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


i386_get_mtrr, i386_set_mtrraccess Memory Type Range Registers


i386 Architecture Library (libi386, -li386)


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

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

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


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 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:

uncached memory
use write-combining
use write-through caching
write-protected memory
use write-back caching

Valid values for flags are:

own range, reset the MTRR when the current process exits
use fixed range MTRR
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.


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.


The currently running kernel or CPU has no MTRR support.
The currently running kernel has no MTRR support, or one of the mtrr structures pointed to by mtrrp is invalid.
No unused MTRRs are available.


The i386_get_mtrr() and i386_set_mtrr() functions appeared in NetBSD 1.6.
November 10, 2001 NetBSD 7.0