This appendage creates a list of the addresses of the areas that must be fixed to prevent paging exceptions during the execution of the current input/output (I/O) request. While this appendage can be entered more than once for one I/O request, each time it is entered it must create the same list of areas to be fixed. The appendage can use the 16-word save area pointed to by register 13. Registers 10, 11, and 13 can be used as work registers.

Each page-fix entry placed in the list by the appendage must have the following doubleword format:

     ³0³1                           31³32³33                           63³
     ³─³──────────────────────────────³──³───────────────────────────────³
     ³0³       Starting virtual       ³ 0³        Ending virtual         ³
     ³ ³       address of area        ³  ³        address of area        ³
     ³ ³       to be fixed            ³  ³        to be fixed + 1        ³
     ³─³──────────────────────────────³──³───────────────────────────────³
     ³ ³                              ³  ³                               ³

On return from the PGFX appendage to the system (via the return address provided in register 14), register 10 must point to the first page-fix entry and register 11 must contain the number of page-fix entries in the work area. The system then fixes the pages corresponding to the areas listed by the PGFX appendage. The pages remain fixed until the associated EXCPVR request terminates.

If either the channel end appendage or the abnormal end appendage returns via the return address in register 14 plus 8, the PGFX appendage is not normally reentered. Instead, the SIO appendage is entered, and the page-fix list built by the PGFX appendage is still active. When a PURGE macro has been issued (for instance, when a storage swap has occurred), the PGFX appendage is entered after either the channel end appendage or the abnormal end appendage returns via the return address in register 14 plus 8. When I/O is restored, the PGFX appendage is entered. The page-fix list must be in page-fixed storage.