Can It Tetris?

I say hello to you again, my near and dear viewers, as I begin this devlog with a frustratingly long amount of time logged. Let’s dive in so I can put the last few days behind me.

DMing the Memory

Last time I menioned that there was a feature I needed to add that allowed the cpu to write a byte to a register in memory to trigger a transfer of data from the address specified by the written byte to the OAM (object attribute memory — sprites and stuff; this transfer is the DMA OAM tranfer) because that was the main way stuff was written to oam b/c otherwise it would be too slow.
Originally, I wrote a doubly recursive function kinda thing b/c it called read_byte() when it itself was called indirectly by read_byte() and it also called itself, but I scrapped it due to unguarenteed tail recursion and the slowness of repeatedly matching the section of memory being written to/read from, so I just used copy_within(). This also somehow fixed my input issues, so idk.

Double Bug Whammy

This is the part of the last few days that has been a slog and a horror. After adding the DMA OAM transfer, I tried running Tetris, and it worked…until I got into gameplay. The sprite showing the selected player amount on the start screen was working and it would respond to user input. However, when it hit actual gameplay, it wouldn’t render the tetromino sprites until they were baked into the background as part of setting in place. To try and diagnose the problem, I ran blarrg’s big cpu_instrs.gb test ROM, and it stopped rendering frames in the middle of the 3rd test, the standalone ROM for which had been working without a flaw. Testing in mGBA through RetroArch (I don’t have mGBA installed standalone on this device), I found that the test takes a second to run, but doesn’t cause the whole emulator to hang. Also, since new frames weren’t being completed, it wouldn’t poll for input since it only polls on a frame completion, which meant I had to close the terminal session to quit the emulator. I spent hours running tests just to find that it was getting hit by a STOP instruction it couldn’t escape. I checked Pan Docs and found there had been some unimplemented STOP behaviours. After implementing those and it still not working, I set STOP to only stop for a single cycle, which let the test ROM pass all the tests with no timing issues. After that, within 2 minutes, I was able to find an incorrect == (should have been !=) in my sprite attributes bit 7 sprite priority control, allowing Tetris to finally work!!!

Next up is:

• MBC2
• MBC3
• MBC5
• Saving
• Audio
• TUI rather than bare CLI
• a lib.rs so others can write a frontend on top of Rusty Game Girl
• A style guide and making my code compliant (hard to do while writing and over a codebase who’s size is at least 4.5x bigger than anything else I’ve ever worked with)
• Unit tests

also here’s a video of me selling at Tetris b/c I wasn’t listening to Jamie Paige while playing