_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.