinister, an evil scientist who created robots to help him destroy the world. Unfortunately his robots have gone wrong and he’s lost control of them. The Chuckie Egg hens have become robots, the eggs have become vacuum tubes, and the chicken feed has become little power packs.

I’ve added parallax scrolling and a little chiptune. As per usual there’s a large list of things left to do, but here’s a video of the state of things so far:

Though the graphics are all original, they draw from a number of different influences. I wanted the robot to look something like a cross between the robot from “A Grand Day Out with Wallace and Gromit” and the robot from “Forbidden Planet”. The palette and background was inspired by Rick Dangerous 2. I referenced the Master System version of “Wonder Boy in Monster Land” when drawing the life indicator. The original sketches for Professor Sinister looked like something from “The Far Side”, but as a 16x16 sprite that doesn’t really come across.

2018-06-06

Chuckie Egg GBA

Here’s a video of something old-but-new I’ve been working on:

It’s Chuckie Egg, but for the GBA.

How did I manage to squeeze the bitmap-mode UI library built for a dual-CPU 66/16MHz DS into a single-CPU 16MHz GBA? With full-screen scrolling?

I didn’t even try.

Extracting A Library

I mentioned before that the games I’ve been writing with SZLib adhere reasonably strictly to the MVC architecture. The game logic forms the “model” portion of that architecture. If done correctly, the game logic could live in an entirely separate library from the code that handles graphics, sound and user input. That’s what I’ve done to get this working: I moved the model to a new “SZChuckie” library and created a new front-end project that uses the GBA’s tile-and-sprite graphics modes.

Splitting out the game library was mostly trivial, but somehow took far longer than I expected. References to bitmaps, sounds and user input handling had all found their way into the game logic. For example, each of the sprite-esque things in the game (eggs, chicken feed, hens, ducks, Harry) stored an array of bitmaps that represented their animations. The view controller could ask for the current bitmap in order to display the correct image in a view. I had to replace the bitmap reference with a more abstract “animation frame” object, which ditched the bitmap data and stored just a size and a frame number. The view controller now stores the bitmaps and uses SZLib’s type introspection to figure out which bitmap to display for any incoming didChangeFrame events emitted by the game object. The class that represents a level had a function for generating a bitmap from its level data which the view controller used as the game’s background; I moved that into a separate file.

Pulling the sounds into a higher layer meant adding more callbacks to the game object’s delegate. The delegate - in this case, the view controller - can respond appropriately and play the correct sounds.

Input was similarly straightforward. I added a new d-pad/button class specific to Chuckie Egg (so there’s a “jump” button instead of “A” and “B” buttons) and pass its state to the game whenever I call its update() function. As a side-effect, this fixed a few places where the game wasn’t responding correctly to some button presses or the analog stick.

The most long-winded changes were purely administrative: renaming all of the files/classes/etc; creating makefiles and build scripts; and testing that nothing was broken.

With all that done I could move onto the GBA port, which of course started with more administrative work. I needed to get three libraries from SZLib building for the GBA: the core library, which includes reference-counted objects, boxed primitives and collections; the geometry library, which includes points, sizes and rectangles; and the hardware abstraction library, which includes abstractions for initializing the hardware, sound, vwaits and hardware division. Those all needed makefiles. I used the makefile from libgba as a template. With the makefiles in place I could add GBA support to the hardware abstraction library, which involved digging through libgba’s headers to find the appropriate function calls (where available) or disabling the functionality (where not). The core and geometry libraries just worked.

That all put the project in a very weird place. Typically when I start a game I get enough of the logic working to start pushing pixels around on the screen, and from there the logic and graphics code evolve in parallel. Somewhere near the end of the project I’ll add sound. However, in this project I just had to link the game library and hook up a few callbacks to get a fully-working and audible, but invisible, version of Chuckie Egg.

The GBA

Programming the GBA isn’t particularly difficult. Well, it shouldn’t be difficult because it’s a very simple platform. It’s a little harder than it needs to be because libgba has no documentation and the canonical tonc guide references constants that don’t match libgba, so I find myself trying to interpret memory addresses rather than looking for the names of those addresses. The command line graphics conversion utility that ships with devKitARM (grit) has a tendency to either get things wrong (it refuses to generate a correct palette for me) or be unhelpfully helpful (some combination of settings causes it to insert a blank tile at the start of a tile map, causing me to spend far too long trying to figure out why I couldn’t fill the screen with the tile I wanted).

Once I really get the hang of the GBA I’ll hide all of its features behind an abstraction library that I can make cross-platform, which will let me debug my code on my development machine. With neither a debugger nor the ability to printf() on the GBA I’m wasting far too much time running C in my head.

The Port

The GBA’s screen is 240x160 pixels, which is about 20% smaller than the Dragon 32’s resolution of 256x192 (this is a remake of the Dragon 32 version). The DS port was very fortunate in that the DS’ resolution is also 256x192, so there was no need to scroll or scale the game to get it to fit. For the GBA port I decided to opt for scrolling as it’s so simple to achieve (just change a couple of registers). It refused to work for a while until I dug up a comment in one of the examples that noted that the registers are write-only. Duh.

I think that the scrolling works really well. The levels aren’t much larger than the GBA’s screen so it’s impossible to get disoriented or lost. It is possible to get into a “leap of faith” situation where a player might jump from the top of the level hoping that there’s something at the bottom to catch them, but it doesn’t make the game less enjoyable (at least, not for me, but then I know these levels backwards so I’m not a great test subject).

The unmodifiable parts of the levels - the floors and ladders - are implemented in a tiled background. Everything else - the hens, duck, eggs, chicken feed and Harry himself - are all sprites. That gives me around 30 sprites per level, which the GBA throws around without breaking a sweat. I’m considering changing the eggs and feed to become tiles within the background if performance becomes a problem.

Sound is powered by MaxMod. The API is slightly different to the DS’ version, but they’re similar enough that making sound work was simple.

I wasn’t particularly worried about performance. From the considerable amount of perf work I’ve done on the DS version I know that the overwhelming amount of work the game does is in the UI libraries that I’m not using.

Disaster Strikes

Everything was progressing well until I hit a massive problem. The game worked perfectly in mGBA and VBA-M, but it refused to do anything but display a green screen on a real GBA. Every time I’ve encountered something like this on the DS it was caused by a memory problem. The DS emulators I use don’t accurately model the behavior of memory on a real device, so problems like allocating the wrong amount of memory, forgetting to initialize variables, and using memory after it’s freed (etc) can crash real hardware but cause no problems under emulation. I set about investigating.

First step: Code inspection. Was I allocating the wrong amount of memory for my objects? That’s a mistake I’ve made repeatedly in SZLib: allocating enough memory for a pointer to a struct rather than the struct itself. Nope. Forgetting to initialize variables? Nope. Leaking memory? Nope.

Next: Run the SDL version through a static analyzer. No problems.

Next: Run with the address sanitizer enabled. No problems.

Maybe the problem is in the GBA front-end code. I #ifdef-ed out enough of the GBA-specific code to get it compiling on my dev machine and ran that through all of the available code/memory checking tools. No problems.

Let’s dig back through the Git history to find a version that did work on the GBA. Rolling back a few days gave me a version that worked. It seemed that I’d subsequently made a change where I stopped writing to an intermediate buffer representing the OAM (object attribute memory; for sprites) and wrote directly to it. Undoing that change didn’t fix the bug.

I started to remove arbitrary pieces of code that caused memory allocation to occur. Sometimes a change would allow the game to run; sometimes a different bug would manifest. It made no sense.

I enabled libgba’s console output and wrapped malloc() so that it would print the address of the allocated memory. No clues there: all of the addresses were safely contained within the correct region of RAM. Then I started printing out the addresses of stack variables. It seemed that I was getting close to the end of the stack, which perhaps made sense. Stack variables are stored in a 32K block of memory (IWRAM) which is immediately adjacent to a 1K block of memory that controls I/O. Overflowing the stack would cause weird behavior. Immediately following the 1K I/O block is a 1K palette block. One of the changes I made to the game caused the palette to start cycling. Aha, that seems likely. But how am I overflowing the stack? And why doesn’t that bug show up in the emulators?

I started up mGBA and took a look at its memory viewer. According to that the game uses a tiny fraction of the 256K and 32K RAM blocks. There’s no way it should be overflowing the stack or encountering problems related to memory shortages.

Perhaps there’s something wrong with the GBA itself. I recently got hold of another backlit GBA which is the device I’ve been testing on. I haven’t used it for playing games; maybe it’s defective? Nope, same problem on another device.

At this point I’d wasted three evenings on this ridiculous problem and, although I’d learned a significant amount about how the GBA’s memory is laid out, I’d made no progress in figuring out the cause of the issue.

I put everything away and thought about the problem some more.

There’s nothing wrong with the majority of the code. It’s running on 4 other platforms without issue. I’ve tested it with every analysis tool at my disposal and can’t find any problems. The code that’s specific to the GBA amounts to less than 1,000 simple lines, and I analyzed most of that too. The fact that the game runs perfectly under emulation suggests it isn’t a library or compiler problem. It isn’t a problem with the GBA hardware because it exhibits the same behavior on two different devices. It isn’t a memory exhaustion problem. So what’s the difference between running the game in an emulator and running it on my GBA?

The flash cart.

Could it be the flash cart?

I’ve got an EZ-Flash IV (one of the older ones that uses mini-SD cards). It runs its own code and patches ROM files in order for them to work correctly. Could the flash cart be breaking the ROM? Fortunately I’ve got a couple of older flash carts lying around. Copy the ROM to my Supercard SD and…

It works. Perfectly.

The EZ-Flash is doing something bad to the ROM, causing it to write to bad memory addresses, causing bad things to happen. There’s nothing wrong with my code. And there was no way I was ever going to debug that problem.

To Do

There’s plenty of work left to do before the game is finished:

  • Add a “Simian Zombie” screen.
  • Add a title/high score screen.
  • Add level/life/score/time/bonus indicators.
  • Show the “Game over” screen when the player dies.
  • Show the score increasing when the player completes a level.
  • Show the “Get ready” screen before each level.

I also want to add an attract mode showing little demos of gameplay that appear after a few seconds idling on the title screen. I’m thinking of using that system to show how to perform various techniques in the game (like grabbing ladders easily, bouncing between ladders, etc). Oh, and then there’s the level editor that I didn’t finish, too, but that makes less sense on the GBA as it doesn’t have a writable file system.

2017-05-24

Fake DS Lite

I had some problems with devKitARM recently that broke Chuckie Egg and Hanky Alien. Thinking that the problem might be related to some DSi-specific code that got added to libnds, I decided to try out the games on my DS Lite. Unfortunately, the console no longer recognized DS carts. No amount of fiddling would get it to read either of the two carts I tested.

Nuts.

I’d heard that AliExpress - purveyor of fine knock-off Chinese electronics like the GB Boy Colour - had a selection of fake DS Lites on offer, so I went digging. I eventually settled on a cobalt blue DS Lite for the very reasonable price of $40. Given that an original Nintendo console would have cost more than double that back in the day, and given that it’s difficult to find an original, new DS Lite for less than ten times that, it seemed like a chance worth taking.

Here’s what arrived:

Fake DS Lite

Let’s start with the bad first. Side by side with a real DS, the shell is obviously not original. The top case is slightly larger than it should be and bulges outwards slightly along the edge where the clamshell opens. The square black rubber pads that are dotted around the upper screen are fitted imprecisely. The plastic feels cheap, especially when compared to the beautifully glossy Nintendo device. The A, B, X and Y buttons are horribly mushy, and the Start and Select buttons are too small for the holes that they sit in. The cover over the top screen is fitted incorrectly, allowing light from the screen to bleed through its top edge. The screens themselves are set deeper into the casing than on the original. The DS cartridge slot makes a series of nasty griding sounds when a cartridge is inserted, and the cover over the GBA slot is looser than on the original handheld.

In short, the casing is cheap and poorly assembled. But it’s close enough.

Now the good. My original DS Lite came from the factory with a dead pixel; a common problem. Cracked hinges were another, and the hinges on mine cracked not long after I bought it. The clone device doesn’t exhibit either problem (yet). Both screens are perfect and indistinguishable from the original. We’ll see how long the hinge lasts.

Functionally it appears identical to the Nintendo console. When you consider that the K1-GBA still isn’t a perfect clone of the GBA, and that this thing has a GBA embedded within it, that’s stunning. The only explanations that make any sense are that this clone is from a DS factory secretly making extra devices on the side, or that someone stole and replicated the PCBs like the Everdrive clones, or that it’s assembled from spare OEM parts and a third-party case.

It even comes with a box (missing the serial number, of course), manual, soft case and screen protectors.

I haven’t really tried the stylus yet. It’s possible that the touchscreen is terrible. The screen’s texture feels different to the original, though that could just be because it’s new. I haven’t tried the charger yet as the battery is fully charged. The combination of a knock-off charger and a knock-off battery is a little concerning. Other than that, it looks to be a good alternative to either buying a second-hand DS Lite (and possibly ending up with a device that was mistreated) or paying a crazy amount for a new official device.

Oh, and now that I have a second DS Lite, my original console has started recognizing carts again.

2016-07-24

AGB-101

I need to stop collecting these.

AGB-101

The purple and black GBAs came from eBay, while the yellow version (it looks orange in the photo, but it’s really banana yellow) is from Retro Modding. Like GB modders Deadpan Robot (formerly Gameboy Mods), Retro Modding take used GBAs and refurbish them with a variety of third-party shells and enhancements.

I was very pleased with Retro Modding. In addition to providing great service and doing an excellent job of modifying the GBA - for considerably less than the current prices on eBay - they threw in a neat case and a bunch of stickers.

Aspiring backlit GBA owners should note that a triwing screwdriver may turn out to be an essential purchase. Two of these three handhelds have required adjustments to the voltage potentiometer inside the console in order to fix graphical glitches on the screen. The purple unit displayed flickering colors, whilst the yellow unit was noticably interlaced. There’s a video here that demonstrates where to insert a screwdriver if you don’t mind poking a hole in the sticker on the back of the device, but a triwing screwdriver will allow you to open it up and avoid the hole.

The approach that’s worked best for me is to make small adjustments of around a quarter-turn, testing out the console with each change. It’s a little laborious, as each test requires reassembling the console enough to be able to re-insert the cartridge and batteries, but I’ve seen numerous (unsubstantiated) reports that turning it too far can damage the screen. The purple unit was particularly troublesome to get right because it would work well for a day or so before the flickering re-appeared.