Wes Newell wrote:

Oh, as86 comes with the linux distro I use. I assume if you
got a 64bit linux distro, the assembler would support the new
64 bit instruction set.


AMD64 support was added to FreeBSD a while back as well.
The BSD preference for compiling everything with the comfort of
the ports system may better suit a person who wanted to keep an
eye on optimizations etc.