  We used a set of moderately-deep and high-resolution optical observations
obtained with the Hubble Space Telescope to investigate the properties of the
stellar population in the heavily obscured bulge globular cluster NGC 6256. The
analysis of the color-magnitude diagram revealed a stellar population with an
extended blue horizontal branch and severely affected by differential
reddening, which was corrected taking into account color excess variations up
to \delta E(B-V) ~ 0.51. We implemented a Monte Carlo Markov Chain technique to
perform the isochrone fitting of the observed color-magnitude diagram in order
to derive the stellar age, the cluster distance and the average color excess in
the cluster direction. Using different set of isochrones we found that NGC 6256
is characterized by a very old stellar age around 13.0 Gyr, with a typical
uncertainty of ~ 0.5 Gyr. We also found an average color excess E(B-V) = 1.19
and a distance from the Sun of 6.8 kpc. We then derived the cluster
gravitational center and measured its absolute proper motion using the Gaia-DR2
catalog. All this was used to back-integrate the cluster orbit in a Galaxy-like
potential and measure its integrals of motion. It turned out that NGC 6256 is
currently in a low-eccentricity orbit entirely confined within the bulge and
its integrals of motion are fully compatible with a cluster purely belonging to
the Galaxy native globular cluster population. All these pieces of evidence
suggest that NGC 6256 is an extremely old relic of the past history of the
Galaxy, formed during the very first stages of its assembly.
