# Register Mode
The register addressing mode is used to access the data in registers. It is
identified by byte values 0x50 through 0x5E with the low nibble indicating
the starting register. Data types larger than 32-bits use multiple
registers. 64-bit data types use the next higher register as the most
significant 32-bits (e.g. A quadword at R0 uses R0 as the least significant
32-bits and R1 as the most significant 32-bits). 128-bit data types use 4
registers with the starting register being the least significant 32-bits.
The behavior of 0x5F is always unpredictable. 0x5E is unpredictable for data
types 64-bits and larger. 0x5C and 0x5D are unpredictable for 128-bit data
types.