_Unwind_GetGR returns data at index found in context . The register is identified by its index: 0 to 31 are for the fixed registers, and 32 to 127 are for the stacked registers.
During the two phases of unwinding, only GR1 has a guaranteed value, which is the global pointer of the frame referenced by the unwind context . If the register has its NAT bit set, the behavior is unspecified.