Documentation/x86/mtrr.txt

7   the Memory Type Range Registers (MTRRs) may be used to control
8   processor access to memory ranges. This is most useful when you have
9   a video (VGA) card on a PCI or AGP bus. Enabling write-combining
10   allows bus write transfers to be combined into a larger transfer
12   of image write operations 2.5 times or more.
15   Registers (ARRs) which provide a similar functionality to MTRRs. For
16   these, the ARRs are used to emulate the MTRRs.
18   The AMD K6-2 (stepping 8 and above) and K6-3 processors have two
22   The Centaur C6 (WinChip) has 8 MCRs, allowing write-combining. These
28   to manipulate your MTRRs. Typically the X server should use
34 There are two interfaces to /proc/mtrr: one is an ASCII interface
35 which allows you to read and write. The other is an ioctl()
44 reg00: base=0x00000000 (   0MB), size= 128MB: write-back, count=1
45 reg01: base=0x08000000 ( 128MB), size=  64MB: write-back, count=1
47 Creating MTRRs from the C-shell:
48 # echo "base=0xf8000000 size=0x400000 type=write-combining" >! /proc/mtrr
50 # echo "base=0xf8000000 size=0x400000 type=write-combining" >| /proc/mtrr
54 reg00: base=0x00000000 (   0MB), size= 128MB: write-back, count=1
55 reg01: base=0x08000000 ( 128MB), size=  64MB: write-back, count=1
56 reg02: base=0xf8000000 (3968MB), size=   4MB: write-combining, count=1
58 This is for video RAM at base address 0xf8000000 and size 4 megabytes. To
59 find out your base address, you need to look at the output of your X
63 (--) S3: PCI: 968 rev 0, Linear FB @ 0xf8000000
69 To find out the size of your framebuffer (what, you don't actually
72 (--) S3: videoram:  4096k
77 ioctl() interface, so users won't have to do anything. If you use a
78 commercial X server, lobby your vendor to add support for MTRRs.
82 %echo "base=0xfb000000 size=0x1000000 type=write-combining" >/proc/mtrr
86 reg00: base=0x00000000 (   0MB), size=  64MB: write-back, count=1
87 reg01: base=0xfb000000 (4016MB), size=  16MB: write-combining, count=1
95 region that you created is type=write-combining.
97 Removing MTRRs from the C-shell:
104 /*  mtrr-show.c
106     Source file for mtrr-show (example program to show MTRRs using ioctl()'s)
108     Copyright (C) 1997-1998  Richard Gooch
121     along with this program; if not, write to the Free Software
130     This program will use an ioctl() on /proc/mtrr to show the current MTRR
131     settings. This is an alternative to reading /proc/mtrr.
134     Written by      Richard Gooch   17-DEC-1997
136     Last updated by Richard Gooch   2-MAY-1998
157     "write-combining",          /* 1 */
160     "write-through",            /* 4 */
161     "write-protect",            /* 5 */
162     "write-back",               /* 6 */
170     if ( ( fd = open ("/proc/mtrr", O_RDONLY, 0) ) == -1 )
200 /*  mtrr-add.c
202     Source file for mtrr-add (example programme to add an MTRRs using ioctl())
204     Copyright (C) 1997-1998  Richard Gooch
217     along with this program; if not, write to the Free Software
226     This programme will use an ioctl() on /proc/mtrr to add an entry. The first
227     available mtrr is used. This is an alternative to writing /proc/mtrr.
230     Written by      Richard Gooch   17-DEC-1997
232     Last updated by Richard Gooch   2-MAY-1998
254     "write-combining",          /* 1 */
257     "write-through",            /* 4 */
258     "write-protect",            /* 5 */
259     "write-back",               /* 6 */
269 	fprintf (stderr, "Usage:\tmtrr-add base size type\n");
283     if ( ( fd = open ("/proc/mtrr", O_WRONLY, 0) ) == -1 )
294     if (ioctl (fd, MTRRIOC_ADD_ENTRY, &sentry) == -1)