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/slot_machine/macro/macro.rb

@@ -20,10 +20,9 @@ module SlotMachine
     # Assumes int return value and extracts the fixnum for process exit code
     def self.exit_sequence(builder)
       save_message( builder )
+      unix_return = Risc.syscall_reg(1)
       builder.build do
-        message << message[:return_value]
-        int = message.reduce_int(false) #hack, noo type check
-        message << int
+        unix_return << message[:return_value].reduce_int(false) #hack, noo type check
         add_code Risc::Syscall.new("emit_syscall(exit)", :exit )
       end
     end
@@ -46,7 +45,7 @@ module SlotMachine
       r8 = Risc::RegisterValue.new( :saved_message , :Message).set_compiler(builder.compiler)
       tmp = Risc::RegisterValue.new( :integer_tmp , :Integer).set_compiler(builder.compiler)
       builder.build do
-        tmp << message
+        tmp << Risc.syscall_reg(1)
         message << r8
         message[:return_value][Parfait::Integer.integer_index] << tmp
       end

lib/slot_machine/macro/putstring.rb

@@ -3,9 +3,13 @@ module SlotMachine
     def to_risc(compiler)
       builder = compiler.builder(compiler.source)
       integer = builder.prepare_int_return # makes integer_tmp variable as return
+      word = Risc.syscall_reg(1).set_compiler(compiler)
+      length = Risc.syscall_reg(2).set_compiler(compiler)
       builder.build do
-        word = message[:receiver].to_reg.known_type(:Word)
-        integer << word[:char_length]
+        string = message[:receiver].to_reg.known_type(:Word)
+        integer << string[:char_length]
+        word << string     # into right
+        length << integer  # registers
       end
       SlotMachine::Macro.emit_syscall( builder , :putstring )
       compiler