diff --git a/artiq/devices/runtime.py b/artiq/devices/runtime.py
index 0cdf14910..05e0e567c 100644
--- a/artiq/devices/runtime.py
+++ b/artiq/devices/runtime.py
@@ -4,20 +4,46 @@ from llvm import target as lt
 lt.initialize_all()
 
 _syscalls = [
-	("print_int",	lc.Type.void(),		[lc.Type.int()]),
-	("gpio_set",	lc.Type.void(),		[lc.Type.int(), lc.Type.int()])
+	("rpc",			"i+:i"),
+	("gpio_set",	"ii:v")
 ]
 
+def _str_to_functype(s):
+	_chr_to_type = {
+		"v": lc.Type.void,
+		"i": lc.Type.int
+	}
+	assert(s[-2] == ":")
+	type_ret = _chr_to_type[s[-1]]()
+
+	var_arg_fixcount = None
+	type_args = []
+	for n, c in enumerate(s[:-2]):
+		if c == "+":
+			type_args.append(lc.Type.int())
+			var_arg_fixcount = n
+		else:
+			type_args.append(_chr_to_type[c]())
+	return var_arg_fixcount, lc.Type.function(type_ret, type_args, var_arg=var_arg_fixcount is not None)
+
 class Environment:
 	def set_module(self, module):
-		for func_name, func_type_ret, func_type_args in _syscalls:
-			function_type = lc.Type.function(func_type_ret, func_type_args)
-			module.add_function(function_type, "__syscall_"+func_name)
+		self.var_arg_fixcount = dict()
+		for func_name, func_type_str in _syscalls:
+			var_arg_fixcount, func_type = _str_to_functype(func_type_str)
+			if var_arg_fixcount is not None:
+				self.var_arg_fixcount[func_name] = var_arg_fixcount
+			module.add_function(func_type, "__syscall_"+func_name)
 
 		self.module = module
 
 	def emit_syscall(self, builder, syscall_name, args):
-		builder.call(self.module.get_function_named("__syscall_"+syscall_name), args)
+		if syscall_name in self.var_arg_fixcount:
+			fixcount = self.var_arg_fixcount[syscall_name]
+			args = args[:fixcount] \
+				+ [lc.Constant.int(lc.Type.int(), len(args) - fixcount)] \
+				+ args[fixcount:]
+		return builder.call(self.module.get_function_named("__syscall_"+syscall_name), args)
 
 	def emit_object(self):
 		tm = lt.TargetMachine.new(triple="or1k", cpu="generic")