The ASN1C compiler has a special processing mode that contains extensions to handle items in the older 1990 version of ASN.1 (i.e. the now deprecated X.208 and X.209 standards). This mode is activated by using the -asnstd x208 command-line option.

Although the X.208 and X.209 standards are no longer supported by the ITU-T, they are still in use today. This version of ASN1C contains logic to parse some common MACRO definitions that are still in widespread use despite the fact that MACRO syntax was retired with this version of the standard. The types of MACRO definitions that are supported are ROSE OPERATION and ERROR and SNMP OBJECT-TYPE.