diff --git a/examples/hello/hello.q b/examples/hello/hello.q
index d3e8c5c..60f65e4 100644
--- a/examples/hello/hello.q
+++ b/examples/hello/hello.q
@@ -3,9 +3,8 @@ main() {
 }
 
 hello() {
-	one := 1
-	write := one
-	stdout := one
+	write := 1
+	stdout := 1
 	address := 4194305
 	length := 3
 
diff --git a/src/build/Function.go b/src/build/Function.go
index 3ba0646..efdac96 100644
--- a/src/build/Function.go
+++ b/src/build/Function.go
@@ -142,13 +142,18 @@ func (f *Function) CompileVariableDefinition(expr *expression.Expression) error
 		return err
 	}
 
-	// All expressions are returned to the memory pool.
-	// To avoid losing variable values, we will detach it from the expression.
-	expr.RemoveChild(value)
+	reg, exists := f.CPU.FindFree()
+
+	if !exists {
+		panic("no free registers")
+	}
+
+	f.ExpressionToRegister(value, reg)
+	f.CPU.Use(reg)
 
 	f.Variables[name] = &Variable{
-		Name:  name,
-		Value: value,
+		Name:     name,
+		Register: reg,
 	}
 
 	return nil
@@ -197,7 +202,8 @@ func (f *Function) TokenToRegister(t token.Token, register cpu.Register) error {
 			return errors.New(&errors.UnknownIdentifier{Name: name}, f.File, t.Position)
 		}
 
-		return f.ExpressionToRegister(variable.Value, register)
+		f.Assembler.MoveRegisterRegister(register, variable.Register)
+		return nil
 
 	case token.Number:
 		value := t.Text()
diff --git a/src/build/Variable.go b/src/build/Variable.go
index a37b2b5..c8189f1 100644
--- a/src/build/Variable.go
+++ b/src/build/Variable.go
@@ -1,9 +1,11 @@
 package build
 
-import "git.akyoto.dev/cli/q/src/build/expression"
+import (
+	"git.akyoto.dev/cli/q/src/build/cpu"
+)
 
 // Variable represents a variable in a function.
 type Variable struct {
-	Name  string
-	Value *expression.Expression
+	Name     string
+	Register cpu.Register
 }
diff --git a/src/build/arch/x64/ModRM.go b/src/build/arch/x64/ModRM.go
new file mode 100644
index 0000000..b6ab57f
--- /dev/null
+++ b/src/build/arch/x64/ModRM.go
@@ -0,0 +1,9 @@
+package x64
+
+// ModRM is used to generate a ModRM suffix.
+// - mod: 2 bits
+// - reg: 3 bits
+// - rm:  3 bits
+func ModRM(mod byte, reg byte, rm byte) byte {
+	return (mod << 6) | (reg << 3) | rm
+}
diff --git a/src/build/arch/x64/ModRM_test.go b/src/build/arch/x64/ModRM_test.go
new file mode 100644
index 0000000..e9d470f
--- /dev/null
+++ b/src/build/arch/x64/ModRM_test.go
@@ -0,0 +1,34 @@
+package x64_test
+
+import (
+	"testing"
+
+	"git.akyoto.dev/cli/q/src/build/arch/x64"
+	"git.akyoto.dev/go/assert"
+)
+
+func TestModRM(t *testing.T) {
+	testData := []struct{ mod, reg, rm, expected byte }{
+		{0b_00, 0b_111, 0b_000, 0b_00_111_000},
+		{0b_00, 0b_110, 0b_001, 0b_00_110_001},
+		{0b_00, 0b_101, 0b_010, 0b_00_101_010},
+		{0b_00, 0b_100, 0b_011, 0b_00_100_011},
+		{0b_00, 0b_011, 0b_100, 0b_00_011_100},
+		{0b_00, 0b_010, 0b_101, 0b_00_010_101},
+		{0b_00, 0b_001, 0b_110, 0b_00_001_110},
+		{0b_00, 0b_000, 0b_111, 0b_00_000_111},
+		{0b_11, 0b_111, 0b_000, 0b_11_111_000},
+		{0b_11, 0b_110, 0b_001, 0b_11_110_001},
+		{0b_11, 0b_101, 0b_010, 0b_11_101_010},
+		{0b_11, 0b_100, 0b_011, 0b_11_100_011},
+		{0b_11, 0b_011, 0b_100, 0b_11_011_100},
+		{0b_11, 0b_010, 0b_101, 0b_11_010_101},
+		{0b_11, 0b_001, 0b_110, 0b_11_001_110},
+		{0b_11, 0b_000, 0b_111, 0b_11_000_111},
+	}
+
+	for _, test := range testData {
+		modRM := x64.ModRM(test.mod, test.reg, test.rm)
+		assert.Equal(t, modRM, test.expected)
+	}
+}
diff --git a/src/build/arch/x64/Move.go b/src/build/arch/x64/Move.go
index 6d9fd57..63ffaf0 100644
--- a/src/build/arch/x64/Move.go
+++ b/src/build/arch/x64/Move.go
@@ -1,13 +1,43 @@
 package x64
 
+import "git.akyoto.dev/cli/q/src/build/cpu"
+
 // MoveRegNum32 moves a 32 bit integer into the given register.
-func MoveRegNum32(code []byte, register uint8, number uint32) []byte {
+func MoveRegNum32(code []byte, destination cpu.Register, number uint32) []byte {
+	if destination >= 8 {
+		code = append(code, REX(0, 0, 0, 1))
+		destination -= 8
+	}
+
 	return append(
 		code,
-		0xb8+register,
+		0xb8+byte(destination),
 		byte(number),
 		byte(number>>8),
 		byte(number>>16),
 		byte(number>>24),
 	)
 }
+
+// MoveRegReg64 moves a register value into another register.
+func MoveRegReg64(code []byte, destination cpu.Register, source cpu.Register) []byte {
+	r := byte(0) // Extension to the "reg" field in ModRM.
+	b := byte(0) // Extension to the "rm" field in ModRM or the SIB base (r8 up to r15 use this).
+
+	if source >= 8 {
+		r = 1
+		source -= 8
+	}
+
+	if destination >= 8 {
+		b = 1
+		destination -= 8
+	}
+
+	return append(
+		code,
+		REX(1, r, 0, b),
+		0x89,
+		ModRM(0b11, byte(source), byte(destination)),
+	)
+}
diff --git a/src/build/arch/x64/REX.go b/src/build/arch/x64/REX.go
new file mode 100644
index 0000000..ba1fa1a
--- /dev/null
+++ b/src/build/arch/x64/REX.go
@@ -0,0 +1,7 @@
+package x64
+
+// REX is used to generate a REX prefix.
+// w, r, x and b can only be set to either 0 or 1.
+func REX(w, r, x, b byte) byte {
+	return 0b_0100_0000 | (w << 3) | (r << 2) | (x << 1) | b
+}
diff --git a/src/build/arch/x64/REX_test.go b/src/build/arch/x64/REX_test.go
new file mode 100644
index 0000000..e48e3c5
--- /dev/null
+++ b/src/build/arch/x64/REX_test.go
@@ -0,0 +1,34 @@
+package x64_test
+
+import (
+	"testing"
+
+	"git.akyoto.dev/cli/q/src/build/arch/x64"
+	"git.akyoto.dev/go/assert"
+)
+
+func TestREX(t *testing.T) {
+	testData := []struct{ w, r, x, b, expected byte }{
+		{0, 0, 0, 0, 0b_0100_0000},
+		{0, 0, 0, 1, 0b_0100_0001},
+		{0, 0, 1, 0, 0b_0100_0010},
+		{0, 0, 1, 1, 0b_0100_0011},
+		{0, 1, 0, 0, 0b_0100_0100},
+		{0, 1, 0, 1, 0b_0100_0101},
+		{0, 1, 1, 0, 0b_0100_0110},
+		{0, 1, 1, 1, 0b_0100_0111},
+		{1, 0, 0, 0, 0b_0100_1000},
+		{1, 0, 0, 1, 0b_0100_1001},
+		{1, 0, 1, 0, 0b_0100_1010},
+		{1, 0, 1, 1, 0b_0100_1011},
+		{1, 1, 0, 0, 0b_0100_1100},
+		{1, 1, 0, 1, 0b_0100_1101},
+		{1, 1, 1, 0, 0b_0100_1110},
+		{1, 1, 1, 1, 0b_0100_1111},
+	}
+
+	for _, test := range testData {
+		rex := x64.REX(test.w, test.r, test.x, test.b)
+		assert.Equal(t, rex, test.expected)
+	}
+}
diff --git a/src/build/asm/Assembler.go b/src/build/asm/Assembler.go
index 5af72a6..f159bc4 100644
--- a/src/build/asm/Assembler.go
+++ b/src/build/asm/Assembler.go
@@ -28,8 +28,13 @@ func (a *Assembler) Finalize() ([]byte, []byte) {
 	for _, x := range a.Instructions {
 		switch x.Mnemonic {
 		case MOVE:
-			regNum := x.Data.(*RegisterNumber)
-			code = x64.MoveRegNum32(code, uint8(regNum.Register), uint32(regNum.Number))
+			switch operands := x.Data.(type) {
+			case *RegisterNumber:
+				code = x64.MoveRegNum32(code, operands.Register, uint32(operands.Number))
+
+			case *RegisterRegister:
+				code = x64.MoveRegReg64(code, operands.Destination, operands.Source)
+			}
 
 		case RETURN:
 			code = x64.Return(code)
diff --git a/src/build/asm/Instructions.go b/src/build/asm/Instructions.go
index aa74d0e..82aebdb 100644
--- a/src/build/asm/Instructions.go
+++ b/src/build/asm/Instructions.go
@@ -13,6 +13,17 @@ func (a *Assembler) MoveRegisterNumber(reg cpu.Register, number uint64) {
 	})
 }
 
+// MoveRegisterRegister moves a register value into another register.
+func (a *Assembler) MoveRegisterRegister(destination cpu.Register, source cpu.Register) {
+	a.Instructions = append(a.Instructions, Instruction{
+		Mnemonic: MOVE,
+		Data: &RegisterRegister{
+			Destination: destination,
+			Source:      source,
+		},
+	})
+}
+
 // Label adds a label at the current position.
 func (a *Assembler) Label(name string) {
 	a.Instructions = append(a.Instructions, Instruction{
diff --git a/src/build/asm/RegisterRegister.go b/src/build/asm/RegisterRegister.go
new file mode 100644
index 0000000..5acb084
--- /dev/null
+++ b/src/build/asm/RegisterRegister.go
@@ -0,0 +1,18 @@
+package asm
+
+import (
+	"fmt"
+
+	"git.akyoto.dev/cli/q/src/build/cpu"
+)
+
+// RegisterRegister operates with two registers.
+type RegisterRegister struct {
+	Destination cpu.Register
+	Source      cpu.Register
+}
+
+// String returns a human readable version.
+func (data *RegisterRegister) String() string {
+	return fmt.Sprintf("%s, %s", data.Destination, data.Source)
+}