any tutorials for x86-64 assembly under freebsd?

[iam at sdf.org]

i am running freebsd-12 under x86-64.
i have been pointed to 'as' and 'ld' for tools.
i have also been pointed to the 'url' for intel
documentation about x86-64 assembly.
i also have a sample "hello world" program in
x86-64 assembly language.
may i know of any tutorials which can teach me
to work with assembly using x86-64?
i saw one book on amazon.com, but that uses the
assembler from microsoft ('masm') which i think
only runs under windows-10+.
any pointers and/or help would be appreciated.
thank you.