LBU

assembly

[
  "lbu [SrcL, SrcR<{.sw,.uw,.neg}><<<shamt>], ->{t, u, Rd}"
]

block

[]

catalog_indices

[
  311
]

catalog_records

[
  {
    "agu": {
      "action": "Load",
      "address_kind": "Register",
      "offset_scale": 0,
      "prefetch_returns_address": false,
      "signed_load": false,
      "size_bytes": 1,
      "update_mode": "None"
    },
    "asm": "lbu [SrcL, SrcR<{.sw,.uw,.neg}><<<shamt>], ->{t, u, Rd}",
    "constraints": [],
    "encoding": [
      {
        "index": 0,
        "mask": "0x0000707f",
        "match": "0x00004009",
        "width_bits": 32
      }
    ],
    "encoding_kind": "L32",
    "fields": [
      {
        "name": "RegDst",
        "pieces": [
          {
            "instruction_lsb": 7,
            "value_lsb": 0,
            "width": 5
          }
        ],
        "signedness": "encoding-defined",
        "width": 5
      },
      {
        "name": "SrcL",
        "pieces": [
          {
            "instruction_lsb": 15,
            "value_lsb": 0,
            "width": 5
          }
        ],
        "signedness": "encoding-defined",
        "width": 5
      },
      {
        "name": "SrcR",
        "pieces": [
          {
            "instruction_lsb": 20,
            "value_lsb": 0,
            "width": 5
          }
        ],
        "signedness": "encoding-defined",
        "width": 5
      },
      {
        "name": "SrcRType",
        "pieces": [
          {
            "instruction_lsb": 25,
            "value_lsb": 0,
            "width": 2
          }
        ],
        "signedness": "encoding-defined",
        "width": 2
      },
      {
        "name": "shamt",
        "pieces": [
          {
            "instruction_lsb": 27,
            "value_lsb": 0,
            "width": 5
          }
        ],
        "signedness": "encoding-defined",
        "width": 5
      }
    ],
    "form_id": "lbu_32_a9a58ab4ea22",
    "length_bits": 32,
    "mnemonic": "LBU",
    "semantic_family": "AGU",
    "semantic_group": "LDA/BASE_REG",
    "semantic_handler": "ExecuteScalarLoad",
    "semantic_summary": "LBU snapshots its scalar sources, forms its encoded address, and loads one aligned little-endian 1-byte value.",
    "status": "accepted"
  }
]

classification

[
  "agu"
]

contract

{
  "block_composition": [
    "none"
  ],
  "canonical_assembly": [
    "lbu [SrcL, SrcR<{.sw,.uw,.neg}><<<shamt>], ->{t, u, Rd}"
  ],
  "defaults": [
    "Every displayed operand field is encoded explicitly; encoded zero is a value and never denotes omission.",
    "SrcRType=0 leaves SrcR unchanged, SrcRType=1 sign-extends SrcR[31:0], SrcRType=2 zero-extends SrcR[31:0], and SrcRType=3 negates the full PTO_XLEN value. Encoded shamt zero performs no shift."
  ],
  "encoding_class": "standalone-encoded",
  "examples": [
    "lbu [SrcL, SrcR<{.sw,.uw,.neg}><<<shamt>], ->{t, u, Rd}"
  ],
  "exceptions": [
    "A fixed-bit mismatch, reserved field value, or unavailable selected T/U source raises Fault_IllegalInstruction before instruction effects.",
    "A misaligned 1-byte address raises Fault_DataAlignment before translation or permission. A later permission or bounded-memory failure raises Fault_DataPage at the original address.",
    "A fault emits no successful memory event, performs no partial memory or destination effect, preserves pending writeback, and leaves TPC at the faulting instruction.",
    "Recovery performs a full reissue: every address, source snapshot, preflight, memory operation, and destination is recomputed with no retained progress."
  ],
  "field_contracts": {},
  "field_zero_meanings": {
    "RegDst": "Encoded zero discards this result without suppressing the instruction's other effects.",
    "SrcL": "Encoded zero reads the architectural zero GPR.",
    "SrcR": "Encoded zero reads the architectural zero GPR.",
    "SrcRType": "Encoded zero leaves the complete PTO_XLEN register-offset value unchanged.",
    "shamt": "Encoded zero performs no shift."
  },
  "legality": [
    "Every encoded Reg5 source uses the complete domain: codes 0..23 select absolute GPRs, codes 24..27 select T#1..T#4, and codes 28..31 select U#1..U#4 without consumption.",
    "Every Reg5 destination is assigned: codes 1..23 write GPRs, code 30 pushes U, code 31 pushes T, and codes 0 and 24..29 discard only that result.",
    "All four SrcRType values and all shamt values 0..31 are assigned; apply the modifier before the shift.",
    "Each memory address must be aligned to the 1-byte access size; a 1-byte access is the complete transfer unit."
  ],
  "memory_effects": [
    "After complete preflight, perform one little-endian 1-byte load and record one relaxed load event.",
    "The load preserves memory and reservation state."
  ],
  "operands": [
    {
      "field": "RegDst",
      "role": "Reg5 loaded-value destination or discard"
    },
    {
      "field": "SrcL",
      "role": "Reg5 address-base source"
    },
    {
      "field": "SrcR",
      "role": "Reg5 register-offset source"
    },
    {
      "field": "SrcRType",
      "role": "register-offset transformation selector"
    },
    {
      "field": "shamt",
      "role": "post-transformation logical-left-shift amount"
    }
  ],
  "ordering": [
    "Snapshot all explicit and implicit scalar sources before destination or memory effects; duplicate and source/destination aliases observe pre-instruction values.",
    "Complete the relaxed 1-byte memory operation, publish any result or writeback, and then advance TPC."
  ],
  "standalone_opcode": true,
  "state_effects": [
    "Form offset = LSL(Modify(SrcR, SrcRType), the encoded shamt) and add it modulo 2^PTO_XLEN to the SrcL base.",
    "After a successful 1-byte load, zero-extend the loaded value to PTO_XLEN and publish it through the destination.",
    "Successful execution advances TPC by 4 bytes; a rejected or faulting attempt does not retire."
  ]
}

depends_on

[
  "PTO-BLOCK-MODEL-SCHEMA-PROFILE-ENCODING"
]

id

PTO-SCALAR-LBU

mnemonic

LBU

summary

LBU snapshots its scalar sources, forms its encoded address, and loads one aligned little-endian 1-byte value.

surface

scalar

{
  "classification": [
    "agu"
  ],
  "documentation": "docs/scalar/agu/LBU.md",
  "id": "PTO-SCALAR-LBU",
  "instruction_contract": {
    "artifact": "spec/evidence/instruction-contract-closure.json",
    "mnemonic": "LBU",
    "ndf_clause": "PTO-INST-SCALAR-LBU"
  },
  "mnemonic": "LBU",
  "readiness_subjects": [
    "ADR-0029",
    "ADR-0059"
  ],
  "semantic_tests": [
    "PTO-AVS-AGU-LBU-ALIAS-001",
    "PTO-AVS-AGU-LBU-BOUND-001",
    "PTO-AVS-AGU-LBU-EXEC-001",
    "PTO-AVS-AGU-LBU-FAULT-001"
  ],
  "source": "asl/scalar/agu/LBU.asl",
  "surface": "scalar",
  "tests": [
    "PTO-AVS-AGU-LBU-ALIAS-001",
    "PTO-AVS-AGU-LBU-BOUND-001",
    "PTO-AVS-AGU-LBU-EXEC-001",
    "PTO-AVS-AGU-LBU-FAULT-001",
    "PTO-AVS-SCALAR-LBU-DECODE-001"
  ]
}