This is xnu-8019. See this file in:
pexpert/i386/pe_init.c                standard
pexpert/i386/pe_bootargs.c            standard
pexpert/i386/pe_identify_machine.c    standard
pexpert/i386/pe_kprintf.c             standard
pexpert/i386/pe_interrupt.c           standard
pexpert/i386/pe_serial.c              standard