You can download the instruction manual for this machine and many others from the webpage here:
https://www.ifixit.com/Guide/Old+Sewi...

The Bernina 644 was made in Switzerland in the early 1960s, it is a very
rugged looking model that is notable for its precision engineering, build
quality and strength, it was sold by Bernina for both domestic and industrial use - it weighs in at around 18 kg.

The machine can do adjustable straight stitch and reverse, and the feed
dogs can be dropped, via a lever on the base, to facilitate embroidery and
darning. This model does not do zigzag.