save and restore the right registers

lib/arm/constants.rb

@@ -59,7 +59,12 @@ module Arm
       if name.is_a? Vm::Word
         name = "r#{name.register}"
       end
-      REGISTERS[name.to_s]
+      if name.is_a? Fixnum
+        name = "r#{name}"
+      end
+      r = REGISTERS[name.to_s]
+      raise "no reg #{name}" if r == nil
+      r
     end
 
    def calculate_u8_with_rr(arg)

lib/arm/stack_instruction.rb

@@ -58,8 +58,8 @@ module Arm
       regs = @first
       if (regs.is_a?(Array))
         @operand = 0
-        regs.each do |reg |
-          next unless reg
+        regs.each_with_index do |reg , index|
+          raise "nil register in push, index #{index}" if reg == nil
           @operand |= (1 << reg_code(reg))
         end
       else