Decoding the ARM7TDMI Instruction set. Article at https://www.gregorygaines.com/blog/decoding-the-arm7tdmi-instruction-set-game-boy-advance/

arm7tdmi_instruction_set_decoder.go

type Instruction func(opcode uint32, arm *ARM7TDMI)

func DecodeARMInstruction(opcode uint32) Instruction {
  switch {
    case IsBranchAndBranchExchange(opcode):
      return BranchAndBranchExchange;

    case IsBlockDataTransfer(opcode):
      return BlockDataTransfer;

    case IsBranchAndBranchWithLink(opcode):
      return BranchAndBranchWithLink;

    case IsSoftwareInterrupt(opcode):
      return SoftwareInterrupt

    case IsUndefined(opcode):
      return Undefined

    case IsSingleDataTransfer(opcode):
      return SingleDataTransfer

    case IsSingleDataSwap(opcode):
      return SingleDataSwap

    case IsMultiply(opcode):
      return Multiply
      
    case IsHalfwordDataTransferRegister(opcode):
      return HalfwordDataTransferRegister

    case IsHalfwordDataTransferImmediate(opcode):
      return HalfwordDataTransferImmediate

    case IsPSRTransferMRS(opcode):
      return PSRTransferMRS

    case IsPSRTransferMSR(opcode):
      return PSRTransferMSR

    case IsDataProcessing(opcode):
      return DataProcessing
  }
  
  return UnimplementedARMInstruction
}

func IsBranchAndBranchExchange(opcode uint32) bool {
  const branchAndExchangeFormat = 0b0000_0001_0010_1111_1111_1111_0001_0000

  const formatMask = 0b0000_1111_1111_1111_1111_1111_1111_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == branchAndExchangeFormat
}

func IsBlockDataTransfer(opcode uint32) bool {
  const blockDataTransferFormat = 0b0000_1000_0000_0000_0000_0000_0000_0000

  const formatMask = 0b0000_1110_0000_0000_0000_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == blockDataTransferFormat
}

func IsBranchAndBranchWithLink(opcode uint32) bool {
  const branchFormat = 0b0000_1010_0000_0000_0000_0000_0000_0000
  const branchWithLinkFormat = 0b0000_1011_0000_0000_0000_0000_0000_0000

  const formatMask = 0b0000_1111_0000_0000_0000_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == branchFormat || extractedFormat == branchWithLinkFormat
}

func IsSoftwareInterrupt(opcode uint32) bool {
  const softwareInterruptFormat = 0b0000_1111_0000_0000_0000_0000_0000_0000

  const formatMask = 0b0000_1111_0000_0000_0000_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == softwareInterruptFormat
}

func IsUndefined(opcode uint32) bool {
  const undefinedFormat = 0b0000_0110_0000_0000_0000_0000_0001_0000

  const formatMask = 0b0000_1110_0000_0000_0000_0000_0001_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == undefinedFormat
}

func IsSingleDataTransfer(opcode uint32) bool {
  const singleDataTransferFormat = 0b0000_0100_0000_0000_0000_0000_0000_0000

  const formatMask = 0b0000_1100_0000_0000_0000_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == singleDataTransferFormat
}

func IsSingleDataSwap(opcode uint32) bool {
  const singleDataSwapFormat = 0b0000_0001_0000_0000_0000_0000_1001_0000

  const formatMask = 0b0000_1111_1000_0000_0000_1111_1111_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == singleDataSwapFormat
}

func IsMultiply(opcode uint32) bool {
  const multiplyFormat = 0b0000_0000_0000_0000_0000_0000_1001_0000
  const multiplyLongFormat = 0b0000_0000_1000_0000_0000_0000_1001_0000

  const formatMask = 0b0000_1111_1000_0000_0000_0000_1111_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == multiplyFormat || extractedFormat == multiplyLongFormat
}

func IsHalfwordDataTransferRegister(opcode uint32) bool {
  const halfwordDataTransferRegisterFormat = 0b0000_0000_0000_0000_0000_0000_1001_0000

  const formatMask = 0b0000_1110_0100_0000_0000_1111_1001_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == halfwordDataTransferRegisterFormat
}

func IsHalfwordDataTransferImmediate(opcode uint32) bool {
  const halfwordDataTransferImmediateFormat = 0b0000_0000_0100_0000_0000_0000_1001_0000

  const formatMask = 0b0000_1110_0100_0000_0000_0000_1001_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == halfwordDataTransferImmediateFormat
}

func IsPSRTransferMRS(opcode uint32) bool {
  const mrsFormat = 0b0000_0001_0000_1111_0000_0000_0000_0000

  const formatMask = 0b0000_1111_1011_1111_0000_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == mrsFormat
}

func IsPSRTransferMSR(opcode uint32) bool {
  const msrFormat = 0b0000_0001_0010_0000_1111_0000_0000_0000

  const formatMask = 0b0000_1101_1011_0000_1111_0000_0000_0000
  
  var extractedFormat = opcode & formatMask

  return extractedFormat == msrFormat
}

func IsDataProcessing(opcode uint32) bool {
  const dataProcessingFormat = 0b0000_0000_0000_0000_0000_0000_0000_0000

  const formatMask = 0b0000_1100_0000_0000_0000_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == dataProcessingFormat
}

thumb_instruction_set_decoder.go

type Instruction func(opcode uint16, arm *ARM7TDMI)

func DecodeTHUMBInstruction(opcode uint16) Instruction {
  switch {
    case IsTHUMBSoftwareInterrupt(opcode):
      return THUMBSoftwareInterrupt

    case IsUnconditionalBranch(opcode):
      return UnconditionalBranch

    case IsConditionalBranch(opcode):
      return ConditionalBranch

    case IsMultipleLoadStore(opcode):
      return MultipleLoadstore

    case IsLongBranchWithLink(opcode):
      return LongBranchWithLink

    case IsAddOffsetToStackPointer(opcode):
      return AddOffsetToStackPointer

    case IsPushPopRegisters(opcode):
      return PushPopRegisters

    case IsLoadStoreHalfword(opcode):
      return LoadStoreHalfword

    case IsSPRelativeLoadStore(opcode):
      return SPRelatvieLoadStore
        
    case IsLoadAddress(opcode):
      return LoadAddress
        
    case IsLoadStoreWithImmediateOffset(opcode):
      return LoadStoreWithImmediateOffset
    
    case IsLoadStoreWithRegisterOffset(opcode):
      return LoadStoreWithRegisterOffset
        
    case IsLoadStoreSignExtendedByteHalfword(opcode):
      return LoadStoreSignExtendedByteHalfword
      
    case IsPCRelativeLoad(opcode):
      return PCRelativeLoad
      
    case IsHiRegisterOperationsBranchExchange(opcode):
      return HiRegisterOperationsBranchExchange

    case IsALUOperations(opcode):
      return ALUOperations

    case IsMoveCompareAddSubtractImmediate(opcode):
      return MoveCompareAddSubtractImmediate

    case IsAddSubtract(opcode):
      return AddSubtract

    case IsMoveShiftedRegister(opcode):
      return MoveShiftedRegister  
  }
  
  return UnimplementedTHUMBInstruction
}

func IsTHUMBSoftwareInterrupt(opcode uint16) bool {
  const softwareInterruptFormat = 0b1101_1111_0000_0000

  const formatMask = 0b1111_1111_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == softwareInterruptFormat
}

func IsUnconditionalBranch(opcode uint16) bool {
  const unconditionalBranchFormat = 0b1110_0000_0000_0000

  const formatMask = 0b1111_1000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == unconditionalBranchFormat
}

func IsConditionalBranch(opcode uint16) bool {
  const conditionalBranchFormat = 0b1101_0000_0000_0000

  const formatMask = 0b1111_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == conditionalBranchFormat
}

func IsMultipleLoadStore(opcode uint16) bool {
  const multipleLoadStoreFormat = 0b1100_0000_0000_0000

  const formatMask = 0b1111_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == multipleLoadStoreFormat
}

func IsLongBranchWithLink(opcode uint16) bool {
  const longBranchWithLinkFormat = 0b1111_0000_0000_0000

  const formatMask = 0b1111_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == longBranchWithLinkFormat
}

func IsAddOffsetToStackPointer(opcode uint16) bool {
  const addOffsetToStackPointerFormat = 0b1011_0000_0000_0000

  const formatMask = 0b1111_1111_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == addOffsetToStackPointerFormat
}

func IsPushPopRegisters(opcode uint16) bool {
  const pushPopRegistersFormat = 0b1011_0100_0000_0000

  const formatMask = 0b1111_0110_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == pushPopRegistersFormat
}

func IsLoadStoreHalfword(opcode uint16) bool {
  const loadStoreHalfwordFormat = 0b1000_0000_0000_0000

  const formatMask = 0b1111_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == loadStoreHalfwordFormat
}

func IsSPRelativeLoadStore(opcode uint16) bool {
  const spRelativeLoadStoreFormat = 0b1001_0000_0000_0000

  const formatMask = 0b1111_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == spRelativeLoadStoreFormat
}

func IsLoadAddress(opcode uint16) bool {
  const loadAddressFormat = 0b1010_0000_0000_0000
  
  const formatMask = 0b1111_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == loadAddressFormat
}

func IsLoadStoreWithImmediateOffset(opcode uint16) bool {
  const loadStoreImmediateOffsetFormat = 0b0110_0000_0000_0000

  const formatMask = 0b1110_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == loadStoreImmediateOffsetFormat
}

func IsLoadStoreWithRegisterOffset(opcode uint16) bool {
  const loadStoreRegisterOffsetFormat = 0b0101_0000_0000_0000

  const formatMask = 0b1111_0010_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == loadStoreRegisterOffsetFormat
}

func IsLoadStoreSignExtendedByteHalfword(opcode uint16) bool {
  const loadStoreSignExtendedByteHalfwordFormat = 0b0101_0010_0000_0000

  const formatMask = 0b1111_0010_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == loadStoreSignExtendedByteHalfwordFormat
}

func IsPCRelativeLoad(opcode uint16) bool {
  const pcRelativeLoadFormat = 0b0100_1000_0000_0000

  const formatMask = 0b1111_1000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == pcRelativeLoadFormat
}

func IsHiRegisterOperationsBranchExchange(opcode uint16) bool {
  const hiRegisterOperationsBranchExchangeFormat = 0b0100_0100_0000_0000

  const formatMask = 0b1111_1100_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == hiRegisterOperationsBranchExchangeFormat
}

func IsALUOperations(opcode uint16) bool {
  const aluOperationsFormat = 0b0100_0000_0000_0000

  const formatMask = 0b1111_1100_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == aluOperationsFormat
}

func IsMoveCompareAddSubtractImmediate(opcode uint16) bool {
  const moveCompareAddSubtractImmediateFormat = 0b0010_0000_0000_0000

  const formatMask = 0b1110_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == moveCompareAddSubtractImmediateFormat
}

func IsAddSubtract(opcode uint16) bool {
  const addSubtractFormat = 0b0001_1000_0000_0000

  const formatMask = 0b1111_1000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == addSubtractFormat
}

func IsMoveShiftedRegister(opcode uint16) bool {
  const moveShiftedRegistersFormat = 0b0000_0000_0000_0000

  const formatMask = 0b1110_0000_0000_0000

  var extractedFormat = opcode & formatMask

  return extractedFormat == moveShiftedRegistersFormat
}