Inside an interrupt routine that makes a function call, all save-on-call registers (R0-R3) should be saved to the stack regardless of whether they are used in the interrupt. For C2000 with 64-bit floating point registers (--float-support=fpu64 --abi=eabi), only the 32-bit RxH halves of 64-bit floating point registers that are unused in the interrupt are being saved. If the 64-bit FP register is used in the interrupt, the entire register is saved to the stack.