diff --git a/Makefile b/Makefile
index f9917dad..ae0afe1c 100644
--- a/Makefile
+++ b/Makefile
@@ -107,7 +107,7 @@ smoketest-no-avr:
 	tinygo build -size short -o test.elf -target=pca10040            examples/serial
 	tinygo build -size short -o test.elf -target=pca10040            examples/test
 	# test all targets/boards
-	tinygo build             -o test.elf                             examples/blinky2 # TODO: re-enable -size flag with MachO support
+	tinygo build             -o test.wasm -tags=pca10040             examples/blinky2
 	tinygo build -size short -o test.elf -target=microbit            examples/echo
 	tinygo build -size short -o test.elf -target=nrf52840-mdk        examples/blinky1
 	tinygo build -size short -o test.elf -target=pca10031            examples/blinky1
diff --git a/src/machine/board_arduino.go b/src/machine/board_arduino.go
index d64a9de3..b3582b5a 100644
--- a/src/machine/board_arduino.go
+++ b/src/machine/board_arduino.go
@@ -1,4 +1,4 @@
-// +build avr,arduino
+// +build arduino
 
 package machine
 
diff --git a/src/machine/board_bluepill.go b/src/machine/board_bluepill.go
index b950c935..7ec73147 100644
--- a/src/machine/board_bluepill.go
+++ b/src/machine/board_bluepill.go
@@ -1,4 +1,4 @@
-// +build stm32,bluepill
+// +build bluepill
 
 package machine
 
diff --git a/src/machine/board_digispark.go b/src/machine/board_digispark.go
index 1fbd13a5..9a483d50 100644
--- a/src/machine/board_digispark.go
+++ b/src/machine/board_digispark.go
@@ -1,4 +1,4 @@
-// +build attiny85,digispark
+// +build digispark
 
 package machine
 
diff --git a/src/machine/board_microbit.go b/src/machine/board_microbit.go
index ca9b00c8..888c7c96 100644
--- a/src/machine/board_microbit.go
+++ b/src/machine/board_microbit.go
@@ -1,4 +1,4 @@
-// +build nrf51,microbit
+// +build microbit
 
 package machine
 
diff --git a/src/machine/board_pca10031.go b/src/machine/board_pca10031.go
index 9627918c..b2b37c69 100644
--- a/src/machine/board_pca10031.go
+++ b/src/machine/board_pca10031.go
@@ -1,4 +1,4 @@
-// +build nrf51,pca10031
+// +build pca10031
 
 // pca10031 is a nrf51 based dongle, intended for use in wireless applications.
 //
diff --git a/src/machine/board_pca10040.go b/src/machine/board_pca10040.go
index 7ac47582..f00e357b 100644
--- a/src/machine/board_pca10040.go
+++ b/src/machine/board_pca10040.go
@@ -1,4 +1,4 @@
-// +build nrf,pca10040
+// +build pca10040
 
 package machine
 
diff --git a/src/machine/board_stm32.go b/src/machine/board_stm32.go
new file mode 100644
index 00000000..02ab51a6
--- /dev/null
+++ b/src/machine/board_stm32.go
@@ -0,0 +1,16 @@
+// +build bluepill stm32f4disco
+
+package machine
+
+// Peripheral abstraction layer for the stm32.
+
+const (
+	portA Pin = iota * 16
+	portB
+	portC
+	portD
+	portE
+	portF
+	portG
+	portH
+)
diff --git a/src/machine/board_stm32f4disco.go b/src/machine/board_stm32f4disco.go
index 05150cab..36c173b2 100644
--- a/src/machine/board_stm32f4disco.go
+++ b/src/machine/board_stm32f4disco.go
@@ -1,4 +1,4 @@
-// +build stm32,stm32f4disco
+// +build stm32f4disco
 
 package machine
 
diff --git a/src/machine/i2c.go b/src/machine/i2c.go
index c3dfe24c..bd6c1dc2 100644
--- a/src/machine/i2c.go
+++ b/src/machine/i2c.go
@@ -1,4 +1,4 @@
-// +build avr nrf sam stm32f103xx
+// +build !stm32f4disco
 
 package machine
 
diff --git a/src/machine/machine_dummy.go b/src/machine/machine_dummy.go
deleted file mode 100644
index 11a4c9a8..00000000
--- a/src/machine/machine_dummy.go
+++ /dev/null
@@ -1,40 +0,0 @@
-// +build !avr,!nrf,!sam,!stm32
-
-package machine
-
-// Dummy machine package, filled with no-ops.
-
-type PinMode uint8
-
-const (
-	PinInput PinMode = iota
-	PinOutput
-)
-
-// Fake LED numbers, for testing.
-const (
-	LED  Pin = LED1
-	LED1 Pin = 0
-	LED2 Pin = 0
-	LED3 Pin = 0
-	LED4 Pin = 0
-)
-
-// Fake button numbers, for testing.
-const (
-	BUTTON  Pin = BUTTON1
-	BUTTON1 Pin = 0
-	BUTTON2 Pin = 0
-	BUTTON3 Pin = 0
-	BUTTON4 Pin = 0
-)
-
-func (p Pin) Configure(config PinConfig) {
-}
-
-func (p Pin) Set(value bool) {
-}
-
-func (p Pin) Get() bool {
-	return false
-}
diff --git a/src/machine/machine_generic.go b/src/machine/machine_generic.go
new file mode 100644
index 00000000..ccc3e2b7
--- /dev/null
+++ b/src/machine/machine_generic.go
@@ -0,0 +1,184 @@
+// +build !avr,!nrf,!sam,!stm32
+
+package machine
+
+// Dummy machine package that calls out to external functions.
+
+var (
+	SPI0  = SPI{0}
+	I2C0  = I2C{0}
+	UART0 = UART{0}
+)
+
+type PinMode uint8
+
+const (
+	PinInput PinMode = iota
+	PinOutput
+	PinInputPullup
+	PinInputPulldown
+)
+
+func (p Pin) Configure(config PinConfig) {
+	gpioConfigure(p, config)
+}
+
+func (p Pin) Set(value bool) {
+	gpioSet(p, value)
+}
+
+func (p Pin) Get() bool {
+	return gpioGet(p)
+}
+
+//go:export __tinygo_gpio_configure
+func gpioConfigure(pin Pin, config PinConfig)
+
+//go:export __tinygo_gpio_set
+func gpioSet(pin Pin, value bool)
+
+//go:export __tinygo_gpio_get
+func gpioGet(pin Pin) bool
+
+type SPI struct {
+	Bus uint8
+}
+
+type SPIConfig struct {
+	Frequency uint32
+	SCK       Pin
+	MOSI      Pin
+	MISO      Pin
+	Mode      uint8
+}
+
+func (spi SPI) Configure(config SPIConfig) {
+	spiConfigure(spi.Bus, config.SCK, config.MOSI, config.MISO)
+}
+
+// Transfer writes/reads a single byte using the SPI interface.
+func (spi SPI) Transfer(w byte) (byte, error) {
+	return spiTransfer(spi.Bus, w), nil
+}
+
+//go:export __tinygo_spi_configure
+func spiConfigure(bus uint8, sck Pin, mosi Pin, miso Pin)
+
+//go:export __tinygo_spi_transfer
+func spiTransfer(bus uint8, w uint8) uint8
+
+// InitADC enables support for ADC peripherals.
+func InitADC() {
+	// Nothing to do here.
+}
+
+// Configure configures an ADC pin to be able to be used to read data.
+func (adc ADC) Configure() {
+}
+
+// Get reads the current analog value from this ADC peripheral.
+func (adc ADC) Get() uint16 {
+	return adcRead(adc.Pin)
+}
+
+//go:export __tinygo_adc_read
+func adcRead(pin Pin) uint16
+
+// InitPWM enables support for PWM peripherals.
+func InitPWM() {
+	// Nothing to do here.
+}
+
+// Configure configures a PWM pin for output.
+func (pwm PWM) Configure() {
+}
+
+// Set turns on the duty cycle for a PWM pin using the provided value.
+func (pwm PWM) Set(value uint16) {
+	pwmSet(pwm.Pin, value)
+}
+
+//go:export __tinygo_pwm_set
+func pwmSet(pin Pin, value uint16)
+
+// I2C is a generic implementation of the Inter-IC communication protocol.
+type I2C struct {
+	Bus uint8
+}
+
+// I2CConfig is used to store config info for I2C.
+type I2CConfig struct {
+	Frequency uint32
+	SCL       Pin
+	SDA       Pin
+}
+
+// Configure is intended to setup the I2C interface.
+func (i2c I2C) Configure(config I2CConfig) {
+	i2cConfigure(i2c.Bus, config.SCL, config.SDA)
+}
+
+// Tx does a single I2C transaction at the specified address.
+func (i2c I2C) Tx(addr uint16, w, r []byte) error {
+	i2cTransfer(i2c.Bus, &w[0], len(w), &r[0], len(r))
+	// TODO: do something with the returned error code.
+	return nil
+}
+
+//go:export __tinygo_i2c_configure
+func i2cConfigure(bus uint8, scl Pin, sda Pin)
+
+//go:export __tinygo_i2c_transfer
+func i2cTransfer(bus uint8, w *byte, wlen int, r *byte, rlen int) int
+
+type UART struct {
+	Bus uint8
+}
+
+type UARTConfig struct {
+	BaudRate uint32
+	TX       Pin
+	RX       Pin
+}
+
+// Configure the UART.
+func (uart UART) Configure(config UARTConfig) {
+	uartConfigure(uart.Bus, config.TX, config.RX)
+}
+
+// Read from the UART.
+func (uart UART) Read(data []byte) (n int, err error) {
+	return uartRead(uart.Bus, &data[0], len(data)), nil
+}
+
+// Write to the UART.
+func (uart UART) Write(data []byte) (n int, err error) {
+	return uartWrite(uart.Bus, &data[0], len(data)), nil
+}
+
+// Buffered returns the number of bytes currently stored in the RX buffer.
+func (uart UART) Buffered() int {
+	return 0
+}
+
+// ReadByte reads a single byte from the UART.
+func (uart UART) ReadByte() (byte, error) {
+	var b byte
+	uartRead(uart.Bus, &b, 1)
+	return b, nil
+}
+
+// WriteByte writes a single byte to the UART.
+func (uart UART) WriteByte(b byte) error {
+	uartWrite(uart.Bus, &b, 1)
+	return nil
+}
+
+//go:export __tinygo_uart_configure
+func uartConfigure(bus uint8, tx Pin, rx Pin)
+
+//go:export __tinygo_uart_read
+func uartRead(bus uint8, buf *byte, bufLen int) int
+
+//go:export __tinygo_uart_write
+func uartWrite(bus uint8, buf *byte, bufLen int) int
diff --git a/src/machine/machine_stm32.go b/src/machine/machine_stm32.go
index 4e28d03d..89fed477 100644
--- a/src/machine/machine_stm32.go
+++ b/src/machine/machine_stm32.go
@@ -5,14 +5,3 @@ package machine
 // Peripheral abstraction layer for the stm32.
 
 type PinMode uint8
-
-const (
-	portA Pin = iota * 16
-	portB
-	portC
-	portD
-	portE
-	portF
-	portG
-	portH
-)
diff --git a/src/machine/spi.go b/src/machine/spi.go
index 83f16d5e..a3e6235f 100644
--- a/src/machine/spi.go
+++ b/src/machine/spi.go
@@ -1,4 +1,4 @@
-// +build nrf stm32f103xx atsamd21g18a
+// +build !stm32f407,!avr
 
 package machine