v7 man page for phys

PHYS(2) 							System Calls Manual							   PHYS(2)

NAME
       phys - allow a process to access physical addresses

SYNOPSIS
       phys(segreg, size, physadr)

DESCRIPTION
       The  argument  segreg specifies a process virtual (data-space) address range of 8K bytes starting at virtual address segregx8K bytes.  This
       address range is mapped into physical address physadrx64 bytes.	Only the first sizex64 bytes of this mapping is addressable.  If  size	is
       zero, any previous mapping of this virtual address range is nullified.  For example, the call

	    phys(6, 1, 0177775);

       will  map  virtual  addresses  0160000-0160077  into physical addresses 017777500-017777577.  In particular, virtual address 0160060 is the
       PDP-11 console located at physical address 017777560.

       This call may only be executed by the super-user.

SEE ALSO
       PDP-11 segmentation hardware

DIAGNOSTICS
       The function value zero is returned if the physical mapping is in effect.  The value -1 is returned if not super-user, if segreg is not	in
       the range 0-7, if size is not in the range 0-127, or if the specified segreg is already used for other than a previous call to phys.

BUGS
       This  system  call is obviously very machine dependent and very dangerous.  This system call is not considered a permanent part of the sys-
       tem.

ASSEMBLER
       (phys = 52.)
       sys phys; segreg; size; physadr

								       PDP11								   PHYS(2)