1               Video Mode Selection Support 2.13
2            (c) 1995--1999 Martin Mares, <>
51. Intro
7   This small document describes the "Video Mode Selection" feature which
8allows the use of various special video modes supported by the video BIOS. Due
9to usage of the BIOS, the selection is limited to boot time (before the
10kernel decompression starts) and works only on 80X86 machines.
12   ** Short intro for the impatient: Just use vga=ask for the first time,
13   ** enter `scan' on the video mode prompt, pick the mode you want to use,
14   ** remember its mode ID (the four-digit hexadecimal number) and then
15   ** set the vga parameter to this number (converted to decimal first).
17   The video mode to be used is selected by a kernel parameter which can be
18specified in the kernel Makefile (the SVGA_MODE=... line) or by the "vga=..."
19option of LILO (or some other boot loader you use) or by the "vidmode" utility
20(present in standard Linux utility packages). You can use the following values
21of this parameter:
23   NORMAL_VGA - Standard 80x25 mode available on all display adapters.
25   EXTENDED_VGA - Standard 8-pixel font mode: 80x43 on EGA, 80x50 on VGA.
27   ASK_VGA - Display a video mode menu upon startup (see below).
29   0..35 - Menu item number (when you have used the menu to view the list of
30      modes available on your adapter, you can specify the menu item you want
31      to use). 0..9 correspond to "0".."9", 10..35 to "a".."z". Warning: the
32      mode list displayed may vary as the kernel version changes, because the
33      modes are listed in a "first detected -- first displayed" manner. It's
34      better to use absolute mode numbers instead.
36   0x.... - Hexadecimal video mode ID (also displayed on the menu, see below
37      for exact meaning of the ID). Warning: rdev and LILO don't support
38      hexadecimal numbers -- you have to convert it to decimal manually.
402. Menu
42   The ASK_VGA mode causes the kernel to offer a video mode menu upon
43bootup. It displays a "Press <RETURN> to see video modes available, <SPACE>
44to continue or wait 30 secs" message. If you press <RETURN>, you enter the
45menu, if you press <SPACE> or wait 30 seconds, the kernel will boot up in
46the standard 80x25 mode.
48   The menu looks like:
50Video adapter: <name-of-detected-video-adapter>
51Mode: COLSxROWS:
520 0F00 80x25
531 0F01 80x50
542 0F02 80x43
553 0F03 80x26
57Enter mode number or `scan': <flashing-cursor-here>
59   <name-of-detected-video-adapter> tells what video adapter did Linux detect
60-- it's either a generic adapter name (MDA, CGA, HGC, EGA, VGA, VESA VGA [a VGA
61with VESA-compliant BIOS]) or a chipset name (e.g., Trident). Direct detection
62of chipsets is turned off by default (see CONFIG_VIDEO_SVGA in chapter 4 to see
63how to enable it if you really want) as it's inherently unreliable due to
64absolutely insane PC design.
66   "0 0F00 80x25" means that the first menu item (the menu items are numbered
67from "0" to "9" and from "a" to "z") is a 80x25 mode with ID=0x0f00 (see the
68next section for a description of mode IDs).
70   <flashing-cursor-here> encourages you to enter the item number or mode ID
71you wish to set and press <RETURN>. If the computer complains something about
72"Unknown mode ID", it is trying to tell you that it isn't possible to set such
73a mode. It's also possible to press only <RETURN> which leaves the current mode.
75   The mode list usually contains a few basic modes and some VESA modes. In
76case your chipset has been detected, some chipset-specific modes are shown as
77well (some of these might be missing or unusable on your machine as different
78BIOSes are often shipped with the same card and the mode numbers depend purely
79on the VGA BIOS).
81   The modes displayed on the menu are partially sorted: The list starts with
82the standard modes (80x25 and 80x50) followed by "special" modes (80x28 and
8380x43), local modes (if the local modes feature is enabled), VESA modes and
84finally SVGA modes for the auto-detected adapter.
86   If you are not happy with the mode list offered (e.g., if you think your card
87is able to do more), you can enter "scan" instead of item number / mode ID. The
88program will try to ask the BIOS for all possible video mode numbers and test
89what happens then. The screen will be probably flashing wildly for some time and
90strange noises will be heard from inside the monitor and so on and then, really
91all consistent video modes supported by your BIOS will appear (plus maybe some
92`ghost modes'). If you are afraid this could damage your monitor, don't use this
95   After scanning, the mode ordering is a bit different: the auto-detected SVGA
96modes are not listed at all and the modes revealed by `scan' are shown before
97all VESA modes.
993. Mode IDs
101   Because of the complexity of all the video stuff, the video mode IDs
102used here are also a bit complex. A video mode ID is a 16-bit number usually
103expressed in a hexadecimal notation (starting with "0x"). You can set a mode
104by entering its mode directly if you know it even if it isn't shown on the menu.
106The ID numbers can be divided to three regions:
108   0x0000 to 0x00ff - menu item references. 0x0000 is the first item. Don't use
109    outside the menu as this can change from boot to boot (especially if you
110    have used the `scan' feature).
112   0x0100 to 0x017f - standard BIOS modes. The ID is a BIOS video mode number
113    (as presented to INT 10, function 00) increased by 0x0100.
115   0x0200 to 0x08ff - VESA BIOS modes. The ID is a VESA mode ID increased by
116    0x0100. All VESA modes should be autodetected and shown on the menu.
118   0x0900 to 0x09ff - Video7 special modes. Set by calling INT 0x10, AX=0x6f05.
119    (Usually 940=80x43, 941=132x25, 942=132x44, 943=80x60, 944=100x60,
120    945=132x28 for the standard Video7 BIOS)
122   0x0f00 to 0x0fff - special modes (they are set by various tricks -- usually
123    by modifying one of the standard modes). Currently available:
124    0x0f00 standard 80x25, don't reset mode if already set (=FFFF)
125    0x0f01 standard with 8-point font: 80x43 on EGA, 80x50 on VGA
126    0x0f02 VGA 80x43 (VGA switched to 350 scanlines with a 8-point font)
127    0x0f03 VGA 80x28 (standard VGA scans, but 14-point font)
128    0x0f04 leave current video mode
129    0x0f05 VGA 80x30 (480 scans, 16-point font)
130    0x0f06 VGA 80x34 (480 scans, 14-point font)
131    0x0f07 VGA 80x60 (480 scans, 8-point font)
132    0x0f08 Graphics hack (see the CONFIG_VIDEO_HACK paragraph below)
134   0x1000 to 0x7fff - modes specified by resolution. The code has a "0xRRCC"
135    form where RR is a number of rows and CC is a number of columns.
136    E.g., 0x1950 corresponds to a 80x25 mode, 0x2b84 to 132x43 etc.
137    This is the only fully portable way to refer to a non-standard mode,
138    but it relies on the mode being found and displayed on the menu
139    (remember that mode scanning is not done automatically).
141   0xff00 to 0xffff - aliases for backward compatibility:
142    0xffff equivalent to 0x0f00 (standard 80x25)
143    0xfffe equivalent to 0x0f01 (EGA 80x43 or VGA 80x50)
145   If you add 0x8000 to the mode ID, the program will try to recalculate
146vertical display timing according to mode parameters, which can be used to
147eliminate some annoying bugs of certain VGA BIOSes (usually those used for
148cards with S3 chipsets and old Cirrus Logic BIOSes) -- mainly extra lines at the
149end of the display.
1514. Options
153   Some options can be set in the source text (in arch/i386/boot/video.S).
154All of them are simple #define's -- change them to #undef's when you want to
155switch them off. Currently supported:
157   CONFIG_VIDEO_SVGA - enables autodetection of SVGA cards. This is switched
158off by default as it's a bit unreliable due to terribly bad PC design. If you
159really want to have the adapter autodetected (maybe in case the `scan' feature
160doesn't work on your machine), switch this on and don't cry if the results
161are not completely sane. In case you really need this feature, please drop me
162a mail as I think of removing it some day.
164   CONFIG_VIDEO_VESA - enables autodetection of VESA modes. If it doesn't work
165on your machine (or displays a "Error: Scanning of VESA modes failed" message),
166you can switch it off and report as a bug.
168   CONFIG_VIDEO_COMPACT - enables compacting of the video mode list. If there
169are more modes with the same screen size, only the first one is kept (see above
170for more info on mode ordering). However, in very strange cases it's possible
171that the first "version" of the mode doesn't work although some of the others
172do -- in this case turn this switch off to see the rest.
174   CONFIG_VIDEO_RETAIN - enables retaining of screen contents when switching
175video modes. Works only with some boot loaders which leave enough room for the
176buffer. (If you have old LILO, you can adjust heap_end_ptr and loadflags
177in setup.S, but it's better to upgrade the boot loader...)
179   CONFIG_VIDEO_LOCAL - enables inclusion of "local modes" in the list. The
180local modes are added automatically to the beginning of the list not depending
181on hardware configuration. The local modes are listed in the source text after
182the "local_mode_table:" line. The comment before this line describes the format
183of the table (which also includes a video card name to be displayed on the
184top of the menu).
186   CONFIG_VIDEO_400_HACK - force setting of 400 scan lines for standard VGA
187modes. This option is intended to be used on certain buggy BIOSes which draw
188some useless logo using font download and then fail to reset the correct mode.
189Don't use unless needed as it forces resetting the video card.
191   CONFIG_VIDEO_GFX_HACK - includes special hack for setting of graphics modes
192to be used later by special drivers (e.g., 800x600 on IBM ThinkPad -- see
194Allows to set _any_ BIOS mode including graphic ones and forcing specific
195text screen resolution instead of peeking it from BIOS variables. Don't use
196unless you think you know what you're doing. To activate this setup, use
197mode number 0x0f08 (see section 3).
1995. Still doesn't work?
201   When the mode detection doesn't work (e.g., the mode list is incorrect or
202the machine hangs instead of displaying the menu), try to switch off some of
203the configuration options listed in section 4. If it fails, you can still use
204your kernel with the video mode set directly via the kernel parameter.
206   In either case, please send me a bug report containing what _exactly_
207happens and how do the configuration switches affect the behaviour of the bug.
209   If you start Linux from M$-DOS, you might also use some DOS tools for
210video mode setting. In this case, you must specify the 0x0f04 mode ("leave
211current settings") to Linux, because if you don't and you use any non-standard
212mode, Linux will switch to 80x25 automatically.
214   If you set some extended mode and there's one or more extra lines on the
215bottom of the display containing already scrolled-out text, your VGA BIOS
216contains the most common video BIOS bug called "incorrect vertical display
217end setting". Adding 0x8000 to the mode ID might fix the problem. Unfortunately,
218this must be done manually -- no autodetection mechanisms are available.
220   If you have a VGA card and your display still looks as on EGA, your BIOS
221is probably broken and you need to set the CONFIG_VIDEO_400_HACK switch to
222force setting of the correct mode.
2246. History
2261.0 (??-Nov-95) First version supporting all adapters supported by the old
227        setup.S + Cirrus Logic 54XX. Present in some 1.3.4? kernels
228        and then removed due to instability on some machines.
2292.0 (28-Jan-96) Rewritten from scratch. Cirrus Logic 64XX support added, almost
230        everything is configurable, the VESA support should be much more
231        stable, explicit mode numbering allowed, "scan" implemented etc.
2322.1 (30-Jan-96) VESA modes moved to 0x200-0x3ff. Mode selection by resolution
233        supported. Few bugs fixed. VESA modes are listed prior to
234        modes supplied by SVGA autodetection as they are more reliable.
235        CLGD autodetect works better. Doesn't depend on 80x25 being
236        active when started. Scanning fixed. 80x43 (any VGA) added.
237        Code cleaned up.
2382.2 (01-Feb-96) EGA 80x43 fixed. VESA extended to 0x200-0x4ff (non-standard 02XX
239        VESA modes work now). Display end bug workaround supported.
240        Special modes renumbered to allow adding of the "recalculate"
241        flag, 0xffff and 0xfffe became aliases instead of real IDs.
242        Screen contents retained during mode changes.
2432.3 (15-Mar-96) Changed to work with 1.3.74 kernel.
2442.4 (18-Mar-96) Added patches by Hans Lermen fixing a memory overwrite problem
245        with some boot loaders. Memory management rewritten to reflect
246        these changes. Unfortunately, screen contents retaining works
247        only with some loaders now.
248        Added a Tseng 132x60 mode.
2492.5 (19-Mar-96) Fixed a VESA mode scanning bug introduced in 2.4.
2502.6 (25-Mar-96) Some VESA BIOS errors not reported -- it fixes error reports on
251        several cards with broken VESA code (e.g., ATI VGA).
2522.7 (09-Apr-96) - Accepted all VESA modes in range 0x100 to 0x7ff, because some
253          cards use very strange mode numbers.
254        - Added Realtek VGA modes (thanks to Gonzalo Tornaria).
255        - Hardware testing order slightly changed, tests based on ROM
256          contents done as first.
257        - Added support for special Video7 mode switching functions
258          (thanks to Tom Vander Aa).
259        - Added 480-scanline modes (especially useful for notebooks,
260          original version written by, patched by
261          Jeff Chua, rewritten by me).
262        - Screen store/restore fixed.
2632.8 (14-Apr-96) - Previous release was not compilable without CONFIG_VIDEO_SVGA.
264        - Better recognition of text modes during mode scan.
2652.9 (12-May-96) - Ignored VESA modes 0x80 - 0xff (more VESA BIOS bugs!)
2662.10 (11-Nov-96)- The whole thing made optional.
267        - Added the CONFIG_VIDEO_400_HACK switch.
268        - Added the CONFIG_VIDEO_GFX_HACK switch.
269        - Code cleanup.
2702.11 (03-May-97)- Yet another cleanup, now including also the documentation.
271        - Direct testing of SVGA adapters turned off by default, `scan'
272          offered explicitly on the prompt line.
273        - Removed the doc section describing adding of new probing
274          functions as I try to get rid of _all_ hardware probing here.
2752.12 (25-May-98)- Added support for VESA frame buffer graphics.
2762.13 (14-May-99)- Minor documentation fixes.

Archive Download this file