|
HP OpenVMS Systemsask the wizard |
|
The Question is: We are currently migrating from Vax to Alpha. We have a subroutine (Get_Argcnt) that returns the argument count and also an argument list indicating whether any arguments were omitted. We have many subroutines that call Get_Argcnt and rely on the argume nt list to determine if an agrument should be referenced. It's crutial that we can continue to use this subroutine. There is an FAQ, Prog9, referring to this, but I can't believe that the same functionality can't be accomplished on the Alpha. R25 appare ntly contains the argument count of the current subroutine. R16-R21 contain the addresses of the first 6 arguments. I don't understand how to reference arguments 7 and beyond. What I really need is the argument count and argument list of the previous ca ll. I accomplished this on the Vax by getting the previous ap from 8(fp). How can this be done on the Alpha? Thank you, Jerry Ryan Bissell, Inc ITS 616-453-4451 x.352 The Answer is : The implementation of argument passing is quite different between OpenVMS VAX and OpenVMS Alpha -- applications that are dependant on the contents of the VAX call stack will require changes when porting to OpenVMS Alpha. On OpenVMS VAX systems, many programmers used a small routine written in Macro32 to fetch the argument count via the saved AP register. Though this technique was unsupported and not guaranteed to work in the VAX architecture, it worked well enough that various applications have come to depend upon it. This AP-based technique does not work on OpenVMS Alpha systems, even if the Macro32 routine is compiled using the OpenVMS Alpha Macro32 compiler, due to the different way arguments are passed on OpenVMS Alpha. Compaq Fortran offers an IARGCOUNT intrinsic function -- which takes no arguments and which returns the actual argument count. Compaq C has full stdargs.h support for processing variable argument lists, and C provides an extension (va_count) which retrieves the argument count. These compiler-provided mechanisms all have the side-effect of causing the compiler to copy and transfer the argument list via memory -- this memory-based argument list is called "homed" -- which then allows the Fortran programs to test %LOC(arg) .EQ. 0 or the C programs to check va_count to see if any argument(s) were omitted. IARGCOUNT and va_count are available in Compaq Fortran OpenVMS VAX and in Compaq C for OpenVMS VAX, and are recommended as a replacement for the Macro32 stack-cogniscent routines on both VAX and Alpha platforms.
|