Added missing X86::Encoder#write_register_byte method.

lib/ronin/asm/x86/encoder.rb

@@ -150,6 +150,23 @@ def write_modrm_byte(mode,reg,rm)
           write_byte((mode << 6) | ((reg & 0b111) << 3) | (rm & 0b111))
         end
 
+        #
+        # Writes a register byte to the output stream.
+        #
+        # @param [Register] register
+        # @param [Immediate, nil] payload
+        # @return [1]
+        #
+        def write_register_byte(register,payload=nil)
+          byte = register.number & 0b1111
+
+          if payload
+            byte |= (payload.value & 0b1111) << 4
+          end
+
+          write_byte(byte)
+        end
+
         def write_immediate(value,size) = write_int(value.to_i,size)
         def write_code_offset(value,size) = write_int(value.to_i,size)
         def write_data_offset(value,size) = write_int(value.to_i,size)

spec/x86/encoder_spec.rb

@@ -723,6 +723,40 @@
     end
   end
 
+  describe "#write_register_byte" do
+    let(:reg) { Ronin::ASM::X86::Registers::ECX }
+
+    it "must write a single byte with the register number encoded into the lower four bits" do
+      subject.write_register_byte(reg)
+
+      byte = output.string.getbyte(0)
+
+      expect(byte & 0b1111).to eq(reg.number)
+    end
+
+    context "when no payload argument is given" do
+      it "must not set the high four bits of the byte" do
+        subject.write_register_byte(reg)
+
+        byte = output.string.getbyte(0)
+
+        expect((byte & 0b11110000) >> 4).to eq(0)
+      end
+    end
+
+    context "when given the optional payload argument" do
+      let(:immediate) { Ronin::ASM::Immediate.new(3) }
+
+      it "must encode the Immediate's value into the high four bits of the byte" do
+        subject.write_register_byte(reg)
+
+        byte = output.string.getbyte(0)
+
+        expect(byte & 0b1111).to eq(reg.number)
+      end
+    end
+  end
+
   describe "#write_opcode" do
     let(:byte) { 0xc3 }