I don't mean that sound causes lag. I mean to say that when there is a lag frame, the game compensates by only doing sound operations during that frame.
In fact, if the previous frame was still working on sound (001B=81) at the time the lag frame occurs, the game will do basically nothing at all during that frame.
You are right though sound seems to be the least well managed element in this game. There are some fairly obvious audio glitches but unfortunately they don't seem to impact much else.
EDIT: With a bit more testing I have found that lag is a bit more common then I previously thought. Still testing though.
So I thought I might post to say that I will no longer be actively looking into the stage end glitch. Everything I check comes up clear, so I am categorizing this as hardware error unless some new information comes up.
The code suffers from a weakness in that flipping ANY bit in $003B will end the level immediately, with no sanity checking of any kind. So while still a rare occurrence, this is very susceptible to a random bit flip.
There are few instances in the code that change 003B and none of them show up abnormally in anything I test. At the time the glitch occurs, nothing in particular is happening, and I have looked at lots of trace logs to try and find anything out of the ordinary.
Level end is delayed by a timer at 002A. At most this can be FE and countdown, setting glitch initiation back a few seconds, but here to nothing of interest is happening.
While I suppose something like DelayStageClear from megaman is still possible, I don't see anyway a bad bank swap would lead to the level ending. Even when I modified the ROM to purposely call the code I wanted, I still got results that were distinguishable from what happened in the video, because that block of code does other things like clear enemies and trigger sounds. I don't really see ACE potential either, but obviously I didn't check everything so who knows.
So if anyone is still interested in this, I will gladly help answer any specific questions, but will not be investigating further. To anyone still hunting, good luck!
Here is another TAS I recently did. 1P Contra (US) in 9:47.33 with SDA timing. TAS timing I think is 9:51 still better than the SDA WR by DK28 by 2 seconds.
Link to video
If you press start or select on frame 11, you can press start on frame 16 to start the game. This saves scrolling the title screen and lets you start playing much sooner.
I am still the wizard that did it.
"On my business card, I am a corporate president. In my mind, I am a game developer. But in my heart, I am a gamer." -- Satoru Iwata
<scrimpy> at least I now know where every map, energy and save room in this game is
The pacifist run uses the JPN version, which saves about half a minute by skipping pre-level screens.
The obsoleted run from 2004 does use the USA version though. TheMechanicalKoopa, you may want to compare your run to this one to see where you're losing time (besides what Invariel pointed out).
http://www.youtube.com/Noxxa
<dwangoAC> This is a TAS (...). Not suitable for all audiences. May cause undesirable side-effects. May contain emulator abuse. Emulator may be abusive. This product contains glitches known to the state of California to cause egg defects.
<Masterjun> I'm just a guy arranging bits in a sequence which could potentially amuse other people looking at these bits
<adelikat> In Oregon Trail, I sacrificed my own family to save time. In Star trek, I killed helpless comrades in escape pods to save time. Here, I kill my allies to save time. I think I need help.
it seems like this one is a modded rom. notice the cores in the base stages dont even take 8 hits to be destroyed. it;s like the enemies and bosses have way less HP than the original US made Contra. I think this was done with manipulating the games values. I still think the one I did is more legit.
You're welcome to think that, but you have to also remember that the judging process involves somebody that is not a creator of the run synching the gameplay with their own setup, including their own ROM, which drastically reduces the probability of a bad dump being used on both sides. Then there are the other members of the site who might try to optimize gameplay on their own machines, with their own ROM dumps, further reducing the possibility of someone sneaking a run made from a bad dump onto the site.
While there may be an emulation error that is resulting in things possibly having fewer hit points, I have difficulty believing that is the case, because it would mean critically mis-reading specific values in the ROM consistently.
Lastly, as noted, your submission has a known optimization, namely the thing I posted just above.
I am still the wizard that did it.
"On my business card, I am a corporate president. In my mind, I am a game developer. But in my heart, I am a gamer." -- Satoru Iwata
<scrimpy> at least I now know where every map, energy and save room in this game is
that is a good point I am sure there are people who monitor this stuff pretty closely. I will have to do some research and slow down the video to see if I can see how the cores were beat so quickly. how do you sync runs tho? video software?
Japanese version not only allows you to skip cutscenece and it have BG animations. It also harder. I'm sure it starts on the same difficulty as the second loop of the USA Contra. So it better in every aspect. Also every time you jump on horizontal scrolling levels - you loose time. This is like one of the famous fact about Contra. And also you can do those ledge skips on level 3 on emulator. And because you can use memory watch and manipulate that value at 0094 as you want - you can do a lot of those skips, much more that RTA speedrunners can do.
By having a copy of the exact version of the emulator used, and playing back the input file in that emulator.
In the case of the quite obsoleted run, that emulator is Famtasia 5.1. In the case of the existing pacifist run, that emulator is FCEU 0.98.28, but the downloadable .fm2 file synchs with FCEUX 2.1.5.
Since the .fm2 file in the downloadable zip is just a text file, you can read the inputs at each frame and compare what was done in that run to what you did in yours.
There may be some timing issues between FCEUX 2.1.5 and whichever emulator you are using, but you should still be able to see the strategies that were employed by carefully reading the input file and using frame advance with the emulator to see the results on-screen.
I am still the wizard that did it.
"On my business card, I am a corporate president. In my mind, I am a game developer. But in my heart, I am a gamer." -- Satoru Iwata
<scrimpy> at least I now know where every map, energy and save room in this game is
Joined: 4/17/2010
Posts: 11495
Location: Lake Chargoggagoggmanchauggagoggchaubunagungamaugg
Video to draw some attention to the above
Link to video
Warning: When making decisions, I try to collect as much data as possible before actually deciding. I try to abstract away and see the principles behind real world events and people's opinions. I try to generalize them and turn into something clear and reusable. I hate depending on unpredictable and having to make lottery guesses. Any problem can be solved by systems thinking and acting.
After the Summoning Salt video showing the level skip bug, I decided to start with Trax's disassembly and take it further. I've made a complete disassembly of Contra with proper labels which allows for modification without breaking jumps and branching. It includes supplemental documentation, diagrams, lua scripts, and tooling to build the rom. This code is on github at https://github.com/vermiceli/nes-contra-us/
I didn't see anything obvious that would cause the level skip bug and I agree with Alyosha that it probably was an NMI before some appropriate memory could be cleared. I think that this repo would help anyone who's interested in the bug as it's much easier to read and parse.
Joined: 4/17/2010
Posts: 11495
Location: Lake Chargoggagoggmanchauggagoggchaubunagungamaugg
vermiceli wrote:
After the Summoning Salt video showing the level skip bug, I decided to start with Trax's disassembly and take it further. I've made a complete disassembly of Contra with proper labels which allows for modification without breaking jumps and branching. It includes supplemental documentation, diagrams, lua scripts, and tooling to build the rom. This code is on github at https://github.com/vermiceli/nes-contra-us/
Warning: When making decisions, I try to collect as much data as possible before actually deciding. I try to abstract away and see the principles behind real world events and people's opinions. I try to generalize them and turn into something clear and reusable. I hate depending on unpredictable and having to make lottery guesses. Any problem can be solved by systems thinking and acting.
After the Summoning Salt video showing the level skip bug, I decided to start with Trax's disassembly and take it further. I've made a complete disassembly of Contra with proper labels which allows for modification without breaking jumps and branching. It includes supplemental documentation, diagrams, lua scripts, and tooling to build the rom. This code is on github at https://github.com/vermiceli/nes-contra-us/
vermiceli, this is absolutely amazing! Great work.
Back when I saw Post #499433 I tried doing some reverse engineering, starting from Trax's disassembly. I gave it up because I was not making much progress. I got as far as identifying, as you did, that ENEMY_VAR_3 and ENEMY_VAR_4form a doubly linked list of the dragon tentacle arm orbs. As I recall, the freeze in Post #499433/Post #499462 was caused by one of the lists somehow developing a cycle (one of the elements pointing to itself). I had hoped it was some wild PC branch, which might offer possibilities of code execution, but it was just an infinite loop over a linked list that had failed to maintain one of its invariants, probably in @enemy_orb_loop or nearby.
Anyway, this disassembly probably opens a lot of doors, to better romhacks, mods, randomizers… pretty exciting.
vermiceli, this is absolutely amazing! Great work.
Back when I saw Post #499433 I tried doing some reverse engineering, starting from Trax's disassembly. I gave it up because I was not making much progress. I got as far as identifying, as you did, that ENEMY_VAR_3 and ENEMY_VAR_4form a doubly linked list of the dragon tentacle arm orbs. As I recall, the freeze in Post #499433/Post #499462 was caused by one of the lists somehow developing a cycle (one of the elements pointing to itself). I had hoped it was some wild PC branch, which might offer possibilities of code execution, but it was just an infinite loop over a linked list that had failed to maintain one of its invariants, probably in @enemy_orb_loop or nearby.
Anyway, this disassembly probably opens a lot of doors, to better romhacks, mods, randomizers… pretty exciting.
Thanks for the compliments. I hope it does open the door to some cool romhacks. I never looked into the freeze on the dragon tentacle orb, but the infinite loop on that list makes sense to me. I wonder what would cause the misconfigured linked list. Is it reproducible from the fm2 file?
It is. I don't still have my notes, but I think I just opened the .fm2, let it play until the freeze, then took a look at where PC was in the built-in debugger. I tried setting a few watchpoints to find out what causes the list structure to get messed up, but didn't notice anything obvious.
It is. I don't still have my notes, but I think I just opened the .fm2, let it play until the freeze, then took a look at where PC was in the built-in debugger. I tried setting a few watchpoints to find out what causes the list structure to get messed up, but didn't notice anything obvious.
I didn't forget about this, but did just now get some time to spend on it and found the root cause. The reason this freeze happens is due to a race condition where the left 'hand' (red dragon arm orb) is destroyed, but before the next frame happens where the 'orb destroyed' routine is executed, another orb on the left arm changes the routine of the left 'hand' to be a different routine. Since the expected 'orb destroyed' routine wasn't run, the rest of the arm didn't get the notice to self-destruct. Then, a few frames later, the left shoulder creates a projectile, which takes over the same slot where the left 'hand' was. Finally, one frame later, when the left shoulder tries to animate the arm, the left 'hand' not having correct data (because it is now a projectile), causes the game to get stuck in an infinite loop.
Here is my best attempt to provide a clear detailed explanation. Below is a diagram of the dragon boss and its arm orbs. Each number below is the enemy slot, i.e. the enemy number. #$06 and #$05 are the left and right 'hands' respectively, and are red. #$0d and #$0a are the left and right 'shoulders' respectively. () represents the dragon's mouth and is uninvolved in this bug. In fact, only the left arm is involved in this bug.
06 08 0c 0f 0d () 0a 0e 0b 07 05
1. Frame #$aa - Enemy #$06 (the left 'hand') is destroyed, the memory address specifying which routine to execute is updated to point to `dragon_arm_orb_routine_04`.
2. Frame #$ab - Enemy #$0f has a timer elapse in `dragon_arm_orb_routine_02`. Enemy #$0d updates the enemy routine for all orbs on the left arm. It does this by incrementing a pointer. Usually, this updates the routine from `dragon_arm_orb_routine_02` to `dragon_arm_orb_routine_03`. However, since arm orb #$06 (the left 'hand') was no longer pointing to `dragon_arm_orb_routine_02`, but instead to `dragon_arm_orb_routine_04`, incrementing this pointer, set #$06's routine to `enemy_routine_init_explosion`.
3. Frames #$ac-#$d1 - The animation for the left 'hand' explosion completes and the 'hand' is removed from memory (`enemy_routine_remove_enemy`)
4. Frame #$d2 - The #$0d (left shoulder) decides that it should create a projectile. The game logic finds an empty enemy slot where the left 'hand' originally was (slot #$06). A bullet is created and initialized. This initialization clears the data that linked the hand to the rest of the arm, in particular `ENEMY_VAR_3` and `ENEMY_VAR_4`.
5. Frame #$d3 - When #$0d (left shoulder) executes, it animates the rest of the orbs to make an attack pattern. It loops down to the hand by following the links among the orbs. When it gets to the hand, it expects that the hand's will have its `ENEMY_VAR_3` set to `#$ff` indicating there aren't any more orbs to process. However, since the enemy at slot #$06 is no longer a hand, but instead a projectile, the value at `ENEMY_VAR_3` has been cleared and is #$00. This causes the logic to get stuck in `@arm_orb_loop` as an infinite loop.
Step (2) caused `dragon_arm_orb_routine_04` to be skipped. Since this routine was not executed as expected, the rest of the arm didn't get updated to know that the 'hand' was destroyed. `dragon_arm_orb_routine_04` is responsible for updating each orb on the arm to be begin its self destruct routine. However, that never happens. So, the shoulder doesn't know to destroy itself.
Instead the shoulder operates as if it wasn't destroyed and when it decides that a projectile should be created, that overwrites the hand with a different enemy type, and clears all the links between the hand and the arm.
Regarding the usefulness of this bug for exploitation, the underlying issue was that one enemy updated the routine of another enemy without checking which routine that enemy was on. In this instance, and like almost all of Contra, it wasn't a pointer to a memory address that was updated, but instead an offset in a table of pointers. It'd be hard to find a place in code where incrementing/decrementing the index into a table causes you to be able to control code execution, but it is something I will look into.
I didn't forget about this, but did just now get some time to spend on it and found the root cause. The reason this freeze happens is due to a race condition where the left 'hand' (red dragon arm orb) is destroyed, but before the next frame happens where the 'orb destroyed' routine is executed, another orb on the left arm changes the routine of the left 'hand' to be a different routine.
I greatly appreciate the detailed explanation. If I understand correctly, then, if the timer had not elapsed just when it did in Step (2), no softlock would have occurred. (I guess the timer is the one that controls the transition between different "phases" of arm movement.) The window for the softlock to occur must be only 1 frame, because otherwise, dragon_arm_orb_routine_04 presumably would have run on the next frame and maintained the expected invariants. That could explain why it's so uncommon.
I agree that the way enemy routine state machines work, it's unlikely to cause the PC to jump to an uncontrolled address. Perhaps there's a situation somewhere, where a racy state machine update causes an enemy to transition into a death state without first taking the necessary damage. Here it might be unlikely, because it requires destroying the hand anyway, as well as waiting out a timer, apparently. Good work in any case.
I greatly appreciate the detailed explanation. If I understand correctly, then, if the timer had not elapsed just when it did in Step (2), no softlock would have occurred. (I guess the timer is the one that controls the transition between different "phases" of arm movement.) The window for the softlock to occur must be only 1 frame, because otherwise, dragon_arm_orb_routine_04 presumably would have run on the next frame and maintained the expected invariants. That could explain why it's so uncommon.
I agree that the way enemy routine state machines work, it's unlikely to cause the PC to jump to an uncontrolled address. Perhaps there's a situation somewhere, where a racy state machine update causes an enemy to transition into a death state without first taking the necessary damage. Here it might be unlikely, because it requires destroying the hand anyway, as well as waiting out a timer, apparently. Good work in any case.
You're exactly right. If the timer that controls the phases of animation didn't elapse exactly when it did, then no softlock would have occurred. If it didn't happen at that exact frame, then dragon_arm_orb_routine_04 would have run the next frame to keep the enemy state correct.
This bug is somewhat similar to the bug that allows the level 4 boss to remain vulnerable while uncombined (Level 4 Boss Gemini Vulnerability). When a delay timer is decremented from 1 to 0, the code should make the boss invincible by calling disable_enemy_collision. However, when the bullet collision happens at the right time, then the regular game code will decrement the timer to 1, and then next frame will have a different routine set the timer to 0, but that routine doesn't call disable_enemy_collision, leaving the boss in a vulnerable state.
So, I think we should be looking at places where timers are involved along with enemy state changes. If there is any luck, then there is somewhere that can transition an enemy to a death state routine earlier than expected.
I lost my old account for andrewg so i'm here on this one.
Anyway, I wish I had heard about this contra glitch sooner to warp from stage 1 to stage 2...
This might be useful information. When I was a kid I was playing a 2 player game with my friend on Super C. After the Spider Boss when the floor starts dropping out, the end music played, forced us to run off the cliffs, and then warped us to the next stage (jungle). I find it interesting since this is the exact sequence seen in the Contra glitch. Imean it's identical to what I remember happening in Super C when I was like 11. I don't know if the code here differs from Super C, but if it does, maybe it would be useful in somehow solving this glitch.
