serial console in early boot

I'm using qemu like this: qemu -nographic -kernel /home/john/dev-linux/arch/x86/boot/bzImage \ -append "console=ttyS0" I noticed that I can do a printk in arch/x86/kernel/head64.c right after the call to clear_bss (line 162 in x86_64_start_kernel). I see the output of the printk on the serial console. This seems to be before any uart/console has been initialized. I'm curious how the printk can make it to the serial console. Anyone know why printk is sending output to the serial console this early in the boot process? I would expect that the serial line wouldn't be ready and that qemu would behave like actual hardware.