diff --git a/src/device/arm/arm.go b/src/device/arm/arm.go
new file mode 100644
index 00000000..5374a5d3
--- /dev/null
+++ b/src/device/arm/arm.go
@@ -0,0 +1,59 @@
+// CMSIS abstraction functions.
+//
+// Original copyright:
+//
+//     Copyright (c) 2009 - 2015 ARM LIMITED
+//
+//     All rights reserved.
+//     Redistribution and use in source and binary forms, with or without
+//     modification, are permitted provided that the following conditions are met:
+//     - Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     - Redistributions in binary form must reproduce the above copyright
+//       notice, this list of conditions and the following disclaimer in the
+//       documentation and/or other materials provided with the distribution.
+//     - Neither the name of ARM nor the names of its contributors may be used
+//       to endorse or promote products derived from this software without
+//       specific prior written permission.
+//
+//     THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+//     AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+//     IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+//     ARE DISCLAIMED. IN NO EVENT SHALL COPYRIGHT HOLDERS AND CONTRIBUTORS BE
+//     LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+//     CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+//     SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+//     INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+//     CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+//     ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+//     POSSIBILITY OF SUCH DAMAGE.
+package arm
+
+type __reg uint32
+type RegValue = __reg
+
+const (
+	SCS_BASE  = 0xE000E000
+	NVIC_BASE = SCS_BASE + 0x0100
+)
+
+// Nested Vectored Interrupt Controller (NVIC).
+var NVIC = struct {
+	ISER [8]__reg
+}{
+	ISER: [8]__reg{
+		NVIC_BASE + 0x000,
+		NVIC_BASE + 0x004,
+		NVIC_BASE + 0x008,
+		NVIC_BASE + 0x00C,
+		NVIC_BASE + 0x010,
+		NVIC_BASE + 0x014,
+		NVIC_BASE + 0x018,
+		NVIC_BASE + 0x01C,
+	},
+}
+
+// Enable the given interrupt number.
+func EnableIRQ(irq uint32) {
+	NVIC.ISER[irq >> 5] = 1 << (irq & 0x1F)
+}
diff --git a/src/runtime/runtime_nrf.c b/src/runtime/runtime_nrf.c
index b9b48123..ab8bb3fe 100644
--- a/src/runtime/runtime_nrf.c
+++ b/src/runtime/runtime_nrf.c
@@ -6,30 +6,6 @@
 #include "runtime_nrf.h"
 #include <string.h>
 
-void uart_init(uint32_t pin_tx) {
-	NRF_UART0->ENABLE        = UART_ENABLE_ENABLE_Enabled;
-	NRF_UART0->BAUDRATE      = UART_BAUDRATE_BAUDRATE_Baud115200;
-	NRF_UART0->TASKS_STARTTX = 1;
-	NRF_UART0->PSELTXD       = 6;
-}
-
-void uart_send(uint8_t c) {
-	NRF_UART0->TXD = c;
-	while (NRF_UART0->EVENTS_TXDRDY != 1) {}
-	NRF_UART0->EVENTS_TXDRDY = 0;
-}
-
-void rtc_init() {
-	// Make sure the low-frequency clock is running.
-	NRF_CLOCK->TASKS_LFCLKSTART = 1;
-	while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0) {}
-	NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
-
-	NRF_RTC0->TASKS_START = 1;
-	NVIC_SetPriority(RTC0_IRQn, 3);
-	NVIC_EnableIRQ(RTC0_IRQn);
-}
-
 static volatile bool rtc_wakeup;
 
 void rtc_sleep(uint32_t ticks) {
diff --git a/src/runtime/runtime_nrf.go b/src/runtime/runtime_nrf.go
index cb3d1550..8278da05 100644
--- a/src/runtime/runtime_nrf.go
+++ b/src/runtime/runtime_nrf.go
@@ -6,15 +6,43 @@ package runtime
 // #include "runtime_nrf.h"
 import "C"
 
-func init() {
-	C.uart_init(6) // pin_tx = 6, for NRF52840-DK
-	C.rtc_init()
-}
+import (
+	"device/arm"
+	"device/nrf"
+)
 
 const Microsecond = 1
 
+func init() {
+	initUART()
+	initLFCLK()
+	initRTC()
+}
+
+func initUART() {
+	nrf.UART0.ENABLE        = nrf.UART0_ENABLE_ENABLE_Enabled
+	nrf.UART0.BAUDRATE      = nrf.UART0_BAUDRATE_BAUDRATE_Baud115200
+	nrf.UART0.TASKS_STARTTX = 1
+	nrf.UART0.PSELTXD       = 6 // pin 6 for NRF52840-DK
+}
+
+func initLFCLK() {
+	nrf.CLOCK.LFCLKSRC = nrf.CLOCK_LFCLKSTAT_SRC_Xtal
+	nrf.CLOCK.TASKS_LFCLKSTART = 1
+	for nrf.CLOCK.EVENTS_LFCLKSTARTED == 0 {}
+	nrf.CLOCK.EVENTS_LFCLKSTARTED = 0
+}
+
+func initRTC() {
+	nrf.RTC0.TASKS_START = 1
+	// TODO: set priority
+	arm.EnableIRQ(nrf.IRQ_RTC0)
+}
+
 func putchar(c byte) {
-	C.uart_send(C.uint8_t(c))
+	nrf.UART0.TXD = nrf.RegValue(c)
+	for nrf.UART0.EVENTS_TXDRDY == 0 {}
+	nrf.UART0.EVENTS_TXDRDY = 0
 }
 
 func Sleep(d Duration) {
diff --git a/src/runtime/runtime_nrf.h b/src/runtime/runtime_nrf.h
index 945dcc7c..1ae8572e 100644
--- a/src/runtime/runtime_nrf.h
+++ b/src/runtime/runtime_nrf.h
@@ -2,10 +2,5 @@
 #pragma once
 
 #include <stdint.h>
-#include <stdbool.h>
 
-void uart_init(uint32_t pin_tx);
-void uart_send(uint8_t c);
-
-void rtc_init();
 void rtc_sleep(uint32_t ticks);