\input ptb-utils

\_checkloaded{math}

\newcount\_mathcountA   \newcount\_mathcountB

\def\newnumber#1{%
    \_xp\def\csname num@#1\endcsname{{0}}%
    \_xp\def\csname num@#1@exp\endcsname{0}%
}

\def\__setnumber#1#2#3#4#5{%
    \let\_next=\_gobbletilnul%
    \ifx\relax#2%
    \else\ifx\relax#3%
        \_xp\edef\csname num@#1\endcsname{\csname num@#1\endcsname{#2}}%
    \else\ifx\relax#4%
        \_xp\edef\csname num@#1\endcsname{\csname num@#1\endcsname{#3#2}}%
    \else\ifx\relax#5%
        \_xp\edef\csname num@#1\endcsname{\csname num@#1\endcsname{#4#3#2}}%
    \else
        \_xp\edef\csname num@#1\endcsname{\csname num@#1\endcsname{#5#4#3#2}}%
        \def\_next{\__setnumber{#1}}%
    \fi\fi\fi\fi%
    \_next%
}

\def\_setnumber#1#2.#3.#4\_nul{%
    \reverse\_regA{#2#3}%
    \def\_regB{\__setnumber{#1}}%
    \_xp\_regB\_regA\relax\relax\relax\relax\_nul%
}

\def\setnumber#1#2{%
    \_xp\def\csname num@#1\endcsname{}%
    \_setnumber{#1}#2..\_nul%
}

\newnumber{test}
\setnumber{test}{1234567890.69420}

\say{\meaning\num@test}

\bye