Your first program: main() + while(1)
Every Embedded C program = #include, int main(void), setup once, then a while(1) loop that never ends.
Your chip's program has NO END. 🔁
Every Embedded C program has the same 4 parts:
1️⃣ #include <avr/io.h> → the chip's register names
2️⃣ int main(void) → the chip starts here
3️⃣ setup lines → run ONCE (D13 = output, LED ON)
4️⃣ while (1) { } → runs FOREVER
An empty while(1) is just ONE machine instruction that jumps to itself (rjmp .-2). At 16 MHz that's 8 million laps every second.
What if main() returns? There's no operating system to go back to. So avr-gcc adds its own ending: cli (interrupts off) + an endless loop. The chip can't stop.
And Arduino? Same shape. setup() and loop() live inside a hidden main(): setup() once, then for(;;) { loop(); }.
// EC L02 - the shape of every Embedded C program
#include <avr/io.h> // chip's register names
int main(void) {
// setup: runs ONCE
DDRB |= (1 << DDB5); // D13 = output
PORTB |= (1 << PORTB5); // LED ON
while (1) {
// loop: runs FOREVER
}
// main() never returns - nothing to return to
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