Added x86 package

src/x86/ModRM.go

@@ -0,0 +1,19 @@
+package x86
+
+// AddressMode encodes the addressing mode.
+type AddressMode = byte
+
+const (
+	AddressMemory         = AddressMode(0b00)
+	AddressMemoryOffset8  = AddressMode(0b01)
+	AddressMemoryOffset32 = AddressMode(0b10)
+	AddressDirect         = AddressMode(0b11)
+)
+
+// ModRM is used to generate a mode-register-memory suffix.
+// - 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
+}