diff --git a/src/Makefile b/src/Makefile
index fdf975a..8a62758 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -12,6 +12,7 @@ LDFLAGS += $(OPT_FLAGS)
 
 SRC =			\
 	fake6502.c	\
+	acia6850.c	\
 	cdcacm.c	\
 	main.c		\
 
diff --git a/src/acia6850.c b/src/acia6850.c
new file mode 100644
index 0000000..957ad40
--- /dev/null
+++ b/src/acia6850.c
@@ -0,0 +1,60 @@
+#include <libopencm3/usb/usbd.h>
+#include <libopencm3/usb/cdc.h>
+
+#include "acia6850.h"
+#include "cdcacm.h"
+
+extern usbd_device *usbd_dev;
+
+// Motorola MC6850 Asynchronous Communications Interface Adapter (ACIA) emulation
+
+// http://www.cpcwiki.eu/index.php/6850_ACIA_chip
+// http://www.cpcwiki.eu/imgs/3/3f/MC6850.pdf
+//
+
+#define ACIAControl 0
+#define ACIAStatus 1
+#define ACIAData 2
+
+// "MC6850 Data Register (R/W) Data can be read when Status.Bit0=1, and written when Status.Bit1=1."
+#define RDRF 1
+#define TDRE 2
+
+uint8_t read6850(uint16_t address) {
+	switch(address & 3) {
+		case ACIAControl:
+			break;
+		case ACIAStatus:
+            return TDRE; // writable
+			break;
+		case ACIAData:
+            // TODO: read buffer from serial port
+			break;
+		default:
+            break;
+	}
+
+	return 0xff;
+}
+
+void write6850(uint16_t address, uint8_t value) {
+	switch(address & 3) {
+		case ACIAControl:
+            // TODO: decode baudrate, mode, break control, interrupt
+			break;
+		case ACIAStatus:
+            (void) value;
+			break;
+		case ACIAData: {
+            static char buf[1];
+            buf[0] = value;
+            cdcacm_send_chunked_blocking(buf, sizeof(buf), usbd_dev);
+			break;
+        }
+		default:
+            break;
+	}
+}
+
+
+
diff --git a/src/acia6850.h b/src/acia6850.h
new file mode 100644
index 0000000..e7abdc5
--- /dev/null
+++ b/src/acia6850.h
@@ -0,0 +1,4 @@
+#include <stdint.h>
+
+uint8_t read6850(uint16_t address);
+void write6850(uint16_t address, uint8_t value);
diff --git a/src/cdcacm.c b/src/cdcacm.c
index 8dbf58a..b5491ee 100644
--- a/src/cdcacm.c
+++ b/src/cdcacm.c
@@ -174,16 +174,16 @@ static int cdcacm_control_request(usbd_device *dev,
 	return 0;
 }
 
-static void send_chunked_blocking(char *buf, int len, usbd_device *dev, int endpoint, int max_packet_length) {
+void cdcacm_send_chunked_blocking(char *buf, int len, usbd_device *dev) {
 	uint16_t bytes_written = 0;
 	uint16_t total_bytes_written = 0;
 	uint16_t bytes_remaining = len;
 
 	do {
 		uint16_t this_length = bytes_remaining;
-		if (this_length > max_packet_length) this_length = max_packet_length;
+		if (this_length > CDCACM_PACKET_SIZE) this_length = CDCACM_PACKET_SIZE;
 
-		bytes_written = usbd_ep_write_packet(dev, endpoint, buf + total_bytes_written, this_length);
+		bytes_written = usbd_ep_write_packet(dev, CDCACM_UART_ENDPOINT, buf + total_bytes_written, this_length);
 		bytes_remaining -= bytes_written;
 		total_bytes_written += bytes_written;
 	} while (bytes_remaining > 0);
@@ -232,7 +232,7 @@ static void usbuart_usb_out_cb(usbd_device *dev, uint8_t ep)
 		}
 	}
 
-	send_chunked_blocking(reply_buf, j, dev, CDCACM_UART_ENDPOINT, CDCACM_PACKET_SIZE);
+	cdcacm_send_chunked_blocking(reply_buf, j, dev);
 }
 
 static void usbuart_usb_in_cb(usbd_device *dev, uint8_t ep)
diff --git a/src/cdcacm.h b/src/cdcacm.h
index 22183b4..d85008e 100644
--- a/src/cdcacm.h
+++ b/src/cdcacm.h
@@ -7,3 +7,4 @@ extern const struct usb_interface_descriptor uart_data_iface[];
 extern const struct usb_iface_assoc_descriptor uart_assoc;
 
 void cdcacm_set_config(usbd_device *dev, uint16_t wValue);
+void cdcacm_send_chunked_blocking(char *buf, int len, usbd_device *dev);
diff --git a/src/main.c b/src/main.c
index de72e79..1320560 100644
--- a/src/main.c
+++ b/src/main.c
@@ -26,15 +26,15 @@
 #include <libopencm3/stm32/rcc.h>
 #include <libopencm3/stm32/gpio.h>
 #include <libopencm3/usb/usbd.h>
-#include <libopencm3/usb/hid.h>
 #include <libopencm3/usb/cdc.h>
 
 #include "fake6502.h"
+#include "acia6850.h"
 #include "rom.h"
 #include "cdcacm.h"
 #include "version.h"
 
-static usbd_device *usbd_dev;
+usbd_device *usbd_dev;
 
 const struct usb_device_descriptor dev_descr = {
 	.bLength = USB_DT_DEVICE_SIZE,
@@ -83,8 +83,10 @@ static const char *usb_strings[] = {
 	"Pill 6502 UART Port",
 };
 
+static bool running = false;
 void sys_tick_handler(void)
 {
+	if (!running) return;
 	step6502();
 	gpio_toggle(GPIOC, GPIO13);
 }
@@ -100,6 +102,9 @@ char *process_serial_command(char *buf, int len) {
 
 	if (buf[0] == 'v') {
 		return "Pill 6502 version " FIRMWARE_VERSION;
+	} else if (buf[0] == 'r') {
+		running = !running;
+		return running ? "resumed" : "paused";
 	} else if (buf[0] == 't') {
 		static char buf[64];
 		snprintf(buf, sizeof(buf), "%ld ticks\r\n%ld instructions", clockticks6502, instructions);
@@ -138,7 +143,6 @@ uint8_t usbd_control_buffer[128];
 
 // 6502 processor memory, 16KB (< 20KB)
 uint8_t ram[0x4000];
-// TODO: serial interface, 0xa000-0xbfff
 uint8_t read6502(uint16_t address) {
 	// RAM
 	if (address < sizeof(ram)) {
@@ -152,13 +156,25 @@ uint8_t read6502(uint16_t address) {
 		return rom[address - 0xc000];
 	}
 
+	// ACIA
+	if (address >= 0xa000 && address <= 0xbfff) {
+		return read6850(address);
+	}
+
 	return 0xff;
 }
 
 void write6502(uint16_t address, uint8_t value) {
+	// RAM
 	if (address < sizeof(ram)) {
 		ram[address] = value;
 	}
+
+	// ACIA
+	if (address >= 0xa000 && address <= 0xbfff) {
+		write6850(address, value);
+	}
+
 }
 
 int main(void)