Register Bit-field

{
  "reg": [
    {"name":"opcode","bits":4,"attr":"R/W"},
    {"name":"rs",    "bits":3},
    {"name":"rt",    "bits":3},
    {"name":"imm",   "bits":6},
    {"bits":16,"name":"reserved","type":1}
  ],
  "config":{"lanes":1}
}