Where in the 48 bit format the prefix is "0b0011111" in bits 0 to 6, this is
now set to "0b0111111".
-+---------------+---------------+--------------+-----------|
++---------------+---------------+--------------+-----------+
| 63:48 | 47:18 | 17:7 | 6:0 |
-+---------------+---------------+--------------+-----------|
++---------------+---------------+--------------+-----------+
| 64 bit prefix | RV32[31:3] | P48[17:7] | 0b0111111 |
-+---------------+---------------+--------------+-----------|
++---------------+---------------+--------------+-----------+
* The 64 bit prefix format is below
* Bits 18 to 47 contain bits 3 to 31 of a standard RV32 format