| base+4 ... base+2          | base             | number of bits             |
| ------ -----------------   | ---------------- | -------------------------- |
| ..xxxx  xxxxxxxxxxxxxxxx   | xnnnxxxxx1111111 | (80+16\*nnn)-bit, nnn!=111 |
| {ops}{Pred}{Reg}{VL Block} | VBLOCK Prefix    |                            |

A suitable prefix, which fits the Expanded Instruction-Length encoding
for "(80 + 16 times instruction-length)", as defined in Section 1.5
of the RISC-V ISA, is as follows:

| 15    | 14:12 | 11:10 | 9     | 8     | 7    | 6:0     |
| -     | ----- | ----- | ----- | ----  | ---  | ------- |
| vlset | 16xil | rplen | pplen | pmode | rmode | 1111111 |

The VL/MAXVL/SubVL Block format, when 16xil != 0b111, is:

[[!table  data="""
31:30 | 29:28 | 27:22    | 21 | 20:19   | 18:16    | comment.................. |
0b00  | SubVL | imm[5:0] |rsvd| rd[4:0]           || sv.setvl rd, x0, imm      |
0b01  | SubVL | imm[5:0] | rs1[2:0]    || rd[2:0]  | sv.setvl rd, rs1, imm (1) |
0b10  | SubVL | imm[5:0] |rsvd|  rs1[4:0]         || sv.setvl x0, rs1, imm     |
0b11  | rsvd  | rsvd     |rsvd| rsvd              || reserved, all 0s          |
"""]]

Note (1) - Registers are in RVC format (x8-x15)

Note (2) - [[specification/sv.setvl]] behaviour is expected, as if an sv.setvl
instruction had actually been called.

When 16xil is 0b111, this is the "Extended" Format, using the >= 192-bit
RISC-V ISA format.  Note that the length is 96+16\*nnnnn, not 192+

| base+5 ... base+3         | base+1  | base             | no. of bits   |
| ------ -----------------  |         | ---------------- | ------------- |
| ..xxxx  xxxxxxxxxxxxxxxx  |         | x111xxxxx1111111 | 96+16\*nnnnn |
| {ops}{Pred}{Reg}{VL Block}| VBLOCK2 | VBLOCK Prefix    |               |

VBLOCK2 extends the VBLOCK fields:

| 15   | 14:12 | 11:10  | 9:8    | 7:5   | 4:0  |
| ---- | ----- | -----  | ----   | ---   | ---- |
| rsvd | mapsz | rplen2 | pplen2 | swlen | ilen |

* ilen is the instruction length (number of 16-bit blocks)
* swlen specifies the number of "swizzle" blocks
* rplen2 extends rplen by 2 bits
* pplen2 extends pplen by 2 bits
* mapsz indicates the size of the "remap" area.  See table below for size
* 1 bit is reserved for extensions

Mapsz to Remap size is in number of 16-bit blocks:

| mapsz | remap size |
| ----- | ---------- |
| 0     | 0          |
| 1     | 6          |
| 2     | 7          |
| 3     | 8          |
| 4     | 10         |
| 5     | 12         |
| 6     | 14         |
| 7     | 16         |