Constant kernel_config::memory::KERNEL_HEAP_START

pub const KERNEL_HEAP_START: usize = _; // 18_446_742_424_442_109_952usize

The higher-half heap gets the 512GB address range starting at the 509th P4 entry, which is the slot right below the recursive P4 entry (510). Actual value: 0o177777_775_000_000_000_0000, or 0xFFFF_FE80_0000_0000