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.