# build the 4 level page tables for one user page
# everything here only writes small entries we construct, so its f64 safe
const PTE_ADDR = 0x000FFFFFFFFFF000

fn pte_index(virtual_address, level) -> num do
  return (virtual_address shr (12 + 9 * level)) band 0x1ff
end

fn next_table(table, index) -> num do
  const slot = table + index * 8
  const entry = mem_read(slot, 8)
  if (entry band 1) != 0 do
    return entry band PTE_ADDR
  end

  const frame = frame_alloc()
  mem_zero(frame, 4096)
  mem_write(slot, frame bor 7, 8) # present | write | user
  return frame
end

fn as_map(pml4, virtual_address, physical_address, flags) do
  const pdpt = next_table(pml4, pte_index(virtual_address, 3))
  const pd = next_table(pdpt, pte_index(virtual_address, 2))
  const pt = next_table(pd, pte_index(virtual_address, 1))

  mem_write(pt + pte_index(virtual_address, 0) * 8, (physical_address band PTE_ADDR) bor flags bor 1, 8)
  invlpg(virtual_address)
end