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# fusée
fusée is a custom bootloader used to start the Atmosphère environment.
It is divided into three sub-components: fusée-primary, fusée-mtc and fusée-secondary.
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fusée is also capable of chainloading other payloads (e.g.: Android).
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fusée's behavior can be configured via the [BCT.ini ](../features/configurations.md ) file located on the SD card.
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## fusée-primary
fusée-primary is the first piece of Atmosphère's code that runs on the hardware.
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It is distributed as a standalone payload designed to be launched via RCM by abusing the CVE-2018-6242 vulnerability.
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This payload is responsible for all the low-level hardware initialization required by the Nintendo Switch, plus the extra task of initializing the SD card and reading the next fusée sub-components from it.
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## fusée-mtc
fusée-mtc is an optional, but heavily recommended sub-component that performs DRAM memory training.
This ensures a proper environment for running the final fusée sub-component.
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## fusée-secondary
fusée-secondary is the last fusée sub-component that runs on the system.
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It is responsible for configuring and bootstrapping the Atmosphère environment by mimicking the Horizon OS's design.
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This includes setting up the cryptosystem, mounting or emulating the eMMC, injecting or patching system modules and launching the exosphère component.
