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

    Purpose:        Find first - locate first bit in bit field
    Format:         opcode startpos.rl,size.rb,base.vb,findpos.wl
    Operation:      Find first set or clear bit searching left
                    across field
    C. Codes:       N = 0, Z = {bit not found}, V = 0, C = 0
    Exceptions:     Reserved operand
    Opcodes:        EB    FFC      Find first clear
                    EA    FFS      Find first set
    Description:    A field specified by the startpos, size and base
                    operands is extracted. The field is tested for a
                    bit in the indicated state starting at bit 0 and
                    extending to the highest bit in the field. If a
                    bit in the desired state is found, the findpos
                    operand is replaced with the bit position and the
                    Z condition bit is cleared. If no bit in the
                    indicated state is found, findpos is replaced by
                    the position relative to base of a bit one to the
                    left of the specified field and the Z condition
                    bit is set. If size = 0, findpos is replaced by
                    startpos and Z is set.