Hello, I am looking at the AtomicSimpleCPU code in src/cpu/simple for x86 ISA. I am trying to understand the following code snippet. Whenever this condition is true for a given PC, it does NOT follow the regular fetch from the instruction cache and then decode. This results in a macroop called Microcode_ROM, which is not an x86 macroop that has a sequence of uops (can be seen in the O3 CPU). Example: Instruction is: Microcode_ROM : ldst t0, HS:[t0 + t6 + 0x20] (This is taken from the O3 logs running the same workload by checking the same PC in the Debug logs).

I am not sure what this macroop is and what the benefit is from the code below. Does this mean this instruction has no x86 machine instruction binary? If it has an x86 instruction binary, how to get it?

if (isRomMicroPC(pc_state.microPC())) {
    t_info.stayAtPC = false;
    curStaticInst = decoder->fetchRomMicroop(
        pc_state.microPC(), curMacroStaticInst);
}

Thanks,

My answer

Hi,

The ROM contains predefined micro-code routines for purposes such as apic interrupt handler (arch/x86/isa/insts/romutil.py).

If you are simulating a full system then basically the interrupt is triggered by 8254 timer (dev/x86/i8254.py) and when it is triggered the core fetches instructions from the ROM to handle interrupts.

For example, for the timer interrupt the linux updates process cgroup times for, do soft irqs, and reschedule, etc.

Hope this helps.