diff --git a/internal/commands/add.go b/internal/commands/add.go
new file mode 100644
index 0000000..38d0465
--- /dev/null
+++ b/internal/commands/add.go
@@ -0,0 +1,273 @@
+package commands
+
+import (
+	"fmt"
+	"strings"
+
+	"c65gm/internal/compiler"
+	"c65gm/internal/preproc"
+	"c65gm/internal/utils"
+)
+
+// AddCommand handles ADD operations
+// Syntax:
+//
+//	ADD <param1> TO <param2> GIVING <dest>      # old syntax with TO/GIVING
+//	ADD <param1> + <param2> -> <dest>           # old syntax with +/->
+//	<dest> = <param1> + <param2>                # new syntax
+type AddCommand struct {
+	param1VarName string
+	param1VarKind compiler.VarKind
+	param1Value   uint16
+	param1IsVar   bool
+
+	param2VarName string
+	param2VarKind compiler.VarKind
+	param2Value   uint16
+	param2IsVar   bool
+
+	destVarName string
+	destVarKind compiler.VarKind
+}
+
+func (c *AddCommand) WillHandle(line preproc.Line) bool {
+	params, err := utils.ParseParams(line.Text)
+	if err != nil || len(params) == 0 {
+		return false
+	}
+
+	// Old syntax: ADD ... (must have exactly 6 params)
+	if strings.ToUpper(params[0]) == "ADD" && len(params) == 6 {
+		return true
+	}
+
+	// New syntax: <dest> = <param1> + <param2>
+	if len(params) == 5 && params[1] == "=" && params[3] == "+" {
+		return true
+	}
+
+	return false
+}
+
+func (c *AddCommand) Interpret(line preproc.Line, ctx *compiler.CompilerContext) error {
+	// Clear state
+	c.param1VarName = ""
+	c.param1IsVar = false
+	c.param1Value = 0
+	c.param2VarName = ""
+	c.param2IsVar = false
+	c.param2Value = 0
+	c.destVarName = ""
+
+	params, err := utils.ParseParams(line.Text)
+	if err != nil {
+		return err
+	}
+
+	paramCount := len(params)
+	scope := ctx.CurrentScope()
+
+	// Create constant lookup function
+	constLookup := func(name string) (int64, bool) {
+		sym := ctx.SymbolTable.Lookup(name, scope)
+		if sym != nil && sym.IsConst() {
+			return int64(sym.Value), true
+		}
+		return 0, false
+	}
+
+	// Determine syntax and parse accordingly
+	if strings.ToUpper(params[0]) == "ADD" {
+		// Old syntax: ADD <param1> TO/+ <param2> GIVING/-> <dest>
+		if paramCount != 6 {
+			return fmt.Errorf("ADD: wrong number of parameters (%d), expected 6", paramCount)
+		}
+
+		separator1 := strings.ToUpper(params[2])
+		if separator1 != "TO" && separator1 != "+" {
+			return fmt.Errorf("ADD: parameter #3 must be 'TO' or '+', got %q", params[2])
+		}
+
+		separator2 := strings.ToUpper(params[4])
+		if separator2 != "GIVING" && separator2 != "->" {
+			return fmt.Errorf("ADD: parameter #5 must be 'GIVING' or '->', got %q", params[4])
+		}
+
+		// Parse destination
+		destName := params[5]
+		destSym := ctx.SymbolTable.Lookup(destName, scope)
+		if destSym == nil {
+			return fmt.Errorf("ADD: unknown variable %q", destName)
+		}
+		if destSym.IsConst() {
+			return fmt.Errorf("ADD: cannot assign to constant %q", destName)
+		}
+		c.destVarName = destSym.FullName()
+		c.destVarKind = getVarKind(destSym)
+
+		// Parse param1
+		if err := c.parseParam(params[1], &c.param1VarName, &c.param1VarKind, &c.param1Value, &c.param1IsVar, ctx, scope, constLookup); err != nil {
+			return fmt.Errorf("ADD: param1: %w", err)
+		}
+
+		// Parse param2
+		if err := c.parseParam(params[3], &c.param2VarName, &c.param2VarKind, &c.param2Value, &c.param2IsVar, ctx, scope, constLookup); err != nil {
+			return fmt.Errorf("ADD: param2: %w", err)
+		}
+
+	} else {
+		// New syntax: <dest> = <param1> + <param2>
+		if paramCount != 5 {
+			return fmt.Errorf("ADD: wrong number of parameters (%d), expected 5", paramCount)
+		}
+
+		if params[1] != "=" {
+			return fmt.Errorf("ADD: expected '=' at position 2, got %q", params[1])
+		}
+
+		if params[3] != "+" {
+			return fmt.Errorf("ADD: expected '+' at position 4, got %q", params[3])
+		}
+
+		// Parse destination
+		destName := params[0]
+		destSym := ctx.SymbolTable.Lookup(destName, scope)
+		if destSym == nil {
+			return fmt.Errorf("ADD: unknown variable %q", destName)
+		}
+		if destSym.IsConst() {
+			return fmt.Errorf("ADD: cannot assign to constant %q", destName)
+		}
+		c.destVarName = destSym.FullName()
+		c.destVarKind = getVarKind(destSym)
+
+		// Parse param1
+		if err := c.parseParam(params[2], &c.param1VarName, &c.param1VarKind, &c.param1Value, &c.param1IsVar, ctx, scope, constLookup); err != nil {
+			return fmt.Errorf("ADD: param1: %w", err)
+		}
+
+		// Parse param2
+		if err := c.parseParam(params[4], &c.param2VarName, &c.param2VarKind, &c.param2Value, &c.param2IsVar, ctx, scope, constLookup); err != nil {
+			return fmt.Errorf("ADD: param2: %w", err)
+		}
+	}
+
+	return nil
+}
+
+func (c *AddCommand) parseParam(
+	param string,
+	varName *string,
+	varKind *compiler.VarKind,
+	value *uint16,
+	isVar *bool,
+	ctx *compiler.CompilerContext,
+	scope []string,
+	constLookup utils.ConstantLookup,
+) error {
+	// Try variable lookup first
+	sym := ctx.SymbolTable.Lookup(param, scope)
+	if sym != nil {
+		// It's a variable or constant
+		*varName = sym.FullName()
+		*varKind = getVarKind(sym)
+		*value = sym.Value
+		*isVar = true
+		return nil
+	}
+
+	// Not a variable, must be an expression
+	val, err := utils.EvaluateExpression(param, constLookup)
+	if err != nil {
+		return fmt.Errorf("not a valid variable or expression: %w", err)
+	}
+
+	if val < 0 || val > 65535 {
+		return fmt.Errorf("value %d out of range (0-65535)", val)
+	}
+
+	*value = uint16(val)
+	*isVar = false
+	return nil
+}
+
+func (c *AddCommand) Generate(_ *compiler.CompilerContext) ([]string, error) {
+	var asm []string
+
+	// If both params are literals, fold at compile time
+	if !c.param1IsVar && !c.param2IsVar {
+		sum := uint32(c.param1Value) + uint32(c.param2Value)
+		lo := uint8(sum & 0xFF)
+		hi := uint8((sum >> 8) & 0xFF)
+
+		asm = append(asm, fmt.Sprintf("\tlda #$%02x", lo))
+		asm = append(asm, fmt.Sprintf("\tsta %s", c.destVarName))
+
+		if c.destVarKind == compiler.KindWord {
+			asm = append(asm, fmt.Sprintf("\tlda #$%02x", hi))
+			asm = append(asm, fmt.Sprintf("\tsta %s+1", c.destVarName))
+		}
+
+		return asm, nil
+	}
+
+	// At least one param is a variable - generate add code
+	asm = append(asm, "\tclc")
+
+	// Load param1
+	if c.param1IsVar {
+		asm = append(asm, fmt.Sprintf("\tlda %s", c.param1VarName))
+	} else {
+		asm = append(asm, fmt.Sprintf("\tlda #$%02x", uint8(c.param1Value&0xFF)))
+	}
+
+	// Add param2
+	if c.param2IsVar {
+		asm = append(asm, fmt.Sprintf("\tadc %s", c.param2VarName))
+	} else {
+		asm = append(asm, fmt.Sprintf("\tadc #$%02x", uint8(c.param2Value&0xFF)))
+	}
+
+	// Store low byte
+	asm = append(asm, fmt.Sprintf("\tsta %s", c.destVarName))
+
+	// If destination is word, handle high byte
+	if c.destVarKind == compiler.KindWord {
+		// Load high byte of param1
+		if c.param1IsVar {
+			if c.param1VarKind == compiler.KindWord {
+				asm = append(asm, fmt.Sprintf("\tlda %s+1", c.param1VarName))
+			} else {
+				asm = append(asm, "\tlda #0")
+			}
+		} else {
+			hi := uint8((c.param1Value >> 8) & 0xFF)
+			asm = append(asm, fmt.Sprintf("\tlda #$%02x", hi))
+		}
+
+		// Add high byte of param2
+		if c.param2IsVar {
+			if c.param2VarKind == compiler.KindWord {
+				asm = append(asm, fmt.Sprintf("\tadc %s+1", c.param2VarName))
+			} else {
+				asm = append(asm, "\tadc #0")
+			}
+		} else {
+			hi := uint8((c.param2Value >> 8) & 0xFF)
+			asm = append(asm, fmt.Sprintf("\tadc #$%02x", hi))
+		}
+
+		// Store high byte
+		asm = append(asm, fmt.Sprintf("\tsta %s+1", c.destVarName))
+	}
+
+	return asm, nil
+}
+
+// getVarKind extracts VarKind from Symbol
+func getVarKind(sym *compiler.Symbol) compiler.VarKind {
+	if sym.IsByte() {
+		return compiler.KindByte
+	}
+	return compiler.KindWord
+}
diff --git a/internal/commands/add_test.go b/internal/commands/add_test.go
new file mode 100644
index 0000000..cb9c6a6
--- /dev/null
+++ b/internal/commands/add_test.go
@@ -0,0 +1,717 @@
+package commands
+
+import (
+	"strings"
+	"testing"
+
+	"c65gm/internal/compiler"
+	"c65gm/internal/preproc"
+)
+
+func TestAddCommand_WillHandle(t *testing.T) {
+	tests := []struct {
+		name string
+		text string
+		want bool
+	}{
+		// Old syntax
+		{"old syntax with TO/GIVING", "ADD a TO b GIVING c", true},
+		{"old syntax with +/->", "ADD a + b -> c", true},
+		{"old syntax mixed case", "add x to y giving z", true},
+
+		// New syntax
+		{"new syntax basic", "result = a + b", true},
+		{"new syntax with literals", "x = 10 + 20", true},
+
+		// Should not handle
+		{"not add - subtract", "result = a - b", false},
+		{"not add - multiply", "result = a * b", false},
+		{"not add - wrong params", "ADD a b c", false},
+		{"empty", "", false},
+		{"just ADD", "ADD", false},
+		{"assignment without add", "x = y", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			cmd := &AddCommand{}
+			line := preproc.Line{Text: tt.text}
+			if got := cmd.WillHandle(line); got != tt.want {
+				t.Errorf("WillHandle() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestAddCommand_Interpret_OldSyntax(t *testing.T) {
+	tests := []struct {
+		name    string
+		setup   func(*compiler.CompilerContext)
+		text    string
+		wantErr bool
+		check   func(*testing.T, *AddCommand)
+	}{
+		{
+			name: "byte + byte -> byte",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+				ctx.SymbolTable.AddVar("c", "", compiler.KindByte, 0)
+			},
+			text:    "ADD a TO b GIVING c",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if !cmd.param1IsVar || cmd.param1VarName != "a" {
+					t.Errorf("param1 should be var 'a'")
+				}
+				if !cmd.param2IsVar || cmd.param2VarName != "b" {
+					t.Errorf("param2 should be var 'b'")
+				}
+				if cmd.destVarName != "c" || cmd.destVarKind != compiler.KindByte {
+					t.Errorf("dest should be byte 'c'")
+				}
+			},
+		},
+		{
+			name: "word + word -> word",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("x", "", compiler.KindWord, 1000)
+				ctx.SymbolTable.AddVar("y", "", compiler.KindWord, 2000)
+				ctx.SymbolTable.AddVar("z", "", compiler.KindWord, 0)
+			},
+			text:    "ADD x TO y GIVING z",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.destVarKind != compiler.KindWord {
+					t.Errorf("dest should be word")
+				}
+			},
+		},
+		{
+			name: "byte + word -> word",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindWord, 1000)
+				ctx.SymbolTable.AddVar("c", "", compiler.KindWord, 0)
+			},
+			text:    "ADD a TO b GIVING c",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.param1VarKind != compiler.KindByte {
+					t.Errorf("param1 should be byte")
+				}
+				if cmd.param2VarKind != compiler.KindWord {
+					t.Errorf("param2 should be word")
+				}
+				if cmd.destVarKind != compiler.KindWord {
+					t.Errorf("dest should be word")
+				}
+			},
+		},
+		{
+			name: "literal + var -> var",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+				ctx.SymbolTable.AddVar("c", "", compiler.KindByte, 0)
+			},
+			text:    "ADD 10 TO b GIVING c",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.param1IsVar {
+					t.Errorf("param1 should be literal")
+				}
+				if cmd.param1Value != 10 {
+					t.Errorf("param1 value = %d, want 10", cmd.param1Value)
+				}
+			},
+		},
+		{
+			name: "var + literal -> var",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("c", "", compiler.KindByte, 0)
+			},
+			text:    "ADD a TO 20 GIVING c",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.param2IsVar {
+					t.Errorf("param2 should be literal")
+				}
+				if cmd.param2Value != 20 {
+					t.Errorf("param2 value = %d, want 20", cmd.param2Value)
+				}
+			},
+		},
+		{
+			name: "literal + literal -> var",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("c", "", compiler.KindByte, 0)
+			},
+			text:    "ADD 15 TO 25 GIVING c",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.param1IsVar || cmd.param2IsVar {
+					t.Errorf("both params should be literals")
+				}
+				if cmd.param1Value != 15 || cmd.param2Value != 25 {
+					t.Errorf("param values = %d, %d, want 15, 25", cmd.param1Value, cmd.param2Value)
+				}
+			},
+		},
+		{
+			name: "hex literal",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("c", "", compiler.KindByte, 0)
+			},
+			text:    "ADD $10 TO $20 GIVING c",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.param1Value != 0x10 || cmd.param2Value != 0x20 {
+					t.Errorf("param values = %d, %d, want 16, 32", cmd.param1Value, cmd.param2Value)
+				}
+			},
+		},
+		{
+			name: "constant usage",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddConst("MAX", "", compiler.KindByte, 100)
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("c", "", compiler.KindByte, 0)
+			},
+			text:    "ADD MAX TO a GIVING c",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if !cmd.param1IsVar || cmd.param1VarName != "MAX" {
+					t.Errorf("param1 should be constant MAX")
+				}
+			},
+		},
+		{
+			name: "alternative syntax +/->",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+				ctx.SymbolTable.AddVar("c", "", compiler.KindByte, 0)
+			},
+			text:    "ADD a + b -> c",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.destVarName != "c" {
+					t.Errorf("dest should be c")
+				}
+			},
+		},
+		{
+			name: "unknown variable",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+			},
+			text:    "ADD a TO b GIVING c",
+			wantErr: true,
+		},
+		{
+			name: "assign to constant",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+				ctx.SymbolTable.AddConst("MAX", "", compiler.KindByte, 100)
+			},
+			text:    "ADD a TO b GIVING MAX",
+			wantErr: true,
+		},
+		{
+			name: "wrong parameter count",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+			},
+			text:    "ADD a TO b",
+			wantErr: true,
+		},
+		{
+			name: "wrong separator #3",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+				ctx.SymbolTable.AddVar("c", "", compiler.KindByte, 0)
+			},
+			text:    "ADD a AND b GIVING c",
+			wantErr: true,
+		},
+		{
+			name: "wrong separator #5",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+				ctx.SymbolTable.AddVar("c", "", compiler.KindByte, 0)
+			},
+			text:    "ADD a TO b INTO c",
+			wantErr: true,
+		},
+		{
+			name: "invalid expression",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("c", "", compiler.KindByte, 0)
+			},
+			text:    "ADD @#$% TO 10 GIVING c",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ctx := compiler.NewCompilerContext(preproc.NewPragma())
+			if tt.setup != nil {
+				tt.setup(ctx)
+			}
+
+			cmd := &AddCommand{}
+			line := preproc.Line{Text: tt.text}
+
+			err := cmd.Interpret(line, ctx)
+
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Interpret() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+
+			if !tt.wantErr && tt.check != nil {
+				tt.check(t, cmd)
+			}
+		})
+	}
+}
+
+func TestAddCommand_Interpret_NewSyntax(t *testing.T) {
+	tests := []struct {
+		name    string
+		setup   func(*compiler.CompilerContext)
+		text    string
+		wantErr bool
+		check   func(*testing.T, *AddCommand)
+	}{
+		{
+			name: "dest = var1 + var2",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+				ctx.SymbolTable.AddVar("result", "", compiler.KindByte, 0)
+			},
+			text:    "result = a + b",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.destVarName != "result" {
+					t.Errorf("dest = %q, want 'result'", cmd.destVarName)
+				}
+				if !cmd.param1IsVar || cmd.param1VarName != "a" {
+					t.Errorf("param1 should be var 'a'")
+				}
+				if !cmd.param2IsVar || cmd.param2VarName != "b" {
+					t.Errorf("param2 should be var 'b'")
+				}
+			},
+		},
+		{
+			name: "dest = literal + var",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("x", "", compiler.KindByte, 5)
+				ctx.SymbolTable.AddVar("result", "", compiler.KindByte, 0)
+			},
+			text:    "result = 100 + x",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.param1IsVar {
+					t.Errorf("param1 should be literal")
+				}
+				if cmd.param1Value != 100 {
+					t.Errorf("param1 = %d, want 100", cmd.param1Value)
+				}
+			},
+		},
+		{
+			name: "dest = var + literal",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("x", "", compiler.KindByte, 5)
+				ctx.SymbolTable.AddVar("result", "", compiler.KindByte, 0)
+			},
+			text:    "result = x + 50",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.param2IsVar {
+					t.Errorf("param2 should be literal")
+				}
+				if cmd.param2Value != 50 {
+					t.Errorf("param2 = %d, want 50", cmd.param2Value)
+				}
+			},
+		},
+		{
+			name: "dest = literal + literal",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("result", "", compiler.KindByte, 0)
+			},
+			text:    "result = 30 + 70",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.param1IsVar || cmd.param2IsVar {
+					t.Errorf("both params should be literals")
+				}
+			},
+		},
+		{
+			name: "word destination",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindWord, 1000)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindWord, 2000)
+				ctx.SymbolTable.AddVar("result", "", compiler.KindWord, 0)
+			},
+			text:    "result = a + b",
+			wantErr: false,
+			check: func(t *testing.T, cmd *AddCommand) {
+				if cmd.destVarKind != compiler.KindWord {
+					t.Errorf("dest should be word")
+				}
+			},
+		},
+		{
+			name: "unknown dest variable",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+			},
+			text:    "result = a + b",
+			wantErr: true,
+		},
+		{
+			name: "assign to constant",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+				ctx.SymbolTable.AddConst("MAX", "", compiler.KindByte, 100)
+			},
+			text:    "MAX = a + b",
+			wantErr: true,
+		},
+		{
+			name: "wrong operator (not +)",
+			setup: func(ctx *compiler.CompilerContext) {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+				ctx.SymbolTable.AddVar("result", "", compiler.KindByte, 0)
+			},
+			text:    "result = a - b",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ctx := compiler.NewCompilerContext(preproc.NewPragma())
+			if tt.setup != nil {
+				tt.setup(ctx)
+			}
+
+			cmd := &AddCommand{}
+			line := preproc.Line{Text: tt.text}
+
+			err := cmd.Interpret(line, ctx)
+
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Interpret() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+
+			if !tt.wantErr && tt.check != nil {
+				tt.check(t, cmd)
+			}
+		})
+	}
+}
+
+func TestAddCommand_Generate(t *testing.T) {
+	tests := []struct {
+		name      string
+		setup     func(*compiler.CompilerContext) *AddCommand
+		wantLines []string
+	}{
+		{
+			name: "constant folding - both literals to byte",
+			setup: func(ctx *compiler.CompilerContext) *AddCommand {
+				ctx.SymbolTable.AddVar("result", "", compiler.KindByte, 0)
+				return &AddCommand{
+					param1IsVar: false,
+					param1Value: 10,
+					param2IsVar: false,
+					param2Value: 20,
+					destVarName: "result",
+					destVarKind: compiler.KindByte,
+				}
+			},
+			wantLines: []string{
+				"\tlda #$1e",
+				"\tsta result",
+			},
+		},
+		{
+			name: "constant folding - both literals to word",
+			setup: func(ctx *compiler.CompilerContext) *AddCommand {
+				ctx.SymbolTable.AddVar("result", "", compiler.KindWord, 0)
+				return &AddCommand{
+					param1IsVar: false,
+					param1Value: 100,
+					param2IsVar: false,
+					param2Value: 200,
+					destVarName: "result",
+					destVarKind: compiler.KindWord,
+				}
+			},
+			wantLines: []string{
+				"\tlda #$2c",
+				"\tsta result",
+				"\tlda #$01",
+				"\tsta result+1",
+			},
+		},
+		{
+			name: "constant folding with overflow",
+			setup: func(ctx *compiler.CompilerContext) *AddCommand {
+				ctx.SymbolTable.AddVar("result", "", compiler.KindWord, 0)
+				return &AddCommand{
+					param1IsVar: false,
+					param1Value: 200,
+					param2IsVar: false,
+					param2Value: 100,
+					destVarName: "result",
+					destVarKind: compiler.KindWord,
+				}
+			},
+			wantLines: []string{
+				"\tlda #$2c",
+				"\tsta result",
+				"\tlda #$01",
+				"\tsta result+1",
+			},
+		},
+		{
+			name: "byte + byte -> byte",
+			setup: func(ctx *compiler.CompilerContext) *AddCommand {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+				ctx.SymbolTable.AddVar("result", "", compiler.KindByte, 0)
+				return &AddCommand{
+					param1IsVar:   true,
+					param1VarName: "a",
+					param1VarKind: compiler.KindByte,
+					param2IsVar:   true,
+					param2VarName: "b",
+					param2VarKind: compiler.KindByte,
+					destVarName:   "result",
+					destVarKind:   compiler.KindByte,
+				}
+			},
+			wantLines: []string{
+				"\tclc",
+				"\tlda a",
+				"\tadc b",
+				"\tsta result",
+			},
+		},
+		{
+			name: "word + word -> word",
+			setup: func(ctx *compiler.CompilerContext) *AddCommand {
+				ctx.SymbolTable.AddVar("x", "", compiler.KindWord, 1000)
+				ctx.SymbolTable.AddVar("y", "", compiler.KindWord, 2000)
+				ctx.SymbolTable.AddVar("z", "", compiler.KindWord, 0)
+				return &AddCommand{
+					param1IsVar:   true,
+					param1VarName: "x",
+					param1VarKind: compiler.KindWord,
+					param2IsVar:   true,
+					param2VarName: "y",
+					param2VarKind: compiler.KindWord,
+					destVarName:   "z",
+					destVarKind:   compiler.KindWord,
+				}
+			},
+			wantLines: []string{
+				"\tclc",
+				"\tlda x",
+				"\tadc y",
+				"\tsta z",
+				"\tlda x+1",
+				"\tadc y+1",
+				"\tsta z+1",
+			},
+		},
+		{
+			name: "byte + word -> word",
+			setup: func(ctx *compiler.CompilerContext) *AddCommand {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("w", "", compiler.KindWord, 1000)
+				ctx.SymbolTable.AddVar("result", "", compiler.KindWord, 0)
+				return &AddCommand{
+					param1IsVar:   true,
+					param1VarName: "a",
+					param1VarKind: compiler.KindByte,
+					param2IsVar:   true,
+					param2VarName: "w",
+					param2VarKind: compiler.KindWord,
+					destVarName:   "result",
+					destVarKind:   compiler.KindWord,
+				}
+			},
+			wantLines: []string{
+				"\tclc",
+				"\tlda a",
+				"\tadc w",
+				"\tsta result",
+				"\tlda #0",
+				"\tadc w+1",
+				"\tsta result+1",
+			},
+		},
+		{
+			name: "literal + var -> byte",
+			setup: func(ctx *compiler.CompilerContext) *AddCommand {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("result", "", compiler.KindByte, 0)
+				return &AddCommand{
+					param1IsVar:   false,
+					param1Value:   5,
+					param2IsVar:   true,
+					param2VarName: "a",
+					param2VarKind: compiler.KindByte,
+					destVarName:   "result",
+					destVarKind:   compiler.KindByte,
+				}
+			},
+			wantLines: []string{
+				"\tclc",
+				"\tlda #$05",
+				"\tadc a",
+				"\tsta result",
+			},
+		},
+		{
+			name: "var + literal -> word",
+			setup: func(ctx *compiler.CompilerContext) *AddCommand {
+				ctx.SymbolTable.AddVar("w", "", compiler.KindWord, 1000)
+				ctx.SymbolTable.AddVar("result", "", compiler.KindWord, 0)
+				return &AddCommand{
+					param1IsVar:   true,
+					param1VarName: "w",
+					param1VarKind: compiler.KindWord,
+					param2IsVar:   false,
+					param2Value:   300,
+					destVarName:   "result",
+					destVarKind:   compiler.KindWord,
+				}
+			},
+			wantLines: []string{
+				"\tclc",
+				"\tlda w",
+				"\tadc #$2c",
+				"\tsta result",
+				"\tlda w+1",
+				"\tadc #$01",
+				"\tsta result+1",
+			},
+		},
+		{
+			name: "byte + byte -> word (promotion)",
+			setup: func(ctx *compiler.CompilerContext) *AddCommand {
+				ctx.SymbolTable.AddVar("a", "", compiler.KindByte, 10)
+				ctx.SymbolTable.AddVar("b", "", compiler.KindByte, 20)
+				ctx.SymbolTable.AddVar("result", "", compiler.KindWord, 0)
+				return &AddCommand{
+					param1IsVar:   true,
+					param1VarName: "a",
+					param1VarKind: compiler.KindByte,
+					param2IsVar:   true,
+					param2VarName: "b",
+					param2VarKind: compiler.KindByte,
+					destVarName:   "result",
+					destVarKind:   compiler.KindWord,
+				}
+			},
+			wantLines: []string{
+				"\tclc",
+				"\tlda a",
+				"\tadc b",
+				"\tsta result",
+				"\tlda #0",
+				"\tadc #0",
+				"\tsta result+1",
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ctx := compiler.NewCompilerContext(preproc.NewPragma())
+			cmd := tt.setup(ctx)
+
+			got, err := cmd.Generate(ctx)
+			if err != nil {
+				t.Fatalf("Generate() error = %v", err)
+			}
+
+			if len(got) != len(tt.wantLines) {
+				t.Errorf("Generate() got %d lines, want %d lines\nGot:\n%s\nWant:\n%s",
+					len(got), len(tt.wantLines),
+					strings.Join(got, "\n"), strings.Join(tt.wantLines, "\n"))
+				return
+			}
+
+			for i := range got {
+				if got[i] != tt.wantLines[i] {
+					t.Errorf("Line %d:\ngot:  %q\nwant: %q", i, got[i], tt.wantLines[i])
+				}
+			}
+		})
+	}
+}
+
+func TestAddCommand_Scopes(t *testing.T) {
+	ctx := compiler.NewCompilerContext(preproc.NewPragma())
+
+	// Global variables
+	ctx.SymbolTable.AddVar("globalA", "", compiler.KindByte, 10)
+	ctx.SymbolTable.AddVar("globalB", "", compiler.KindByte, 20)
+	ctx.SymbolTable.AddVar("result", "", compiler.KindByte, 0)
+
+	// Simulate function declaration to enter scope
+	line := preproc.Line{Text: "FUNC myFunc"}
+	_, err := ctx.FunctionHandler.HandleFuncDecl(line)
+	if err != nil {
+		t.Fatalf("HandleFuncDecl() error = %v", err)
+	}
+
+	// Function scope variables
+	ctx.SymbolTable.AddVar("localA", "myFunc", compiler.KindByte, 5)
+	ctx.SymbolTable.AddVar("localB", "myFunc", compiler.KindByte, 15)
+
+	// Test local variables
+	cmd := &AddCommand{}
+	cmdLine := preproc.Line{Text: "result = localA + localB"}
+	if err := cmd.Interpret(cmdLine, ctx); err != nil {
+		t.Fatalf("Interpret() error = %v", err)
+	}
+
+	if cmd.param1VarName != "myFunc_localA" {
+		t.Errorf("param1 = %q, want 'myFunc_localA'", cmd.param1VarName)
+	}
+	if cmd.param2VarName != "myFunc_localB" {
+		t.Errorf("param2 = %q, want 'myFunc_localB'", cmd.param2VarName)
+	}
+
+	// Test global variables while in function scope
+	cmd2 := &AddCommand{}
+	cmdLine2 := preproc.Line{Text: "result = globalA + globalB"}
+	if err := cmd2.Interpret(cmdLine2, ctx); err != nil {
+		t.Fatalf("Interpret() error = %v", err)
+	}
+
+	if cmd2.param1VarName != "globalA" {
+		t.Errorf("param1 = %q, want 'globalA'", cmd2.param1VarName)
+	}
+}
diff --git a/internal/compiler/compiler.go b/internal/compiler/compiler.go
index 4ddccbe..08ac382 100644
--- a/internal/compiler/compiler.go
+++ b/internal/compiler/compiler.go
@@ -68,6 +68,7 @@ func (c *Compiler) Compile(lines []preproc.Line) ([]string, error) {
 			return nil, fmt.Errorf("%s:%d: %w", line.Filename, line.LineNo, err)
 		}
 
+		codeOutput = append(codeOutput, fmt.Sprintf("; %s", line.Text))
 		codeOutput = append(codeOutput, asmLines...)
 	}
 
diff --git a/main.go b/main.go
index cd444ca..0a7646a 100644
--- a/main.go
+++ b/main.go
@@ -78,12 +78,7 @@ func registerCommands(comp *compiler.Compiler) {
 
 	comp.Registry().Register(&commands.ByteCommand{})
 	comp.Registry().Register(&commands.WordCommand{})
-
-	// TODO: Add more commands as they're implemented:
-	// comp.Registry().Register(&commands.LetCommand{})
-	// comp.Registry().Register(&commands.IfCommand{})
-	// comp.Registry().Register(&commands.WhileCommand{})
-	// etc.
+	comp.Registry().Register(&commands.AddCommand{})
 }
 
 func writeOutput(filename string, lines []string) error {