That's when the TASBot team found that current NES emulation couldn't handle the new wave of subframe inputs they wanted to send. With the hardware and the theory in place, micro500 started discussing the idea online with fellow TAS-makers total_, ais523, and TASBot manager dwangoAC last Thursday, with SGDQ already in full swing. For this year's SGDQ, though, micro500 developed a new TASLink board that could handle the vast flow of data necessary. That's obviously out of the realm of possibility for a human, and it was even beyond the limited hardware TASBot used in previous speedrun tournaments.

AdvertisementĮnlarge / The custom-made TASLink hardware that lets TASBot flood the NES with thousands of inputs per second.TASBot developer micro500 (one of a number of TASers that primarily go by their handles online) tells Ars that exploiting this DPCM glitch requires hardware that can flood the NES' input wire with a full 7,984 inputs per second. At that point, an issue with the game's screen-splitting raster interrupt causes it to start reading instructions from the very beginning of the RAM. If that happens, the game will go into an idle loop, constantly polling for input until it sees a non-maskable interrupt call asking for the next frame. At that point, it figures the repeated input is a 'true input' rather than a phantom from the DPCM glitch and passes that along as the real button being pressed for that frame.Īll TASBot has to do, then, is ensure that the game never sees the same input twice in a row when polling the controller within a frame.

3, the developers accounted for this problem by simply polling the controller input multiple times per frame, until the system sees the same input twice in a row. Uncorrected, this hardware vagary would lead to a lot of 'phantom inputs,' where a button press would register when none had occurred.įor Super Mario Bros. 3.Īs it turns out, the NES hardware itself has a small bug, such that reading sound data from this channel results in the CPU sometimes making an extra 'read' request from one of the controller inputs. This one-bit data stream was used to play extremely basic audio samples in select games, including Super Mario Bros.

Further Reading Pokémon plays Twitch: How a robot got IRC running on an unmodified SNESTASBot's newest bit of game-breaking magic relies on the vagaries of the NES' DPCM (differential pulse code modulation) sound channel.

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