Dualbank Flash Programming
The ST STM32L5 series devices come with a dual-bank flash memory. The layout of the dual-bank flash memory can be configured by the user through the option byte DUALBANK. By default, the value of this option byte is DUALBANK == 1, which means that the flash is configured as dual bank memory while DUALBANK == 0 means that the flash is configured as single bank memory flash. In dual-bank mode, the flash boot source can be switched between bank 1 or bank 2. In single-bank mode, flash always boots from bank 1. The total flash size is exactly the same for both modes. Another advantage of using DUALBANK is that both banks can be accessed in parallel (e.g. reading bank 1 while programming bank 2).
The flash algorithm has to behave differently, depending on the active flash bank mode configuration as the flash controller expects different page indices for the same destination address.
When using the J-Link flash loader, make sure that the flash controller is configured for dual bank flash layout (DUALBANK == 1), which is the default.