diff --git a/src/build/arch/x64/ModRM.go b/src/build/arch/x64/ModRM.go
index ca6a327..bb6852e 100644
--- a/src/build/arch/x64/ModRM.go
+++ b/src/build/arch/x64/ModRM.go
@@ -1,16 +1,19 @@
 package x64
 
+// AddressMode encodes the addressing mode.
+type AddressMode = byte
+
 const (
-	AddressMemory         = byte(0b00)
-	AddressMemoryOffset8  = byte(0b01)
-	AddressMemoryOffset32 = byte(0b10)
-	AddressDirect         = byte(0b11)
+	AddressMemory         = AddressMode(0b00)
+	AddressMemoryOffset8  = AddressMode(0b01)
+	AddressMemoryOffset32 = AddressMode(0b10)
+	AddressDirect         = AddressMode(0b11)
 )
 
 // 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 {
+// - mod: 2 bits. The addressing mode.
+// - reg: 3 bits. Register reference or opcode extension.
+// - rm:  3 bits. Register operand.
+func ModRM(mod AddressMode, reg byte, rm byte) byte {
 	return (mod << 6) | (reg << 3) | rm
 }
diff --git a/src/build/arch/x64/SIB.go b/src/build/arch/x64/SIB.go
index 030ffac..438024d 100644
--- a/src/build/arch/x64/SIB.go
+++ b/src/build/arch/x64/SIB.go
@@ -1,16 +1,19 @@
 package x64
 
+// ScaleFactor encodes the scale factor.
+type ScaleFactor = byte
+
 const (
-	Scale1 = byte(0b00)
-	Scale2 = byte(0b01)
-	Scale4 = byte(0b10)
-	Scale8 = byte(0b11)
+	Scale1 = ScaleFactor(0b00)
+	Scale2 = ScaleFactor(0b01)
+	Scale4 = ScaleFactor(0b10)
+	Scale8 = ScaleFactor(0b11)
 )
 
 // SIB is used to generate an SIB byte.
-// - scale: 2 bits
-// - index: 3 bits
-// - base:  3 bits
-func SIB(scale byte, index byte, base byte) byte {
+// - scale: 2 bits. Multiplies the value of the index.
+// - index: 3 bits. Specifies the index register.
+// - base:  3 bits. Specifies the base register.
+func SIB(scale ScaleFactor, index byte, base byte) byte {
 	return (scale << 6) | (index << 3) | base
 }
diff --git a/src/build/arch/x64/encode.go b/src/build/arch/x64/encode.go
index 606f6e2..570076d 100644
--- a/src/build/arch/x64/encode.go
+++ b/src/build/arch/x64/encode.go
@@ -3,7 +3,7 @@ package x64
 import "git.akyoto.dev/cli/q/src/build/cpu"
 
 // encode is the core function that encodes an instruction.
-func encode(code []byte, mod byte, reg cpu.Register, rm cpu.Register, numBytes byte, opCodes ...byte) []byte {
+func encode(code []byte, mod AddressMode, reg cpu.Register, rm cpu.Register, numBytes byte, opCodes ...byte) []byte {
 	w := byte(0) // Indicates a 64-bit register.
 	r := byte(0) // Extension to the "reg" field in ModRM.
 	x := byte(0) // Extension to the SIB index field.
diff --git a/src/build/arch/x64/encodeNum.go b/src/build/arch/x64/encodeNum.go
index 15efe03..2053dfb 100644
--- a/src/build/arch/x64/encodeNum.go
+++ b/src/build/arch/x64/encodeNum.go
@@ -7,7 +7,7 @@ import (
 )
 
 // encodeNum encodes an instruction with up to two registers and a number parameter.
-func encodeNum(code []byte, mod byte, reg cpu.Register, rm cpu.Register, number int, opCode8 byte, opCode32 byte) []byte {
+func encodeNum(code []byte, mod AddressMode, reg cpu.Register, rm cpu.Register, number int, opCode8 byte, opCode32 byte) []byte {
 	if SizeOf(int64(number)) == 1 {
 		code = encode(code, mod, reg, rm, 8, opCode8)
 		return append(code, byte(number))