I386_GET_MTRR(2)					   BSD/i386 System Calls Manual 					  I386_GET_MTRR(2)

NAME

     i386_get_mtrr, i386_set_mtrr -- access Memory Type Range Registers

LIBRARY

     i386 Architecture Library (libi386, -li386)

SYNOPSIS

     #include <sys/types.h>
     #include <machine/sysarch.h>
     #include <machine/mtrr.h>

     int
     i386_get_mtrr(struct mtrr *mtrrp, int *n);

     int
     i386_set_mtrr(struct mtrr *mtrrp, int *n);

DESCRIPTION

     These functions provide an interface to the MTRR registers found on 686-class processors for controlling processor access to memory ranges.
     This is most useful for accessing devices such as video accelerators on pci(4) and agp(4) buses.  For example, enabling write-combining
     allows bus-write transfers to be combined into a larger transfer before bursting over the bus.  This can increase performance of write opera-
     tions 2.5 times or more.

     mtrrp is a pointer to one or more mtrr structures, as described below.  The n argument is a pointer to an integer containing the number of
     structures pointed to by mtrrp.  For i386_set_mtrr() the integer pointed to by n will be updated to reflect the actual number of MTRRs suc-
     cessfully set.  For i386_get_mtrr() no more than n structures will be copied out, and the integer value pointed to by n will be updated to
     reflect the actual number of valid structures retrieved.  A NULL argument to mtrrp will result in just the number of MTRRs available being
     returned in the integer pointed to by n.

     The argument mtrrp has the following structure:

     struct mtrr {
	     uint64_t base;
	     uint64_t len;
	     uint8_t type;
	     int flags;
	     pid_t owner;
     };

     The location of the mapping is described by its physical base address base and length len.  Valid values for type are:

	   MTRR_TYPE_UC      uncached memory
	   MTRR_TYPE_WC      use write-combining
	   MTRR_TYPE_WT      use write-through caching
	   MTRR_TYPE_WP      write-protected memory
	   MTRR_TYPE_WB      use write-back caching

     Valid values for flags are:

	   MTRR_PRIVATE  own range, reset the MTRR when the current process exits
	   MTRR_FIXED	 use fixed range MTRR
	   MTRR_VALID	 entry is valid

     The owner member is the PID of the user process which claims the mapping.	It is only valid if MTRR_PRIVATE is set in flags.  To clear/reset
     MTRRs, use a flags field without MTRR_VALID set.

RETURN VALUES

     Upon successful completion zero is returned, otherwise -1 is returned on failure, and the global variable errno is set to indicate the error.
     The integer value pointed to by n will contain the number of successfully processed mtrr structures in both cases.

ERRORS

     [ENOSYS]  The currently running kernel or CPU has no MTRR support.

     [EINVAL]  The currently running kernel has no MTRR support, or one of the mtrr structures pointed to by mtrrp is invalid.

     [EBUSY]   No unused MTRRs are available.

HISTORY

     The i386_get_mtrr() and i386_set_mtrr() functions appeared in NetBSD 1.6.

BSD
								 November 10, 2001							       BSD