I386_GET_MTRR(System Library Functions Manual (i386 ArchitectuI386_GET_MTRR(3)

NAME    [Toc]    [Back]

     i386_get_mtrr, i386_set_mtrr - access Memory Type Range Registers

LIBRARY    [Toc]    [Back]

     i386 Architecture Library (libi386, -li386)

SYNOPSIS    [Toc]    [Back]

     #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    [Toc]    [Back]

     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) busses.  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 operations 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 successfully 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 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    [Toc]    [Back]

     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 be container the number of successfully
 processed mtrr structures in both cases.

ERRORS    [Toc]    [Back]

     [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    [Toc]    [Back]

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

BSD                            November 10, 2001                           BSD