diff --git a/lib/mjit/x86_64/assembler.rb b/lib/mjit/x86_64/assembler.rb
new file mode 100644
index 0000000000..14f414b33b
--- /dev/null
+++ b/lib/mjit/x86_64/assembler.rb
@@ -0,0 +1,35 @@
+class RubyVM::MJIT::Assembler
+  ByteWriter = RubyVM::MJIT::CType::Immediate.parse('char')
+
+  def initialize
+    @bytes = []
+  end
+
+  def compile(compiler)
+    RubyVM::MJIT::C.mjit_mark_writable
+    write_bytes(compiler.write_addr, @bytes)
+    RubyVM::MJIT::C.mjit_mark_executable
+
+    compiler.write_pos += @bytes.size
+    @bytes.clear
+  end
+
+  def mov(_reg, val)
+    @bytes.push(0xb8, val, 0x00, 0x00, 0x00)
+  end
+
+  def ret
+    @bytes.push(0xc3)
+  end
+
+  private
+
+  def write_bytes(addr, bytes)
+    writer = ByteWriter.new(addr)
+    # If you pack bytes containing \x00, Ruby fails to recognize bytes after \x00.
+    # So writing byte by byte to avoid hitting that situation.
+    bytes.each_with_index do |byte, index|
+      writer[index] = byte
+    end
+  end
+end
diff --git a/lib/ruby_vm/mjit/compiler.rb b/lib/ruby_vm/mjit/compiler.rb
index cb2930246a..c42f75bf4d 100644
--- a/lib/ruby_vm/mjit/compiler.rb
+++ b/lib/ruby_vm/mjit/compiler.rb
@@ -1,6 +1,14 @@
+require 'mjit/x86_64/assembler'
+
 class RubyVM::MJIT::Compiler
-  C = RubyVM::MJIT.const_get(:C, false)
-  INSNS = RubyVM::MJIT.const_get(:INSNS, false)
+  # MJIT internals
+  Assembler = RubyVM::MJIT::Assembler
+  C = RubyVM::MJIT::C
+
+  # Ruby constants
+  Qundef = Fiddle::Qundef
+
+  attr_accessor :write_pos
 
   # @param mem_block [Integer] JIT buffer address
   def initialize(mem_block)
@@ -10,6 +18,16 @@ class RubyVM::MJIT::Compiler
 
   # @param iseq [RubyVM::MJIT::CPointer::Struct]
   def compile(iseq)
-    # TODO: implement
+    return if iseq.body.location.label == '<main>'
+
+    iseq.body.jit_func = write_addr
+    asm = Assembler.new
+    asm.mov(:eax, Qundef)
+    asm.ret
+    asm.compile(self)
+  end
+
+  def write_addr
+    @mem_block + @write_pos
   end
 end
diff --git a/mjit.c b/mjit.c
index d8e7aae152..8226404084 100644
--- a/mjit.c
+++ b/mjit.c
@@ -371,8 +371,8 @@ mjit_compile(FILE *f, const rb_iseq_t *iseq, const char *funcname, int id)
 // New stuff from here
 //
 
-// TODO: Make it configurable
-#define MJIT_CODE_SIZE 16 * 1024 * 1024
+// JIT buffer
+uint8_t *rb_mjit_mem_block = NULL;
 
 void
 rb_mjit_compile(const rb_iseq_t *iseq)
@@ -393,7 +393,7 @@ mjit_init(const struct mjit_options *opts)
     mjit_opts = *opts;
 
     extern uint8_t* rb_yjit_reserve_addr_space(uint32_t mem_size);
-    uint8_t *mem_block = rb_yjit_reserve_addr_space(MJIT_CODE_SIZE);
+    rb_mjit_mem_block = rb_yjit_reserve_addr_space(MJIT_CODE_SIZE);
 
     // MJIT doesn't support miniruby, but it might reach here by MJIT_FORCE_ENABLE.
     rb_mMJIT = rb_const_get(rb_cRubyVM, rb_intern("MJIT"));
@@ -404,7 +404,7 @@ mjit_init(const struct mjit_options *opts)
     }
     rb_mMJITC = rb_const_get(rb_mMJIT, rb_intern("C"));
     VALUE rb_cMJITCompiler = rb_const_get(rb_mMJIT, rb_intern("Compiler"));
-    rb_MJITCompiler = rb_funcall(rb_cMJITCompiler, rb_intern("new"), 1, SIZET2NUM((size_t)mem_block));
+    rb_MJITCompiler = rb_funcall(rb_cMJITCompiler, rb_intern("new"), 1, SIZET2NUM((size_t)rb_mjit_mem_block));
     rb_cMJITIseqPtr = rb_funcall(rb_mMJITC, rb_intern("rb_iseq_t"), 0);
 
     mjit_call_p = true;
diff --git a/mjit.rb b/mjit.rb
index 717ab832a4..7bcb108233 100644
--- a/mjit.rb
+++ b/mjit.rb
@@ -23,15 +23,7 @@ if RubyVM::MJIT.enabled?
     return # miniruby doesn't support MJIT
   end
 
-  # forward declaration for ruby_vm/mjit/compiler
-  RubyVM::MJIT::C = Object.new # :nodoc:
-
-  require 'ruby_vm/mjit/c_type'
-  require 'ruby_vm/mjit/instruction'
-  require 'ruby_vm/mjit/compiler'
-  require 'ruby_vm/mjit/hooks'
-
-  module RubyVM::MJIT
-    private_constant(*constants)
-  end
+  RubyVM::MJIT::C = Object.new # forward declaration for mjit/compiler
+  require 'mjit/c_type'
+  require 'mjit/compiler'
 end
diff --git a/mjit_c.h b/mjit_c.h
index cc4040c9df..8bab98bec9 100644
--- a/mjit_c.h
+++ b/mjit_c.h
@@ -94,4 +94,14 @@ struct compile_status {
     struct inlined_call_context inline_context;
 };
 
+//================================================================================
+//
+// New stuff from here
+//
+
+// TODO: Make it configurable
+#define MJIT_CODE_SIZE 16 * 1024 * 1024
+
+extern uint8_t *rb_mjit_mem_block;
+
 #endif /* MJIT_C_H */
diff --git a/mjit_c.rb b/mjit_c.rb
index 905de6139a..7e352c61d7 100644
--- a/mjit_c.rb
+++ b/mjit_c.rb
@@ -5,6 +5,30 @@ module RubyVM::MJIT # :nodoc: all
   # This `class << C` section is for calling C functions. For importing variables
   # or macros as is, please consider using tool/mjit/bindgen.rb instead.
   class << C
+    #========================================================================================
+    #
+    # New stuff
+    #
+    def mjit_mark_writable
+      Primitive.cstmt! %{
+        extern bool rb_yjit_mark_writable(void *mem_block, uint32_t mem_size);
+        rb_yjit_mark_writable(rb_mjit_mem_block, MJIT_CODE_SIZE);
+        return Qnil;
+      }
+    end
+
+    def mjit_mark_executable
+      Primitive.cstmt! %{
+        extern bool rb_yjit_mark_executable(void *mem_block, uint32_t mem_size);
+        rb_yjit_mark_executable(rb_mjit_mem_block, MJIT_CODE_SIZE);
+        return Qnil;
+      }
+    end
+
+    #========================================================================================
+    #
+    # Old stuff
+    #
     def rb_hash_values(cdhash_addr)
       Primitive.cexpr! 'rb_hash_values((VALUE)NUM2PTR(cdhash_addr))'
     end