import "io"

let twotimes(x) be
{ let sptr, prev;
  if x = 0 then resultis 0;
  prev := twotimes(x-1);
  resultis prev+2 }

let start() be
{ let pdir = 6 << 11, ptablow = 7 << 11, ptabhigh = 8 << 11;
  let a, b;
  for i = 0 to 2047 do
  { pdir ! i := 0;
    ptablow ! i := 0;
    ptabhigh ! i := 0 }

  pdir ! 0 := ptablow bitor 1;
  pdir ! 0x1FF := ptabhigh bitor 1;
  ptablow ! 0 := 1;
  ptabhigh ! 0x7FF := 0x7FFFF801;
  out("pdir = %x\n", pdir);
  out("pdir!0 = %x\n", pdir ! 0);
  out("pdir!1FF = %x\n", pdir ! 0x1FF);

  outs("Dangerous place\n");
  assembly
  { load   r1, [<pdir>]
    setsr  r1, $pdbr
    getsr  r1, $flags
    sbit   r1, $vm
    flagsj r1, pc }

  outs("still alive\n");
  a := 500;  // this works when a is small, but not when it is more than about 350.
             // the stack frames are 6 words long, and we have only allowed one
             // page of 2048 words for the stack.
  b := twotimes(a);
  out("twotimes(%d) = %d\n", a, b) }