The message area header contains statistical, pointer, and general information. Its contents are unrelated to the individual messages. Figure 1 shows the format of the message area header.

The function of the ACB error flag field remains unchanged whether this optional message area is specified. At the end of an open or close, this field contains either X'00' (indicating no error or attention condition occurred) or a nonzero code. The ACB error flag byte contains the nonzero open or close reason code corresponding to the error or attention condition that occurred with the highest severity.

Message area header information is stored only when an attention or error condition is detected. (That is, when the ACB error flag field is set to a nonzero value.) The header information consists of the flag byte only if the message area length (MLEN) is not large enough to accommodate the full message area header. In this case, bit 0 of the flag byte is zero.

Before accessing the message header information (bytes 1 through 19), test byte 0 to see if more information is stored. If MLEN=0, no header information is stored, not even the flag byte. If the full message area header is stored, bytes 1 and 2 contain its actual length. Your program should be sensitive to this length when interrogating the message area header.