diff --git a/lib/asm/arm_assembler.rb b/lib/asm/arm_assembler.rb
index db950a52..50cd2589 100644
--- a/lib/asm/arm_assembler.rb
+++ b/lib/asm/arm_assembler.rb
@@ -25,7 +25,7 @@ module Asm
       end
       attr_reader  :values , :position 
 
-      def instruction(name, *args)
+      def instruction(clazz,name, *args)
         opcode = name.to_s
         arg_nodes = []
 
@@ -43,28 +43,35 @@ module Asm
           end
         }
 
-        add_value Asm::Instruction.new(opcode , arg_nodes)
+        add_value clazz.new(opcode , arg_nodes)
+      end
+      
+      
+      def self.define_instruction(inst , clazz )
+        define_method(inst) do |*args|
+          instruction clazz , inst.to_sym, *args
+        end
+        define_method(inst+'s') do |*args|
+          instruction clazz , (inst+'s').to_sym, *args
+        end
+        %w(al eq ne cs mi hi cc pl ls vc lt le ge gt vs).each do |cond_suffix|
+          define_method(inst+cond_suffix) do |*args|
+            instruction clazz , (inst+cond_suffix).to_sym, *args
+          end
+          define_method(inst+'s'+cond_suffix) do |*args|
+            instruction clazz , (inst+'s'+cond_suffix).to_sym, *args
+          end
+        end
+      end
+
+      ["push", "pop"].each do |inst|
+        define_instruction(inst , StackInstruction)
       end
 
       %w(adc add and bic eor orr rsb rsc sbc sub mov mvn cmn cmp teq tst b bl bx 
-        push pop swi str strb ldr ldrb 
-      ).each { |inst|
-        define_method(inst) { |*args|
-          instruction inst.to_sym, *args
-        }
-        define_method(inst+'s') { |*args|
-          instruction (inst+'s').to_sym, *args
-        }
-        %w(al eq ne cs mi hi cc pl ls vc lt le ge gt vs
-        ).each { |cond_suffix|
-          define_method(inst+cond_suffix) { |*args|
-            instruction (inst+cond_suffix).to_sym, *args
-          }
-          define_method(inst+'s'+cond_suffix) { |*args|
-            instruction (inst+'s'+cond_suffix).to_sym, *args
-          }
-        }
-      }
+         swi str strb ldr ldrb  ).each do |inst|
+        define_instruction(inst , Instruction)
+      end
 
       def assemble_to_string
         #put the strings at the end of the assembled code.
diff --git a/lib/asm/instruction.rb b/lib/asm/instruction.rb
index b29219fa..4016abac 100644
--- a/lib/asm/instruction.rb
+++ b/lib/asm/instruction.rb
@@ -2,7 +2,6 @@ require "asm/assembly_error"
 require "asm/instruction_tools"
 require "asm/normal_builder"
 require "asm/memory_access_builder"
-require "asm/stack_builder"
 require "asm/label"
 
 module Asm
@@ -14,7 +13,7 @@ module Asm
       def initialize(opcode , args)
 
         opcode = opcode.downcase
-        @cond = :al
+        @cond = 0b1011
         if (opcode =~ /(#{COND_POSTFIXES})$/)
           @cond = $1.to_sym
           opcode = opcode[0..-3]
@@ -78,18 +77,6 @@ module Asm
           builder.rd = reg_ref(args[0])
           builder.build_operand args[1]
           builder.assemble io, as, self
-        when :push, :pop
-          # downward growing, decrement before memory access
-          # official ARM style stack as used by gas
-          if (opcode == :push)
-            builder = StackBuilder.new(1,0,1,0) 
-          else
-            builder = StackBuilder.new(0,1,1,1)
-          end
-          builder.cond = COND_CODES[@cond]
-          builder.rn = 13 # sp
-          builder.build_operand args
-          builder.assemble io, as
         when :b, :bl
           arg = args[0]
           if arg.is_a? Label
diff --git a/lib/asm/stack_builder.rb b/lib/asm/stack_builder.rb
deleted file mode 100644
index ac3156bf..00000000
--- a/lib/asm/stack_builder.rb
+++ /dev/null
@@ -1,45 +0,0 @@
-module Asm
-    # ADDRESSING MODE 4
-    class StackBuilder
-      include Asm::InstructionTools
-
-      def initialize(pre_post, up_down, write, store_load)
-        @cond = 0b1110
-        @inst_class = Asm::Instruction::OPC_STACK
-        @pre_post_index = 0
-        @up_down = 0
-        @s = 0
-        @write_base = 0
-        @store_load = 0
-        @rn = 0
-        @operand = 0
-        @pre_post_index = pre_post
-        @up_down = up_down
-        @write_base = write
-        @store_load = store_load
-      end
-      attr_accessor :cond, :inst_class, :pre_post_index, :up_down,
-                    :s, :write_base, :store_load, :rn, :operand
-
-      # Build representation for source value
-      def build_operand(arg)
-        if (arg.is_a?(Array))
-          @operand = 0
-          arg.each do |reg |
-            reg = reg_ref(reg)
-            @operand |= (1 << reg)
-          end
-        else
-          raise Asm::AssemblyError.new("invalid operand argument  #{arg.inspect}")
-        end
-      end
-
-      def assemble(io, as)
-        val = operand | (rn << 16) | (store_load << 16+4) | 
-              (write_base << 16+4+1) | (s << 16+4+1+1) | (up_down << 16+4+1+1+1) |
-              (pre_post_index << 16+4+1+1+1+1) | (inst_class << 16+4+1+1+1+1+2) |
-              (cond << 16+4+1+1+1+1+2+2)
-        io.write_uint32 val
-      end
-    end
-end
diff --git a/lib/asm/stack_instruction.rb b/lib/asm/stack_instruction.rb
new file mode 100644
index 00000000..9533a99c
--- /dev/null
+++ b/lib/asm/stack_instruction.rb
@@ -0,0 +1,65 @@
+require "asm/instruction"
+
+module Asm
+    # ADDRESSING MODE 4
+    class StackInstruction < Instruction
+      include Asm::InstructionTools
+
+      def initialize(opcode , args)
+        super(opcode,args)
+        @operand = 0
+        @cond = 0b1110
+        @inst_class = Asm::Instruction::OPC_STACK
+        @s = 0
+        @rn = 0
+        # downward growing, decrement before memory access
+        # official ARM style stack as used by gas
+        @write_base = 1
+        if (opcode == :push)
+          @pre_post_index = 1
+          @up_down = 0
+          @store_load = 0
+        else  #pop
+          @pre_post_index = 0
+          @up_down = 1
+          @store_load = 1
+        end
+      end
+      attr_accessor :cond, :inst_class, :pre_post_index, :up_down,
+                    :s, :write_base, :store_load, :rn, :operand
+                    
+      def assemble(io, as)
+        cond = @cond.is_a?(Symbol) ?  COND_CODES[@cond]   : @cond
+        rn = 13 # sp
+        build_operand args
+
+        #assemble of old
+        val = @operand
+        val |= (rn <<             16)
+        val |= (store_load <<     16+4) #20
+        val |= (write_base <<     16+4+ 1) 
+        val |= (s <<              16+4+ 1+1) 
+        val |= (up_down <<        16+4+ 1+1+1)
+        val |= (pre_post_index << 16+4+ 1+1+1+1)#24
+        val |= (inst_class <<     16+4+ 1+1+1+1 +2) 
+        val |= (cond <<           16+4+ 1+1+1+1 +2+2)
+        puts "#{self.inspect}"
+        io.write_uint32 val
+      end
+      
+      private 
+      # Build representation for source value
+      def build_operand(arg)
+        if (arg.is_a?(Array))
+          @operand = 0
+          arg.each do |reg |
+            reg = reg_ref(reg)
+            @operand |= (1 << reg)
+          end
+        else
+          raise Asm::AssemblyError.new("invalid operand argument  #{arg.inspect}")
+        end
+      end
+      
+    end
+end
diff --git a/lib/crystal.rb b/lib/crystal.rb
index e604a2b1..f792f601 100644
--- a/lib/crystal.rb
+++ b/lib/crystal.rb
@@ -1,2 +1,3 @@
+require "asm/stack_instruction"
 require "asm/arm_assembler"
 require "elf/object_writer"