Every boot on an x86 machine iterates through the entire sequence of computing evolution in a few milliseconds:
Stage 1: 16-bit Real Mode, acts like an 8086 from ~1978. 500ns; about 15-30 instructions. Goal of this stage is to create a writeable stack+heap.
Stage 2: 32-bit Protected Mode, acts like a 386 from ~1985. 10s of millions of instructions; takes forever on DDR5 with memory training. Temporarily uses L2/L3 “cache-as-ram” to kickstart main RAM (Intel calls this No-Eviction Mode, NEM)—the same concept as an APU/cart start, using the smaller motor (L2/L3) to kickstart the main turbine (main RAM) and execute the training process.
Stage 3: 64-bit Long Mode, acts like AMD64 from ~2003. 100s of millions of instructions; enumerate PCIe devices, GPU, bootloader, etc.
Stage 4+: Secondary core wakeup. Every single core has to go through the same process, but by this point it’s faster because main RAM is warmed up.