\documentclass[a4paper]{article}

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\usepackage{booktabs}
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\usepackage{mathtools}
\usepackage{sectsty}
\usepackage{tabularx}

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\begin{document}

\begin{center}
  \LARGE\sffamily\bfseries Basic AMD64 Linux Syscalls
  \vspace{1em}
\end{center}

\noindent
\begin{center}
  \begin{tabular}{>{\ttfamily}cl}
    \toprule
    \multicolumn{2}{>{\sffamily\bfseries}c}{Register Usage} \\
    \midrule
    rax & Syscall number \\
        & Return value \\
    rdi & First argument \\
    rsi & Second argument \\
    rdx & Third argument \\
    \bottomrule
  \end{tabular}
  \hspace{1em}
  \begin{tabular}{l>{$}r<{$}}
    \toprule
    \multicolumn{2}{>{\sffamily\bfseries}c}{Common open(2) Flags} \\
    \midrule
    Read-only & 0_h \\
    Write-only & 1_h \\
    Read/write & 2_h \\
    Create & 40_h \\
    Truncate & 200_h \\
    Append & 400_h \\
    \bottomrule
  \end{tabular}
\end{center}

\section{File I/O}

\begin{tabularx}{\textwidth}{p{4em}p{2em}>{\ttfamily}cX}
  \toprule
  read & 0 & rdi & File descriptor to read from \\
  & & rsi & Address of buffer \\
  & & rdx & Number of bytes to read \\
  \midrule
  write & 1 & rdi & File descriptor to write to \\
  & & rsi & Address of buffer \\
  & & rdx & Number of bytes to write \\
  \midrule
  open & 2 & rdi & Address of path \\
  & & rsi & Bitwise OR of flags \\
  & & rdx & Mode of file, if call creates a file \\
  \midrule
  close & 3 & rdi & File descriptor to close \\
  \bottomrule
\end{tabularx}

\section{Process Control}

\begin{tabularx}{\textwidth}{p{4em}p{2em}>{\ttfamily}cX}
  \toprule
  fork & 57 & \textrm{\textendash} & No parameters \\
  \midrule
  execve & 59 & rdi & Address of path \\
  & & rsi & Address of array of arguments \\
  & & rsi & Address of array of environment variables \\
  \midrule
  exit & 60 & rdi & Exit code \\
  \bottomrule
\end{tabularx}

\end{document}