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

    Purpose:        Branch on bit interlocked - test and modify selected
                    bit under memory interlock.
    Format:         opcode pos.rl,base.ab,displ.bb
    Operation:      teststate = if {BBSSI} then 1 else 0; newstate =
                    teststate; {set interlock}; temp = FIELD (pos,
                    1, base); FIELD (pos, 1, base) = newstate; {release
                    interlock}; if temp EQL teststate then PC =
                    {PC + SEXT(displ)}
    C. Codes:       Not affected
    Exceptions:     Reserved operand
    Opcodes:        E6    BBSSI    Branch bit set, set interlocked
                    E7    BBCCI    Branch bit clear, clear interlocked
    Description:    The single bit field specified by pos and base is
                    tested. If it is in the state indicated by the
                    instruction, the sign extended branch displacement
                    displ is added to PC. The bit is set to the state
                    indicated by the instruction. The operation is
                    interlocked against other processors or devices in
                    the system.