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

    Purpose:        To find a substring (object) in a character string
    Format:         opcode objlen.rw,objaddr.ab,srclen.rw,srcaddr.ab
    Operation:      {compare object substring with srcstring until match}
    C. Codes:       N = 0, Z = {R0 EQL 0}, Z = 0, C = 0
    Exceptions:     None
    Opcode:         39    MATCHC     Match characters
    Description:    src as specified by srclen and srcaddr is searched
                    for a substring which matches the object string as
                    specified by objlen and objaddr. If the substring
                    is found the Z bit is set, otherwise it is cleared.
                    The search continues until src is exhausted or
                    until a match is found.
    Notes:          After execution:

                    R0 = if a match occurred, 0; otherwise the number
                         of bytes in the object string
                    R1 = if a match occurred, the address of one byte
                         beyond the object string; that is,
                         objaddr + objlen; otherwise, the address of
                         the object string
                    R2 = if a match occurred, the number of bytes
                         remaining in the source string; otherwise 0
                    R3 = if a match occurred, the address of one byte
                         beyond the last byte matched; otherwise, the
                         address of one byte beyond the source string;
                         that is, srcaddr + srclen

                    For zero-length source and object strings, R3 and R1
                    contain the source and object addresses, respectively.

                    If both strings have 0 length or if the object string
                    has length 0, condition code Z is set and registers
                    R0-R3 are left just as though the substring were
                    found.

                    If the source has 0 length and the object string
                    has nonzero length, condition code Z is cleared
                    and registers R0-R3 are left just as though the
                    substring were not found.