5 ms·

Gameboy hello world

188 points by luu 9y ago

wk_end 9y ago
I don't know whether OP is the author of this code, but if they are, or if anyone is keen to use it as a template, there's a small but important quirk of the Game Boy hardware this doesn't take into account: when the Game Boy's screen is rendering (i.e. almost all the time), the CPU doesn't have access to VRAM. In this case, I guess this demo works (if it does, I haven't tested it on my flash cart) because the amount of data transferred to VRAM is small and the timing of things happens to line up so that it all gets sent at the right time, but for anything more complicated you can't rely on that. There's two ways around this: either turn the screen off before copying your tile and map data over (and you can only safely turn the screen off when it's not rendering, too) or set up a system to blast as much data as possible into VRAM as quickly as possible when a frame is finished drawing. To test if the screen is off, either spin until bits 0 and 1 of $FF41 == 01 or set up a interrupt handler that'll get fired on vblank. OP, if you did write this code (or if the author reads this, or if anyone else is interested), feel free to hit me up with any questions. My primary side project is a GB game, and I'd love to talk shop.
- feeb 9y ago
  (author) thanks so much for the comments. i vaguely knew about this but i serendipitously didn't run into it. how does lack of vram access manifest? do vram writes silently fail? this is precisely the sort of quirk i wanted to cover in this template so i appreciate hearing about it.
  - wk_end 9y ago
    Yeah, writes are ignored, and reads return garbage (no idea what). I recommend using the BGB emulator - you can configure it to crash into a debugger whenever it detects an invalid VRAM access. Plus the debugger is insanely useful.
    - posterboy 9y ago
      and the screen is blanked i suppose. that would allow pritive pwm. has that been used? i would imagine that allows some nice fade effects or enhanced bit-depth in the brightness, as is done eg. on c64.
  - moozilla 9y ago
    Adding to what wk_end has said - many emulators (VBA included) will just write to the VRAM without complaining, so you won't even catch the bug unless you try it on a flash cart or in BGB (I'll second that recommendation). For your demo checking $FF41 should be sufficient, unless you're writing a lot of data you'll usually have enough time in one vblank period. You could wait for $FF41 == 01 and then copy all the data, or check it each iteration of the loop (less efficient but will never fail). I've seen actual games do it both ways (decent amount of disassembly/ROM hacking experience).
forvelin 9y ago
Really nice tutorial, thanks ! Aladdin source code article[0] was really nice, I wonder if we have anyone here who was involved in game development for gameboy at its time, who can share a similar article. [0] - https://news.ycombinator.com/item?id=15458200 https://news.ycombinator.com/item?id=15458200
ctw 9y ago
Very cool. Something I really love about assembly programming is that there's no black magic between me and the hardware. I can know exactly what every bit of every instruction means by looking at the instruction set documentation, and I can look at the computer architecture documentation to know exactly how to access each device. Also, the "API"'s you have to learn are incredibly simple compared to learning API's for every framework and protocol in high-level programming. If devices are memory-mapped, than all I'll ever need to know is load and store. I just need to look at the registers that are available for each I/O device, what their addresses are, and what the bits represent in them. From there I access them all the same way: load and store. Couldn't possibly get any easier than just moving things around between registers and memory. Compare that to high level development like web development, where you need an understanding of HTTP on top of one of infinite different web backends, written in different languages, all using different libraries with different naming conventions and different programming paradigms, plus knowledge of HTML, CSS, possibly another smorgasbord of frameworks, plus knowledge of decent security practices, databases, operations, and the list goes on. It's obviously apples to oranges though, and I can produce a lot more real-world value doing web development than I can doing assembly programming, at least just as one person. My point is that I really like the simplicity and straightforwardness of writing assembly, at least in the few very short times I've done it. Maybe large assembly projects have all the same challenges as high-level development or worse.
- ashleyn 9y ago
  I love low-level, assembly-level programming. The big problem is, we're largely past the era of it. All but the most simplest embedded systems are complex enough to require some pre-developed library or abstraction layer to stand between a single tinkerer and the hardware. Otherwise it would take you an eternity just to stand up a "Hello World!" The cost of a Linux-capable SoC is low enough that you can buy one for $5 in the form of the Pi Zero. If a system complex enough to run Linux only costs $5 to sell, and probably even less to make, what incentive is there anymore to produce hardware that's simple enough to work with directly at the assembly layer? The last holdouts are extremely power-efficient embedded systems. And even then you can do those with C. I'm an old fart who can remember the "good ol' days" just as well, and I still program the SNES as a hobby. But really, that's all it'll ever be anymore; a hobby for legacy systems. There's no reason anymore to even skip C, when architectures have been so fine-tuned for code written with C.
iynere 9y ago
!!!!!!!!
iynere 9y ago
!!!!!!!!!!!
RodgerTheGreat 9y ago
A nice simple template like this with explanations of platform quirks and caveats is extremely useful. Well done.
mintplant 9y ago
My brother wrote an alternate Gameboy BIOS for his emulator, including the Nintendo logo animation: https://github.com/Steveice10/GameYob/tree/master/bios https://github.com/Steveice10/GameYob/tree/master/bios (The emulator itself started as a fork of Gameyob, but he ended up rewriting it entirely, piece by piece.)
mathnode 9y ago
Very cool! Eventually I will pick up an Arduboy(1) or a pocketchip(2) with pico8. The games BMO is loaded with are a great source of inspiration: http://adventuretime.wikia.com/wiki/Super_Good_Boys http://adventuretime.wikia.com/wiki/Super_Good_Boys 1) https://arduboy.com/ https://arduboy.com/ 2) https://getchip.com/ https://getchip.com/
pepijndevos 9y ago
I gave a Game Boy programming workshop at SHA2017 if anyone is interested. The material can be found on my blog: http://pepijndevos.nl/2017/08/06/introduction-to-game-boy-hacking-at-sha2017.html http://pepijndevos.nl/2017/08/06/introduction-to-game-boy-ha...

188 points by luu 9y ago

I don't know whether OP is the author of this code, but if they are, or if anyone is keen to use it as a template, there's a small but important quirk of the Game Boy hardware this doesn't take into account: when the Game Boy's screen is rendering (i.e. almost all the time), the CPU doesn't have access to VRAM. In this case, I guess this demo works (if it does, I haven't tested it on my flash cart) because the amount of data transferred to VRAM is small and the timing of things happens to line up so that it all gets sent at the right time, but for anything more complicated you can't rely on that. There's two ways around this: either turn the screen off before copying your tile and map data over (and you can only safely turn the screen off when it's not rendering, too) or set up a system to blast as much data as possible into VRAM as quickly as possible when a frame is finished drawing. To test if the screen is off, either spin until bits 0 and 1 of $FF41 == 01 or set up a interrupt handler that'll get fired on vblank. OP, if you did write this code (or if the author reads this, or if anyone else is interested), feel free to hit me up with any questions. My primary side project is a GB game, and I'd love to talk shop.

feeb 9y ago
(author) thanks so much for the comments. i vaguely knew about this but i serendipitously didn't run into it. how does lack of vram access manifest? do vram writes silently fail? this is precisely the sort of quirk i wanted to cover in this template so i appreciate hearing about it.
- wk_end 9y ago
  Yeah, writes are ignored, and reads return garbage (no idea what). I recommend using the BGB emulator - you can configure it to crash into a debugger whenever it detects an invalid VRAM access. Plus the debugger is insanely useful.
  - posterboy 9y ago
    and the screen is blanked i suppose. that would allow pritive pwm. has that been used? i would imagine that allows some nice fade effects or enhanced bit-depth in the brightness, as is done eg. on c64.
- moozilla 9y ago
  Adding to what wk_end has said - many emulators (VBA included) will just write to the VRAM without complaining, so you won't even catch the bug unless you try it on a flash cart or in BGB (I'll second that recommendation). For your demo checking $FF41 should be sufficient, unless you're writing a lot of data you'll usually have enough time in one vblank period. You could wait for $FF41 == 01 and then copy all the data, or check it each iteration of the loop (less efficient but will never fail). I've seen actual games do it both ways (decent amount of disassembly/ROM hacking experience).

forvelin 9y ago

Really nice tutorial, thanks ! Aladdin source code article[0] was really nice, I wonder if we have anyone here who was involved in game development for gameboy at its time, who can share a similar article. [0] - https://news.ycombinator.com/item?id=15458200 https://news.ycombinator.com/item?id=15458200

ctw 9y ago

Very cool. Something I really love about assembly programming is that there's no black magic between me and the hardware. I can know exactly what every bit of every instruction means by looking at the instruction set documentation, and I can look at the computer architecture documentation to know exactly how to access each device. Also, the "API"'s you have to learn are incredibly simple compared to learning API's for every framework and protocol in high-level programming. If devices are memory-mapped, than all I'll ever need to know is load and store. I just need to look at the registers that are available for each I/O device, what their addresses are, and what the bits represent in them. From there I access them all the same way: load and store. Couldn't possibly get any easier than just moving things around between registers and memory. Compare that to high level development like web development, where you need an understanding of HTTP on top of one of infinite different web backends, written in different languages, all using different libraries with different naming conventions and different programming paradigms, plus knowledge of HTML, CSS, possibly another smorgasbord of frameworks, plus knowledge of decent security practices, databases, operations, and the list goes on. It's obviously apples to oranges though, and I can produce a lot more real-world value doing web development than I can doing assembly programming, at least just as one person. My point is that I really like the simplicity and straightforwardness of writing assembly, at least in the few very short times I've done it. Maybe large assembly projects have all the same challenges as high-level development or worse.

ashleyn 9y ago
I love low-level, assembly-level programming. The big problem is, we're largely past the era of it. All but the most simplest embedded systems are complex enough to require some pre-developed library or abstraction layer to stand between a single tinkerer and the hardware. Otherwise it would take you an eternity just to stand up a "Hello World!" The cost of a Linux-capable SoC is low enough that you can buy one for $5 in the form of the Pi Zero. If a system complex enough to run Linux only costs $5 to sell, and probably even less to make, what incentive is there anymore to produce hardware that's simple enough to work with directly at the assembly layer? The last holdouts are extremely power-efficient embedded systems. And even then you can do those with C. I'm an old fart who can remember the "good ol' days" just as well, and I still program the SNES as a hobby. But really, that's all it'll ever be anymore; a hobby for legacy systems. There's no reason anymore to even skip C, when architectures have been so fine-tuned for code written with C.

iynere 9y ago

!!!!!!!!

iynere 9y ago

!!!!!!!!!!!

RodgerTheGreat 9y ago

A nice simple template like this with explanations of platform quirks and caveats is extremely useful. Well done.

mintplant 9y ago

My brother wrote an alternate Gameboy BIOS for his emulator, including the Nintendo logo animation: https://github.com/Steveice10/GameYob/tree/master/bios https://github.com/Steveice10/GameYob/tree/master/bios (The emulator itself started as a fork of Gameyob, but he ended up rewriting it entirely, piece by piece.)

mathnode 9y ago

Very cool! Eventually I will pick up an Arduboy(1) or a pocketchip(2) with pico8. The games BMO is loaded with are a great source of inspiration: http://adventuretime.wikia.com/wiki/Super_Good_Boys http://adventuretime.wikia.com/wiki/Super_Good_Boys 1) https://arduboy.com/ https://arduboy.com/ 2) https://getchip.com/ https://getchip.com/

pepijndevos 9y ago

I gave a Game Boy programming workshop at SHA2017 if anyone is interested. The material can be found on my blog: http://pepijndevos.nl/2017/08/06/introduction-to-game-boy-hacking-at-sha2017.html http://pepijndevos.nl/2017/08/06/introduction-to-game-boy-ha...