VAX Assembly question

I'm currently perusing _Computer Programming and Architecture: The VAX-11_ by Levy and Eckhouse and I come across the description of the CALLG and CALLS instructions. They go on to describe the stack frame created by the CALL* instruction: Condition handler (initially 0) <- FP SPA | S | 0 | MASK | PSW | 0 FP + 4 Saved AP etc ... Saved FP Saved PC Saved R0 . . . Saved R11 My question is about the condition handler. In the text, it's described as: A longword condition handler address. Here, the calling routine may store the address of an error-handling routine to be called if an exceptional error condition arises in the procedure. This is the only section (as far as I can tell) where this is described, and I'm curious as to how exactly it worked. It appears that while the CPU may set this location to 0, its purpose isn't really dictated by the hardware, but it a convention used by VMS. But then (as I was typing this), I was struct by this bit: ... the calling routine may store ... Huh? How? The calling routine either does a: argsize: .long 0 arg1: .long 0 arg2: .long 0 arg3: .long 0 arg4: .long 0 movl #4,argsize ; use fixed memory movl filehandle,arg1 ; for the arguments moval somememory,arg2 movl #0,arg3 movl #4096,arg4 callg argsize,dump_memory or pushl #4096 ; use the stack pushl #0 ; for the arguments pushal somememory pushl filehandle calls #4,dump_memory (forgive if my assembly isn't quite right, I'm going from the book rather quickly here). How can the calling code set the condition handler? The CPU creates the stack frame (as diagrammed above), not the calling routine. Am I missing something? -spc (Really curious about this ... )