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  1                        Video Mode Selection Support 2.13
  2                     (c) 1995--1999 Martin Mares, <>
  3 --------------------------------------------------------------------------------
  5 1. Intro
  6 ~~~~~~~~
  7    This small document describes the "Video Mode Selection" feature which
  8 allows the use of various special video modes supported by the video BIOS. Due
  9 to usage of the BIOS, the selection is limited to boot time (before the
 10 kernel 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
 18 specified in the kernel Makefile (the SVGA_MODE=... line) or by the "vga=..."
 19 option 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
 21 of 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.
 40 2. Menu
 41 ~~~~~~~
 42    The ASK_VGA mode causes the kernel to offer a video mode menu upon
 43 bootup. It displays a "Press <RETURN> to see video modes available, <SPACE>
 44 to continue or wait 30 secs" message. If you press <RETURN>, you enter the
 45 menu, if you press <SPACE> or wait 30 seconds, the kernel will boot up in
 46 the standard 80x25 mode.
 48    The menu looks like:
 50 Video adapter: <name-of-detected-video-adapter>
 51 Mode:    COLSxROWS:
 52 0  0F00  80x25
 53 1  0F01  80x50
 54 2  0F02  80x43
 55 3  0F03  80x26
 56 ....
 57 Enter 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
 61 with VESA-compliant BIOS]) or a chipset name (e.g., Trident). Direct detection
 62 of chipsets is turned off by default (see CONFIG_VIDEO_SVGA in chapter 4 to see
 63 how to enable it if you really want) as it's inherently unreliable due to
 64 absolutely insane PC design.
 66    "0  0F00  80x25" means that the first menu item (the menu items are numbered
 67 from "0" to "9" and from "a" to "z") is a 80x25 mode with ID=0x0f00 (see the
 68 next section for a description of mode IDs).
 70    <flashing-cursor-here> encourages you to enter the item number or mode ID
 71 you 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
 73 a 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
 76 case your chipset has been detected, some chipset-specific modes are shown as
 77 well (some of these might be missing or unusable on your machine as different
 78 BIOSes are often shipped with the same card and the mode numbers depend purely
 79 on the VGA BIOS).
 81    The modes displayed on the menu are partially sorted: The list starts with
 82 the standard modes (80x25 and 80x50) followed by "special" modes (80x28 and
 83 80x43), local modes (if the local modes feature is enabled), VESA modes and
 84 finally 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
 87 is able to do more), you can enter "scan" instead of item number / mode ID.  The
 88 program will try to ask the BIOS for all possible video mode numbers and test
 89 what happens then. The screen will be probably flashing wildly for some time and
 90 strange noises will be heard from inside the monitor and so on and then, really
 91 all 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
 93 function.
 95    After scanning, the mode ordering is a bit different: the auto-detected SVGA
 96 modes are not listed at all and the modes revealed by `scan' are shown before
 97 all VESA modes.
 99 3. Mode IDs
100 ~~~~~~~~~~~
101    Because of the complexity of all the video stuff, the video mode IDs
102 used here are also a bit complex. A video mode ID is a 16-bit number usually
103 expressed in a hexadecimal notation (starting with "0x"). You can set a mode
104 by entering its mode directly if you know it even if it isn't shown on the menu.
106 The 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
146 vertical display timing according to mode parameters, which can be used to
147 eliminate some annoying bugs of certain VGA BIOSes (usually those used for
148 cards with S3 chipsets and old Cirrus Logic BIOSes) -- mainly extra lines at the
149 end of the display.
151 4. Options
152 ~~~~~~~~~~
153    Some options can be set in the source text (in arch/i386/boot/video.S).
154 All of them are simple #define's -- change them to #undef's when you want to
155 switch them off. Currently supported:
157    CONFIG_VIDEO_SVGA - enables autodetection of SVGA cards. This is switched
158 off by default as it's a bit unreliable due to terribly bad PC design. If you
159 really want to have the adapter autodetected (maybe in case the `scan' feature
160 doesn't work on your machine), switch this on and don't cry if the results
161 are not completely sane. In case you really need this feature, please drop me
162 a mail as I think of removing it some day.
164    CONFIG_VIDEO_VESA - enables autodetection of VESA modes. If it doesn't work
165 on your machine (or displays a "Error: Scanning of VESA modes failed" message),
166 you can switch it off and report as a bug.
168    CONFIG_VIDEO_COMPACT - enables compacting of the video mode list. If there
169 are more modes with the same screen size, only the first one is kept (see above
170 for more info on mode ordering). However, in very strange cases it's possible
171 that the first "version" of the mode doesn't work although some of the others
172 do -- in this case turn this switch off to see the rest.
174    CONFIG_VIDEO_RETAIN - enables retaining of screen contents when switching
175 video modes. Works only with some boot loaders which leave enough room for the
176 buffer. (If you have old LILO, you can adjust heap_end_ptr and loadflags
177 in setup.S, but it's better to upgrade the boot loader...)
179    CONFIG_VIDEO_LOCAL - enables inclusion of "local modes" in the list. The
180 local modes are added automatically to the beginning of the list not depending
181 on hardware configuration. The local modes are listed in the source text after
182 the "local_mode_table:" line. The comment before this line describes the format
183 of the table (which also includes a video card name to be displayed on the
184 top of the menu).
186    CONFIG_VIDEO_400_HACK - force setting of 400 scan lines for standard VGA
187 modes. This option is intended to be used on certain buggy BIOSes which draw
188 some useless logo using font download and then fail to reset the correct mode.
189 Don't use unless needed as it forces resetting the video card.
191    CONFIG_VIDEO_GFX_HACK - includes special hack for setting of graphics modes
192 to be used later by special drivers (e.g., 800x600 on IBM ThinkPad -- see
194 Allows to set _any_ BIOS mode including graphic ones and forcing specific
195 text screen resolution instead of peeking it from BIOS variables. Don't use
196 unless you think you know what you're doing. To activate this setup, use
197 mode number 0x0f08 (see section 3).
199 5. Still doesn't work?
200 ~~~~~~~~~~~~~~~~~~~~~~
201    When the mode detection doesn't work (e.g., the mode list is incorrect or
202 the machine hangs instead of displaying the menu), try to switch off some of
203 the configuration options listed in section 4. If it fails, you can still use
204 your kernel with the video mode set directly via the kernel parameter.
206    In either case, please send me a bug report containing what _exactly_
207 happens 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
210 video mode setting. In this case, you must specify the 0x0f04 mode ("leave
211 current settings") to Linux, because if you don't and you use any non-standard
212 mode, Linux will switch to 80x25 automatically.
214    If you set some extended mode and there's one or more extra lines on the
215 bottom of the display containing already scrolled-out text, your VGA BIOS
216 contains the most common video BIOS bug called "incorrect vertical display
217 end setting". Adding 0x8000 to the mode ID might fix the problem. Unfortunately,
218 this 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
221 is probably broken and you need to set the CONFIG_VIDEO_400_HACK switch to
222 force setting of the correct mode.
224 6. History
225 ~~~~~~~~~~
226 1.0 (??-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.
229 2.0 (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.
232 2.1 (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.
238 2.2 (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.
243 2.3 (15-Mar-96) Changed to work with 1.3.74 kernel.
244 2.4 (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.
249 2.5 (19-Mar-96) Fixed a VESA mode scanning bug introduced in 2.4.
250 2.6 (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).
252 2.7 (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.
263 2.8 (14-Apr-96) - Previous release was not compilable without CONFIG_VIDEO_SVGA.
264                 - Better recognition of text modes during mode scan.
265 2.9 (12-May-96) - Ignored VESA modes 0x80 - 0xff (more VESA BIOS bugs!)
266 2.10 (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.
270 2.11 (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.
275 2.12 (25-May-98)- Added support for VESA frame buffer graphics.
276 2.13 (14-May-99)- Minor documentation fixes.

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