Converts a virtual address to the address of the first byte
    within that page.

    Format

      $START_OF_PAGE  source_va, dest_va, user_mask_addr, quad=YES

1  –  Parameters

 source_va

    Source virtual address.

 dest_va

    Destination of virtual address of first byte within page.

 user_mask_addr

    Location of application-provided byte-within-page mask. If this
    argument is blank, the macro uses MMG$GL_BWP_MASK on an OpenVMS
    Alpha or OpenVMS I64 system and MMG$C_VAX_PAGE_SIZE - 1 (defined
    in $pagedef) on an OpenVMS VAX system.

 quad=YES

    If YES, the conversion supports 64-bit addressing. If NO, the
    conversion does not support 64-bit addressing.