\newcount\x \newcount\nx

\def\mysqrt#1{\x=#1 \nx=\x \isqrt{#1}
  \big\lfloor\sqrt{#1}\big\rfloor=\number\x}

\def\isqrt#1{\nx=#1
  \divide\nx by\x \advance\nx by\x \divide\nx by2
  \ifnum\x=\nx \else\x=\nx \isqrt{#1}\fi}