Bugs & Glitches of High-Level NES Tetris 

"And viewers like you, thank you." That brought me back to watching PBS Kids every saturday

Ah, my good friend "Important table indexed out of bounds resulting in important functions being fed garbage data"

The fact that half a 'byte' is called a 'nybble' is a fact that I often forget and always feel joy upon remembering.

I have no idea why, but I got unreasonably excited when the captions moved to the bottom right corner to make better use of the white space at 24:14.

Don't interrupt me when I'm in my lonely spinning loop.

Babe wake up, new Retro Game Mechanics Explained dropped

Bro really worked out how to smoothly blend a 69/420 joke in an otherwise 100% serious video. Absolutely epic. Edit: go to 22:13 for the sauce

Thanks for covering NES Tetris! I'm blown away with not only how clear your explanations are, but how clean your editing and presentation style is, while showing a lot of very intricate stuff.

The waiting spinning loop feels like a Sisyphus moment, just doing the same thing over and over until its out of its misery

33:17 - It should be noted that a real 6502 CPU has a built-in decimal arithmetic mode allowing it to add and subtract BCD numbers easily. The CPU in the NES is not a true 6502, but a Ricoh 2A03 (or 2A07 in PAL/SECAM machines) made by Ricoh for Nintendo. The 6502's BCD logic was patented and required royalty payments to MOS Technology, the inventor of the 6502. To avoid this, the 2A03/2A07 simply omits the decimal mode, requiring BCD calculations to be done manually, as shown in your example. If the decimal mode had been available, the code to add points to the score would have been much simpler and shorter.

it would take ungodly amounts of foresight to have thought of this fix back when this game was made, but some of the crashes could have been avoided by checking if the score was already "999999" before running the score update routine. or once it's done the check at the end of the routine, then set a flag in memory to say to not bother running the routine again.

Wonderfully explained! This channel was a source of inspiration for my own videos on mechanics, so it coming back around and contributing to one of the videos is an incredibly cool feeling. Even though I already know how everything described worked, I still got some great value out of the visuals in this video. Writing out all the code in the frame at 37:00 is beautiful. Most of all though, I want to give Frieze credit for seemingly deriving lots of the info on his own-- our community has already disassembled the game fully, but when he reached out to me with the script, he'd disassembled the code himself with different labels! I barely had anything to correct him on, which was refreshing compared to a lot of other journalism following the publicity of Blue Scuti crashing the game. Thanks for the excellent work!

the subtitiles/closed captions are very good here, thank you. (like at 27:25, they go to the right where there is space so you can see the cpu and stack info but when that ui is not there, it just goes to the center, very good job)

The smile that came across my face when you said “…and viewers like you! Thank You!” is immeasurable. That was a heavy nostalgia bomb for me!!! 😊

Do i know programming? No. Is this important to my career? No. Do i understand what's going on? No. Will i make a Tetris-based game? Not that i know now. Do i like this? YES.

7:44 This isnt palette glitches. THIS IS PATRICK

i love how this is essentially explaining bugs (mostly use after free, aliasing and out of bounds indexing) in context. still technical, but with enough game elements and diagrams to explain everything.

Finally RGMEx made a video about a Tetris Game!! Wait, what about the 810-line long level?

When we talk about the vblank interrupt on the NES pausing the CPU no matter what it's doing, that's more true than you might think. The PPU signals vblank using NMI, which stands for "non-maskable interrupt", and it's a special signal the CPU is hard-coded to always take. Other interrupts (IRQs) can be ignored by the software during sensitive work. If you're reading the NESDev Wiki or doing research, you'll often hear people talk about their "NMI routine." It's the same thing, that's just the work that starts at the top of vblank. Naming things is hard. :D

As soon as Blue Scuti broke the game, I knew RGM would have to do an episode like this. Well done, sir.

Blessing us with a nearly 1h video

I absolutely LOVE the “viewers like you, thank you!” in the intro. Reminds me of when I used to watch PBS Kids in the late 2000s, such a throwback. Great video as always, keep ‘em coming!

Why did you upload this? I have chores I wanted to handle, but now I have to watch an hour long video about the tetris kill screen and glitches.

This is literally the first channel I name when someone asks what I watch. Thanks for the hard work!

Found this channel last night and I'm endlessly impressed. Fantastic communicative visuals, great voice over, no distracting music -- 10/10, love it.

This video reignites the spark of wonder I get from low-level programming. Things are what we make of them. The numbers aren't "signed integers" unless we treat them like one. It's all just ones and zeroes. Excellent explanation as always ❤

Going beyond the expected address space is like going off the edge of a medieval map - here there be dragons.

Fun fact, the 6502 ISA actually has a BCD mode, but the Ricoh chip has that physically severed and disabled in the NES.

This is exactly the calming content I needed right now, been super stressed out. Time to pop this on and let this super neat info I won't retain wash it away...

Always fun to see the details of why a game breaks, ty for all the good content

really liked the way colors were for the code this time, making it easier to spot different commands. the whole video looked really nice!

Really enjoyed this one! I think Dr Mario has a similar problem getting bogged down in binary-coded decimal scorekeeping when there's an extremely large combo.

i can not describe my excitement when a new video uploads YESSS

I see those choices of the numbers 69 and 420 in your example at 22:39 🧐

I'm very glad you decided to tackle this. I was not satisfied with the technical depth (or lack of, as is the case) with the other explanation video(s).

There is one thing that is missed. One of the levels between 210 and 220 is around 800+ lines before it proceeds to level up.

Welp, time to clear my schedule for an hour! It's always amazing how approachable these videos are, even to people who aren't this type of tech-inclined.

I was busy but not anymore, RGME video!!

In the 200s a "super level" occurs too, due to an issue with the level-up code.

your infographics and special animations while explaining processes really help me understand what’s going on way more than just an explanation on its own. thank you so much for taking the time to make them (especially making them so sleek and nice), it really helps me get the concepts much better!!

With Summoningsalt's new video, I got reminded of this and am rewatching to see just what causes the legendary crashes.

Summoning Salt just put out a new video on NES Tetris, and I already knew some of the stuff he was going to mention like the glitch colors because of this video

Your videos are always excellent and a joy to watch and be intrigued by! Got so excited when I saw a new upload! ^^

Thanks for covering this! With the Tetris records being broken, now I can finally have an idea as to why it does what it does

i always really appreciate how these videos build up the topics on each other as the video progresses! feels really clear and easy to understand.

THANK YOU SO MUCH for the detailed explanation of the line clear crashes, I was confused as to what was happening back in 2019 when I made my mullti-255-level-loop TAS (I made it to around level 700 or so before giving up) After a bit I read somewhere that "the score calculation would take too long", but I wanted to know more details. I've known about this channel even back then, but unfortunately I'm not very savvy when it comes to understanding assembly, so basically I've been waiting for your visualisations in this specific video since early 2020 haha

“And viewers like you, thank you.” I really like that quote since it brought me PBS and PBS kids nostagia!

I was wondering how this happened after watching a StackRabbit video yesterday. Today, I see this. RU-vid recommendations are absolute gold, thank you for making this!

I always love when you post a new video, especially these super long ones. it's so intriguing to learn about how all these older games and systems worked with their limited capabilities. amazing video as always, time to rewatch over and over until I memorize it :)

Thank you so much!!! It is so much fun learning about what's going on behind the digital-curtain!!!

Hi RGME, first of all, amazing video as always! I've been watching your content for a while and thought you would be the perfect guy for diving deep in to the Playstation's personified fear and fearful harmony errors, I've always wondered how they've worked down to specifics and your content style seems perfect for them!

Thank you for all your videos. I've watched them all and learned a lot, then subsequently rewatched them many times for their calming effect. In fact, I've watched them enough that I'm actually starting to retain some of the information through osmosis, despite not having much of a technical background (or brain). Also, I doubt you think about this, but I was glad to see another of your videos with a black background for improved night mode learning!

instaclick as always

I could listen to these all day. Interesting to learn the workings and quirks of deeper hardware/software through the context of gaming.

Even though I know how assembly language works in general, it always seems so daunting when it appears on screen, but you do such a good job of explaining it. Explanations of low-level language like this, especially with the dependence on frames, serve as a reminder of just how much computers are doing each instant, and games like this are the perfect way to demonstrate it. :)

Incredible video! Excellent explanations and visualizations of raw 6502 into game concepts. Your label names are very well chosen. I did a romhack a while back of this game (Tetris Excel on RHDN) and it's great to see these routines all laid out like this so clearly: Perhaps releasing your label file (.MLB?) could help future romhackers, i would have LOVED to have a resource like that. It's also very silly to see that most of these crashes could have been avoided lol, though, humans weren't really supposed to get past level 29. Honestly, it was a fair thing to assume, no one in 1988 could have predicted the controller tapping methods perfected by the Classic Tetris community in the 2020s. I'm excited to see the race to 255, and if humans will ever get there...

Videos like these are the only way I can understand more complicated topics. And insanely entertaining. Thank you

Love this!! Your voice is so calming and I love how you describe these things in detail so we know exactly what's going on behind the scenes instead of a sanitized explanation. For some reason youtube unsubscribed me to you and I think it did this for other ppl too so don't think your vids aren't interesting, youtube is just weird!! This video is awesome and even though I'm def not an expert on NES coding and honestly hex in general (I'm more of a bash and python fella myself) you explained it in a way that I could still mostly understand with limited knowledge!! Awesome vids as always!!

Eventrough it’s all very complicated to understand,but it is well explained,also i absolutely like how you explain stuff,no annoying generic stuff,no overhumorism,no sarcasm,no jokes and that’s how i like it,just focussing on the subject rather then interrupting it with jokes or so, And that’s how it always should be😁

I thought the youtuber TheZZAZZGlitch had some insane machine code analysis, this is just next level, you're showing specific instructions and explaining them one by one and It is way easier to follow, I love the visual component to those videos, since it better utilizes the medium than just having text on screen to pause and read.

Truly fascinating video and extremely well explained, thank you!

I've been blessed to find such a channel that makes incredibly informative and clear explanations on things I am so interested in.

I absolutely love how you explain the code line by line!!!!

Watched the whole thing. Had no idea what you said for any of it. Enjoyed it immensely.

this is such a nice video, absolutely well worth the wait

I'm glad you make such amazing and detailed content, thank you :3

Amazing how it’s very interesting but also this video is great at making me fall asleep. Thank you, one day I’ll finish watching!

Excellent video as always :) Speaking of code waiting for an interrupt to occur - more modern processor designs allow you to effectively turn off the CPU until an external interrupt occurs instead of spinning in a loop, which is useful for reducing energy usage in battery powered devices like phones. Modern operating systems do something similar to this to allow a program to stop running until it's got something it needs, like keyboard/mouse inputs or network messages.

I've been waiting for a Tetris video from RGMechEx for a while now, and I'm not disappointed ☺️

Great vid and thanks for the explaination of the code. I guess one "bug" that was left out of the video on the code would be the one level that goes on for 800+ lines.

36:53 Totally amazing to see the code of a single cycle. What a cool visualization! I wish debuggers could show things like this.

As an old C64 coder, every time I see 6502 code I just have to stop and watch. Dude, you have a good handle on it. Kudos...

Someone in the comments mentioned the captions and i'm going to cry, there's real captions!! Thank you _so_ much for having them. I can actually understand so much more easily now

Beautiful explanation. Great video!!

Amaaaaaaazing video! I would love a part 2 where you propose fixes for these various coding mistakes, to make it failproof

Great video! Some minor corrections: There are actually two lookuptables for the level display the other one goes up to level 49. When the score loop takes so long and is interrupted by the NMI, the counter get overwritten by the BCD represention of the lower 2 digits of the (old *) line counter. That means after returning from the interrupt the score loop runs BCD represention of the lower 2 digits of the line counter - 1 times before exiting. This is a value beetween 0 and 152. Running it 152 time will finish before the NMI trigges again so no further corruption can happen. If the line counter ends in 00 it will run 255 times which is so long it will overrun the next frame too and will cause another lag frame. But it is so long that NMI will never happen to corrupt anything. So it is sort of lucky that can never trigger any corruption. The PAL version has some different effect when reaching high levels. * past some point the displayed numbers of the line counter no longer matches that used in working RAM

I always wondered how that worked, thanks for explaining!

Party time, RGME upload! :D

Omg wait half bytes are called nybbles?? Small bites are nibbles… that’s so cute lmfao

I WAS HOPING YOUD COVER THE LATE TETRIS GLITCHES DUDE!! BLESS

frame_per;DROP TABLE Database admin : Nooooooo!

I was really hoping you would make a video on this

I just realized I havent been subscriibed, I thought I was cause you were always in my feed. Fixed that now lol, thanks for all the great videos!

i love watching game explanations instead of sleeping

Amazing video as always!

awesome video! can’t wait for more :)

Great to see one on Tetris.

I assumed it would be something like this, but thank you for going in extra detail!

It was kinda weird how people somehow just recently discovered the NES Tetris limits for some reason. It is not really news, it has been known for awhile now to people like us. I did not go as far as you because I am just an amateur compared to what you do, but I came up with some game genie codes years ago to prevent the check for losing, and to add 10 lines to the counter after every block drop, so I could quickly see all the colors it goes through. Makes it really easy to hit the game ending glitch, but it is just as easy to get around that too and see the various effects of seemingly clearing thousands of lines. Also found a way to get only long blocks, if anyone is interested in that.

Nice :) I was always wondering how these things came to be. (Also nice PBS Kids reference.)

We have been blessed with nearly an hour of Retro Games Mechanics!

best channel on youtube, love your videos

Gotta be my favorite channel

Yet another consequence of the 2A03 having a neutered decimal mode lol Among other things

Great video! Subbed

hi RGME

Hearing you refer to scratch RAM values getting clobbered reminded me of @PJDiCesare’s “Grand Memory Clobber” speedrun.

These videos give me a better understanding of computers than my computer architecture classes, pretty amazing

Amazing video, thank you

The BMI instruction reminds me of "arithmetic if" in early Fortran.