The $FAB macro allocates storage for a FAB and initializes
    certain FAB fields with defaults and user-specified values. No
    value is returned for this assembly-time operation.

    Format

      $FAB  ALQ=allocation-quantity,

            BKS=bucket-size,

            BLS=block-size,

            CHAN_MODE=channel-access-mode

            CTX=user-context-value,

            DEQ=extension-quantity,

            DNA=default-filespec-address,

            DNM=<filespec>,

            DNS=default-filespec-string-size,

            FAC=<BIO BRO DEL GET PUT TRN UPD>,

            FNA=filespec-string-address,

            FNM=<filespec>,

            FNS=filespec-string-size,

            FOP=<CBT CIF CTG DFW DLT MXV NAM NEF NFS OFP POS RCK RWC

            RWO SCF SPL SQO SUP TEF TMD TMP UFO WCK>,

            FSZ=header-size,

            GBC=global-buffer-count,

            LNM_MODE=logical-name-translation-access-mode,

            MRN=maximum-record-number,

            MRS=maximum-record-size,

            NAM=nam-address,

            ORG={IDX|REL|SEQ},

            RAT=<BLK{CR|FTN|PRN}>,

            RFM={FIX|STM|STMCR|STMLF|UDF|VAR|VFC},

            RTV=window-size,

            SHR=<DEL GET MSE NIL PUT UPD UPI NQL>,

            XAB=xab-address