use crystal calling convention, documented in readme

lib/vm/README.markdown

@@ -7,3 +7,30 @@ Apart from shuffeling things around from one layer to the other, it keeps track
 provides the stack glue. All the stuff a compiler would usually do.
 
 Also all syscalls are abstracted as functions.
+
+The Crystal Convention 
+----------------------
+
+Since we're not in c, we use the regsters more suitably for our job:
+
+- return register is _not_ the same as passing registers
+- we pin one more register (ala stack/fp) for type information (this is used for returns too)
+- one line (8 registers) can be used by a function (caller saved?)
+- rest are scratch and may not hold values during call
+
+For Arm this works out as:
+- 0 type word (for the line)
+- 1-6 argument passing + workspace
+- 7 return value
+
+This means syscalls (using 7 for call number and 0 for return) must shuffle a little, but there's space to do it.
+Some more detail:
+
+1 - returning in the same register as passing makes that one register a special case, which i want to avoid. shuffling it gets tricky and involves 2 moves for what?
+As i see it the benefitd of reusing the same register are one more argument register (not needed) and easy chaining of calls, which doen't really happen so much.
+On the plus side, not using the same register makes saving and restoring registers easy (to implement and understand!). 
+An easy to understand policy is worth gold, as register mistakes are HARD to debug and not what i want to spend my time with just now. So that's settled.
+
+2 - Tagging integers like MRI/BB is a hack which does not extend to other types, such as floats. So we don't use that and instead carry type information externally to the value. This is a burden off course, but then so is tagging. 
+The convention (to make it easier) is to handle data in lines (8 words) and have one of them carry the type info for the other 7. This is also the object layout and so we reuse that code on the stack.
+

lib/vm/call_site.rb

@@ -16,7 +16,7 @@ module Vm
         if arg.is_a?(IntegerConstant) or arg.is_a?(StringConstant)
           function.args[index].load into , arg
         else
-          function.args[index].move( into, arg ) if arg.register != index
+          function.args[index].move( into, arg ) if arg.register != args[index].register
         end
       end
     end

lib/vm/function.rb

@@ -34,9 +34,9 @@ module Vm
       args.each_with_index do |arg , i|
         if arg.is_a?(Value)
           @args[i] = arg
-          raise "arg in non std register #{arg.inspect}" unless i == arg.register
+          raise "arg in non std register #{arg.inspect}" unless (i+1) == arg.register
         else
-          @args[i] = arg.new(i)
+          @args[i] = arg.new(i+1)
         end
       end
       set_return return_type
@@ -53,9 +53,9 @@ module Vm
     def set_return type_or_value
       @return_type = type_or_value || Vm::Integer 
       if @return_type.is_a?(Value)
-        raise "return in non std register #{@return_type.inspect}" unless 0 == @return_type.register
+        raise "return in non std register #{@return_type.inspect}" unless 7 == @return_type.register
       else
-        @return_type = @return_type.new(0)
+        @return_type = @return_type.new(7)
       end
     end
     def arity
@@ -64,22 +64,21 @@ module Vm
 
     def new_local type = Vm::Integer
       register = args.length + @locals.length
-      l = type.new(register)
+      l = type.new(register + 1) # one for the type register 0, TODO add type as arg0 implicitly
+      raise "the red flag #{inspect}" if l.register > 6
       @locals << l
       l
     end
 
     def save_locals context , into
       save = args.collect{|a| a.register } + @locals.collect{|l| l.register}
-      save.delete_at(0)
-      into.push save
+      into.push(save) unless save.empty?
     end
 
     def restore_locals context , into
       #TODO assumes allocation in order, as the pop must be get regs in ascending order (also push)
       restore = args.collect{|a| a.register } + @locals.collect{|l| l.register}
-      restore.delete_at(0)
-      into.pop restore
+      into.pop(restore) unless restore.empty?
     end
 
     def new_block name