Any length specified on EQU must be able to be evaluated at the time that the EQU statement is processed, otherwise message ASMA0182E will be produced. This means not only that the definition must have been seen, but also that the definition can be processed and resolved to an numeric value. If the length value depends on a forward reference, or on unresolved alignment, then it cannot be resolved.
Note for example that if the start of some area is fullword aligned but it contains a doubleword aligned field then the length cannot be resolved until the assembler can tell whether the start is on an odd or even fullword boundary. Jonathan Scott, HLASM IBM Hursley, UK
