The first attempt to support the 64-bit mode had two bugs:
1. The saved general-purpose register 31 value was overwritten with the saved
link register value.
2. The link register was saved and restored using 32-bit instructions.
Use 64-bit store/load instructions to save/restore the link register. Make
sure that the general-purpose register 31 and the link register storage areas
do not overlap.
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