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I386_GET_MTRR(2) FreeBSD System Calls Manual (i386) 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 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 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. FreeBSD 14.1-RELEASE-p8 November 10, 2001 FreeBSD 14.1-RELEASE-p8