The forwarding of interrupts to the application depends on the state of the SoftDevice.
At the lowest level, the MBR receives all interrupts and forwards them to the SoftDevice regardless of whether the SoftDevice is enabled or not. The use of a bootloader introduces some exceptions to this. See Master boot record and bootloader.
Some peripherals and their respective interrupt numbers are reserved for use by the SoftDevice (see Hardware peripherals). Any interrupt handler defined by the application for these interrupts will not be called as long as the SoftDevice is enabled. When the SoftDevice is disabled, these interrupts will be forwarded to the application.
The Supervisor Call (SVC) interrupt is always intercepted by the SoftDevice regardless of whether it is enabled or disabled. The SoftDevice inspects the SVC number, and if it is equal or greater than 0x10, the interrupt is processed by the SoftDevice. SVC numbers below 0x10 are forwarded to the application's SVC interrupt handler. This allows the application to make use of a range of SVC numbers for its own purpose, for example, for an RTOS.
Interrupts not used by the SoftDevice are always forwarded to the application.
For the SoftDevice to locate the application interrupt vectors, the application must define its interrupt vector table at the bottom of the Application Flash Region illustrated in Memory resource map. When the base address of the application code is directly after the top address of the SoftDevice, the code can be developed as a standard ARM® Cortex® -M4 application project with the compiler creating the interrupt vector table.