diff --git a/artiq/devices/runtime.py b/artiq/devices/runtime.py
index 597938931..eb043be08 100644
--- a/artiq/devices/runtime.py
+++ b/artiq/devices/runtime.py
@@ -8,16 +8,24 @@ lt.initialize_all()
 _syscalls = {
 	"rpc":			"i+:i",
 	"gpio_set":		"ii:n",
-	"rtio_set":		"iii:n",
+	"rtio_set":		"Iii:n",
 	"rtio_sync":	"i:n",
 	"dds_program":	"ii:n",
 }
 
+_chr_to_type = {
+	"n": lambda: lc.Type.void(),
+	"i": lambda: lc.Type.int(32),
+	"I": lambda: lc.Type.int(64)
+}
+
+_chr_to_value = {
+	"n": lambda: ir_values.VNone(),
+	"i": lambda: ir_values.VInt(),
+	"I": lambda: ir_values.VInt(64)
+}
+
 def _str_to_functype(s):
-	_chr_to_type = {
-		"n": lc.Type.void,
-		"i": lc.Type.int
-	}
 	assert(s[-2] == ":")
 	type_ret = _chr_to_type[s[-1]]()
 
@@ -42,10 +50,6 @@ class LinkInterface:
 			self.module.add_function(func_type, "__syscall_"+func_name)
 
 	def syscall(self, syscall_name, args, builder):
-		_chr_to_value = {
-			"n": ir_values.VNone,
-			"i": ir_values.VInt
-		}
 		r = _chr_to_value[_syscalls[syscall_name][-1]]()
 		if builder is not None:
 			args = [arg.llvm_value for arg in args]
diff --git a/soc/runtime/main.c b/soc/runtime/main.c
index 2714c1255..01d9a59ac 100644
--- a/soc/runtime/main.c
+++ b/soc/runtime/main.c
@@ -140,7 +140,7 @@ static void gpio_set(int channel, int value)
 	leds_out_write(csr_value);
 }
 
-static void rtio_set(int timestamp, int channel, int value)
+static void rtio_set(long long int timestamp, int channel, int value)
 {
 	rtio_chan_sel_write(channel);
 	rtio_o_timestamp_write(timestamp);