as supports both DEC syntax and BSD syntax. The only
difference is that in DEC syntax, a # character is used to denote
an immediate constants, while in BSD syntax the character for this
purpose is $.
general-purpose registers are named r0 through r7.
Mnemonic alternatives for r6 and r7 are sp and
pc, respectively.
Floating-point registers are named ac0 through ac3, or
alternatively fr0 through fr3.
Comments are started with a # or a / character, and extend
to the end of the line. (FIXME: clash with immediates?)