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

    Purpose:        Add compare and branch - maintain loop count
                    and loop
    Format:         opcode limit.rx,add.rx,index.rx,displ.bw
    Operation:      index = index + add; if {{add GEQ 0} AND {index LEQ
                    limit}} OR {{add LSS 0} AND {index GEQ limit}} then
                    PC = PC + SEXT (displ)
    C. Codes:       N = {index LSS 0}, Z = {index EQL 0}, C = C,
                    V = {integer or floating overflow}
    Exceptions:     Integer or floating overflow, floating
                    underflow, reserved operand
    Opcodes:        9D    ACBB     Add compare and branch byte
                    3D    ACBW     Add compare and branch word
                    F1    ACBL     Add compare and branch long
                    4F    ACBF     Add compare and branch F_floating
                    6F    ACBD     Add compare and branch D_floating
                    4FFD  ACBG     Add compare and branch G_floating
                    6FFD  ACBH     Add compare and branch H_floating
    Description:    add is added to index and index is replaced by the
                    result. index is compared with limit. If add is
                    greater than or equal to zero and the comparison is
                    less than or equal to zero or if add is less than
                    zero and the comparison is greater than or equal,
                    the branch is taken. The condition codes are
                    unpredictable on a reserved operand fault.