VMS Help MACRO, VAX MACRO Assembler, Instructions, BBx  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Purpose:        Branch on bit - test selected bit
    Format:         opcode pos.rl,base.ab,displ.bb
    Operation:      teststate = {if {BBS} then 1 else 0}; if {FIELD
                    (pos, 1, base) EQL teststate} then PC = {PC +
                    SEXT (displ)}
    C. Codes:       Unaffected
    Exceptions:     Reserved operand
    Opcodes:        E0    BBS      Branch on bit set
                    E1    BBC      Branch on bit clear
    Description:    The single bit field specified by pos and base is
                    tested. If it is in the test state indicated by the
                    instruction, the sign extended branch displacement
                    is added to PC and the result is placed in PC.