Wokwi different to real life: interrupts

[alto777]

This on Arduino forum

The inquirer and ppl who helped have explained this in terms of how a continuously present interrupt level is handled differently to real life

In real life, an instruction is executed before the interrupt is taken again.

In the wokwi, the ISR runs over and over with no sunshine on the interrupted code.

Or vice versa.

a7

[drf5n]

The AVR datasheet says it should advance and execute one instruction per interrupt:

One demo in Wokwi:

https://wokwi.com/projects/416353531338279937

versus another test in silicon:

https://forum.arduino.cc/t/physical-uno-and-wokwi-give-different-results-for-a-simple-test/1328159/34?u=davex

I expect that while pin2 is held at a low level, the code in Wokwi should make progress through the program at 1 instruction per serviced interrupt and still blink the pin13 LED.

I observe that while pin2 is held low, the program makes no progress and does not blink the pin13 LED.

My completely uneducated guess about the not-advancing in https://forum.arduino.cc/t/physical-uno-and-wokwi-give-different-results-for-a-simple-test/1328159/39?u=davex is that it might be something in the RETI op-code's -1 backstep:

https://github.com/wokwi/avr8js/blob/6ecba64687e10113ad697860b5f103746a8ef329/src/cpu/instruction.ts#L551-L562

  } else if (opcode === 0x9518) {
    /* RETI, 1001 0101 0001 1000 */
    const { pc22Bits } = cpu;
    const i = cpu.dataView.getUint16(93, true) + (pc22Bits ? 3 : 2);
    cpu.dataView.setUint16(93, i, true);
    cpu.pc = (cpu.data[i - 1] << 8) + cpu.data[i] - 1;
    if (pc22Bits) {
      cpu.pc |= cpu.data[i - 2] << 16;
    }
    cpu.cycles += pc22Bits ? 4 : 3;
    cpu.data[95] |= 0x80; // Enable interrupts
  } else if ((opcode & 0xf000) === 0xc000) {