The bench

// Lesson 1, Arduino version.
// Blinks the UNO R3's LED on pin 13: 1 s on, 1 s off.
void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(1000);
  digitalWrite(13, LOW);
  delay(1000);
}

// Lesson 1, register version: the same blink, written
// directly to the port registers.
// Pin 13 is PORTB bit 5 (PB5) on the UNO R3.
void setup() {
  DDRB |= (1 << DDB5);       // pin 13 is an output
}

void loop() {
  PORTB |= (1 << PORTB5);    // pin 13 HIGH
  delay(1000);
  PORTB &= ~(1 << PORTB5);   // pin 13 LOW
  delay(1000);
}

// Lesson 2, Arduino version: read the knob on A0 and
// print the reading.
void setup() {
  Serial.begin(115200);
}

void loop() {
  int reading = analogRead(A0);  // 0 to 1023 on the UNO R3
  Serial.println(reading);
  delay(20);
}

// Lesson 3, Arduino version: the knob on A0 sets how
// bright the LED on pin 9 is.
void setup() {
  pinMode(9, OUTPUT);
}

void loop() {
  int reading = analogRead(A0);  // 0 to 1023
  analogWrite(9, reading / 4);   // 0 to 255
}

// Simulator: eight LEDs on pins 2 to 9, lit one at a time,
// back and forth.
const int FIRST = 2;
const int LAST = 9;

void setup() {
  for (int pin = FIRST; pin <= LAST; pin++) {
    pinMode(pin, OUTPUT);
  }
  digitalWrite(FIRST, HIGH);
}

void loop() {
  for (int pin = FIRST; pin < LAST; pin++) {
    step(pin, pin + 1);
  }
  for (int pin = LAST; pin > FIRST; pin--) {
    step(pin, pin - 1);
  }
}

// Moves the lit LED from one pin to the next.
void step(int from, int to) {
  digitalWrite(from, LOW);
  digitalWrite(to, HIGH);
  delay(70);
}

// Simulator: the LED on pin 9 breathes. Its brightness rises
// and falls smoothly, set by PWM.
void setup() {
  pinMode(9, OUTPUT);
}

void loop() {
  for (int i = 0; i < 256; i++) {
    analogWrite(9, curve(i));
    delay(8);
  }
}

// A triangle wave, squared: slow near dark, quick near full.
int curve(int i) {
  int t = i < 128 ? i : 255 - i;  // 0 up to 127, back to 0
  return (t * t) >> 6;            // 0 to 252
}

// Simulator: the bare chip. This main() replaces the Arduino
// core's, so nothing is set up: every register keeps its reset
// value until the register console writes to it.
int main(void) {
  for (;;) {
  }
}