  The ATLAS Fast TracKer (FTK) was designed to provide full tracking for the
ATLAS high-level trigger by using pattern recognition based on Associative
Memory (AM) chips and fitting in high-speed field programmable gate arrays. The
tracks found by the FTK are based on inputs from all modules of the pixel and
silicon microstrip trackers. The as-built FTK system and components are
described, as is the online software used to control them while running in the
ATLAS data acquisition system. Also described is the simulation of the FTK
hardware and the optimization of the AM pattern banks. An optimization for
long-lived particles with large impact parameter values is included. A test of
the FTK system with the data playback facility that allowed the FTK to be
commissioned during the shutdown between Run 2 and Run 3 of the LHC is
reported. The resulting tracks from part of the FTK system covering a limited
$\eta$-$\phi$ region of the detector are compared with the output from the FTK
simulation. It is shown that FTK performance is in good agreement with the
simulation.
