16 bit format:

| PrCSR | (15..11) | 10     | 9     | 8   | (7..1)  | 0       |
| ----- | -        | -      | -     | -   | ------- | ------- |
| 0     | predidx  | zero0  | inv0  | i/f | regidx  | ffirst0 |
| 1     | predidx  | zero1  | inv1  | i/f | regidx  | ffirst1 |
| 2     | predidx  | zero2  | inv2  | i/f | regidx  | ffirst2 |
| 3     | predidx  | zero3  | inv3  | i/f | regidx  | ffirst3 |

Note: predidx=x0, zero=1, inv=1 is a RESERVED encoding.  Its use must
generate an illegal instruction trap.

8 bit format:

| PrCSR | 7     | 6     | 5   | (4..0)  |
| ----- | -     | -     | -   | ------- |
| 0     | zero0 | inv0  | i/f | regnum  |

Mapping from 8 to 16 bit format, the table becomes:

| PrCSR | (15..11) | 10     | 9     | 8   | (7..1)  | 0       |
| ----- | -        | -      | -     | -   | ------- | ------- |
| 0     | x9       | zero0  | inv0  | i/f | regnum  | ff=0    |
| 1     | x10      | zero1  | inv1  | i/f | regnum  | ff=0    |
| 2     | x11      | zero2  | inv2  | i/f | regnum  | ff=0    |
| 3     | x12      | zero3  | inv3  | i/f | regnum  | ff=0    |