\begin{Verbatim}[commandchars=\\\{\},codes={\catcode`\$=3\catcode`\^=7\catcode`\_=8\relax}]
i := 0;
M = number of page frames in main memory buffer

repeat
    read M blocks of the relation
    sort M blocks in memory
    write sorted data to file R\PYGZus{}i
until end of relation

read first block of each R\PYGZus{}i into memory
repeat
    choose first (in sort order) from pages
    write tuple to output
    remove tuple from buffer
    if any buffer R\PYGZus{}i empty and not eof(R\PYGZus{}i)
        read next block from R\PYGZus{}i into memory
    until all pages empty
\end{Verbatim}