VSPY(2V)                GENIX Programmer's Manual                VSPY(2V)



      NAME
           vspy - set up mapping to physical memory

      SYNOPSIS
           vspy(virtaddr, physaddr, flag)
           char *virtaddr;
           char *physaddr;

      DESCRIPTION
           Vspy changes the mmu mapping for the current process to allow refer-
           ences directly to physical memory.  This capability allows user mode
           programs to communicate directly with the various peripherals of the
           system (such as a bitmapped graphics screen).

           Virtaddr is the address in the process which is to be used to access
           the  physical memory.  Physaddr is the physical memory address to be
           accessed, or -1 to remove existing access to physical memory.   Flag
           is  0  if  only  read access is desired, or 1 if both read and write
           access is desired.

           Since the lowest mapping granularity for GENIX  is  one  page  (1024
           bytes),  the  low order ten bits of the given addresses are ignored,
           so that the whole virtual page specified by virtaddr will map to the
           whole  physical memory page specified by physaddr. Thus to reference
           a particular offset into the physical memory page, you  must  access
           the same offset into the virtual memory page.

           The virtaddr specified does not have to lie within the  data,  text,
           or  stack segment of the process, so that memory does not have to be
           allocated using sbrk(2) before calling vspy. If the  page  specified
           by  virtaddr  already  exists,  that  page is deleted.  Fork(2) will
           duplicate the physical mapping for the new  process.   A  successful
           exec(2)  or exit(2) removes all existing physical mapping.  Only the
           super-user is allowed to map physical memory arbitrarily.   However,
           certain  physical  memory  addresses corresponding to "safe" devices
           (such as the graphics memory) may be mapped by anyone.

      SEE ALSO
           vlock(2V)

      DIAGNOSTICS
           Zero is returned if the call was successful. -1 is returned  if  any
           of the arguments are illegal or if the user is not privileged.

      BUGS
           I/O cannot be done to pages mapped to physical memory.  Arguments to
           system calls cannot lie in such pages either.








      Printed 11/16/83                                                        1