This program resides at the start of the emmc/ufs flash device. It relies on various things including but not limited to timers, clocks, watchdogs, UART, GPIO, USB, etc and will setup these in it’s init instructions. Preloader executes after BootROM has initialized the storage and validated security, meaning it does not need to perform security checks and has full access to the flash (Note: this information will come handy at a later point). PL also initializes SecureLib which is responsible for Secure Boot setup. After it is done bringing up the platform, it will select a boot mode (usually NORMAL_BOOT) and will load the Android BootLoader.
If during PL init, the Volume Up (+) button is held, Preloader will abort loading and hand control back to BROM, signaling it to load a low-level “Emergency Download” (EDL) mode. This interrupt is only listened for before SecureLib is setup.