During the Quad-SPI boot process, the BootROM configures the controller with 4-bit I/O. This configuration includes a single device and the dual 4-bit stacked case. The BootROM reads the first (or only) Quad-SPI device in x1 mode and reads the width detection parameter in the BootROM Header. If the width detection parameter is equal to 0xAA995566, then the BootROM assumes it found a valid header that is requesting a 4-bit I/O configuration. It might be one device or it might be a dual 4-bit stacked configuration. In the latter case, the second device is always ignored by the BootROM, but it might be accessed by user code. After reading the width detection parameter in x1 mode, the BootROM attempts to read the parameter in x4 mode. If x4 mode fails, it tries x2 mode. After this, the BootROM uses the widest supported I/O bus width to access the Quad-SPI device.