What does Arduino do every 1ms?

Using Arduino Microcontrollers
1

i tried to see how fast an Arduino processes a loop:

void setup() 
{
  // put your setup code here, to run once:
  pinMode(2, OUTPUT); 
}

void loop() 
{
  // put your main code here, to run repeatedly:
  static uint8_t n=B00000000;

    PORTD = n;
    n = ~n;
}

I get about 667 kHz (0.74 microseconds) pulse but
I see every 1 ms there is a gap (about 6 microseconds) in the pulse

Is this the interrupt that updates millis() ? does it do something more?
If it's for millis, why don't i see an similar gap for micros()?

2

I'm not sure of the answer although someone here will help, but your query lead me to wonder what would be the result if you used the bitwise XOR (^) rather than the bitwise NOT. It could toggle only the one bit you are interested in...

3

Timer 0 overflow interrupt. It's the interrupt that drives micros / millis.

4

I knew it drives millis(). Does it also drive micros()? I got the impression that when your code reads micros(), one of the hardware timer/counter registers is read directly.

I also wonder what happens when loop() terminates. I suspect it checks for received serial characters and buffers them. Maybe the same for i2c and SPI. What else happens?

5

I also wonder what happens when loop() terminates. I suspect it checks for received serial characters and buffers them. Maybe the same for i2c and SPI.

No.
You can LOOK, you know...
Arduino Core Source Code

6

PaulRB:
Does it also drive micros()?

Correct.

PaulRB:
I got the impression that when your code reads micros(), one of the hardware timer/counter registers is read directly.

Also correct.