Uninitialized .bss Variables in Simulation

Description

If your program reads the value of an uninitialized .bss variable during HDL simulation, and the BSP (system library) is compiled with the ModelSim only, no hardware support property enabled in Nios II IDE, a warning appears about unfiltered data being 'x'. This warning appears because when this property is enabled, the code that clears the .bss memory region is omitted to speed up HDL simulation so this memory region is uninitialized. The .bss region contains global and static local variables that are not initialized by the application so they default to a value of zero. When the Nios II processor reads uninitialized variables, it displays a warning and converts any of the bits of the uninitialized data to zero which correctly mimics the effect of the missing .bss clearing code. The Hardware Abstraction Layer (HAL) code that executes before and after main() might use .bss variables, so these warnings might appear even if your application does not use the .bss section.