define and use syscall_XX registers

rather than use hardcoded r0 etc use syscall_X
change the syscalls and interpreter to use them
later use platform to map from syscall_X to actually used register (like r0 in arm)

lib/risc/instructions/syscall.rb

@@ -19,6 +19,13 @@ module Risc
     def to_s
       class_source name
     end
+  end
 
+  # return the nth register that a sycall needs
+  # these map to different physical registers in the Platform
+  # first arg is (running 1..) integer, second (optional) type
+  def self.syscall_reg( number , type = nil)
+    type = :Integer unless type
+    RegisterValue.new("syscall_#{number}".to_sym , type)
   end
 end

lib/risc/interpreter.rb

@@ -257,12 +257,12 @@ module Risc
       else
         raise "un-implemented syscall #{name}"
       end
-      set_register( :r0 , ret_value ) # syscalls return into r0 , usually some int
+      set_register( :syscall_1 , ret_value ) # syscalls return into syscall_1
       true
     end
 
     def handle_putstring
-      str = get_register( :r1 ) # should test length, ie r2
+      str = get_register( :"syscall_1" ) # should test length, ie r2
       case str
       when Symbol
         @stdout << str.to_s