1 .. include:: <isonum.txt>
2
3 =================================
4 Video Mode Selection Support 2.13
5 =================================
6
7 :Copyright: |copy| 1995--1999 Martin Mares, <mj@ucw.cz>
8
9 Intro
10 ~~~~~
11
12 This small document describes the "Video Mode Selection" feature which
13 allows the use of various special video modes supported by the video BIOS. Due
14 to usage of the BIOS, the selection is limited to boot time (before the
15 kernel decompression starts) and works only on 80X86 machines that are
16 booted through BIOS firmware (as opposed to through UEFI, kexec, etc.).
17
18 .. note::
19
20 Short intro for the impatient: Just use vga=ask for the first time,
21 enter ``scan`` on the video mode prompt, pick the mode you want to use,
22 remember its mode ID (the four-digit hexadecimal number) and then
23 set the vga parameter to this number (converted to decimal first).
24
25 The video mode to be used is selected by a kernel parameter which can be
26 specified in the kernel Makefile (the SVGA_MODE=... line) or by the "vga=..."
27 option of LILO (or some other boot loader you use) or by the "xrandr" utility
28 (present in standard Linux utility packages). You can use the following values
29 of this parameter::
30
31 NORMAL_VGA - Standard 80x25 mode available on all display adapters.
32
33 EXTENDED_VGA - Standard 8-pixel font mode: 80x43 on EGA, 80x50 on VGA.
34
35 ASK_VGA - Display a video mode menu upon startup (see below).
36
37 0..35 - Menu item number (when you have used the menu to view the list of
38 modes available on your adapter, you can specify the menu item you want
39 to use). 0..9 correspond to "0".."9", 10..35 to "a".."z". Warning: the
40 mode list displayed may vary as the kernel version changes, because the
41 modes are listed in a "first detected -- first displayed" manner. It's
42 better to use absolute mode numbers instead.
43
44 0x.... - Hexadecimal video mode ID (also displayed on the menu, see below
45 for exact meaning of the ID). Warning: LILO doesn't support
46 hexadecimal numbers -- you have to convert it to decimal manually.
47
48 Menu
49 ~~~~
50
51 The ASK_VGA mode causes the kernel to offer a video mode menu upon
52 bootup. It displays a "Press <RETURN> to see video modes available, <SPACE>
53 to continue or wait 30 secs" message. If you press <RETURN>, you enter the
54 menu, if you press <SPACE> or wait 30 seconds, the kernel will boot up in
55 the standard 80x25 mode.
56
57 The menu looks like::
58
59 Video adapter: <name-of-detected-video-adapter>
60 Mode: COLSxROWS:
61 0 0F00 80x25
62 1 0F01 80x50
63 2 0F02 80x43
64 3 0F03 80x26
65 ....
66 Enter mode number or ``scan``: <flashing-cursor-here>
67
68 <name-of-detected-video-adapter> tells what video adapter did Linux detect
69 -- it's either a generic adapter name (MDA, CGA, HGC, EGA, VGA, VESA VGA [a VGA
70 with VESA-compliant BIOS]) or a chipset name (e.g., Trident). Direct detection
71 of chipsets is turned off by default as it's inherently unreliable due to
72 absolutely insane PC design.
73
74 "0 0F00 80x25" means that the first menu item (the menu items are numbered
75 from "0" to "9" and from "a" to "z") is a 80x25 mode with ID=0x0f00 (see the
76 next section for a description of mode IDs).
77
78 <flashing-cursor-here> encourages you to enter the item number or mode ID
79 you wish to set and press <RETURN>. If the computer complains something about
80 "Unknown mode ID", it is trying to tell you that it isn't possible to set such
81 a mode. It's also possible to press only <RETURN> which leaves the current mode.
82
83 The mode list usually contains a few basic modes and some VESA modes. In
84 case your chipset has been detected, some chipset-specific modes are shown as
85 well (some of these might be missing or unusable on your machine as different
86 BIOSes are often shipped with the same card and the mode numbers depend purely
87 on the VGA BIOS).
88
89 The modes displayed on the menu are partially sorted: The list starts with
90 the standard modes (80x25 and 80x50) followed by "special" modes (80x28 and
91 80x43), local modes (if the local modes feature is enabled), VESA modes and
92 finally SVGA modes for the auto-detected adapter.
93
94 If you are not happy with the mode list offered (e.g., if you think your card
95 is able to do more), you can enter "scan" instead of item number / mode ID. The
96 program will try to ask the BIOS for all possible video mode numbers and test
97 what happens then. The screen will be probably flashing wildly for some time and
98 strange noises will be heard from inside the monitor and so on and then, really
99 all consistent video modes supported by your BIOS will appear (plus maybe some
100 ``ghost modes``). If you are afraid this could damage your monitor, don't use
101 this function.
102
103 After scanning, the mode ordering is a bit different: the auto-detected SVGA
104 modes are not listed at all and the modes revealed by ``scan`` are shown before
105 all VESA modes.
106
107 Mode IDs
108 ~~~~~~~~
109
110 Because of the complexity of all the video stuff, the video mode IDs
111 used here are also a bit complex. A video mode ID is a 16-bit number usually
112 expressed in a hexadecimal notation (starting with "0x"). You can set a mode
113 by entering its mode directly if you know it even if it isn't shown on the menu.
114
115 The ID numbers can be divided to those regions::
116
117 0x0000 to 0x00ff - menu item references. 0x0000 is the first item. Don't use
118 outside the menu as this can change from boot to boot (especially if you
119 have used the ``scan`` feature).
120
121 0x0100 to 0x017f - standard BIOS modes. The ID is a BIOS video mode number
122 (as presented to INT 10, function 00) increased by 0x0100.
123
124 0x0200 to 0x08ff - VESA BIOS modes. The ID is a VESA mode ID increased by
125 0x0100. All VESA modes should be autodetected and shown on the menu.
126
127 0x0900 to 0x09ff - Video7 special modes. Set by calling INT 0x10, AX=0x6f05.
128 (Usually 940=80x43, 941=132x25, 942=132x44, 943=80x60, 944=100x60,
129 945=132x28 for the standard Video7 BIOS)
130
131 0x0f00 to 0x0fff - special modes (they are set by various tricks -- usually
132 by modifying one of the standard modes). Currently available:
133 0x0f00 standard 80x25, don't reset mode if already set (=FFFF)
134 0x0f01 standard with 8-point font: 80x43 on EGA, 80x50 on VGA
135 0x0f02 VGA 80x43 (VGA switched to 350 scanlines with a 8-point font)
136 0x0f03 VGA 80x28 (standard VGA scans, but 14-point font)
137 0x0f04 leave current video mode
138 0x0f05 VGA 80x30 (480 scans, 16-point font)
139 0x0f06 VGA 80x34 (480 scans, 14-point font)
140 0x0f07 VGA 80x60 (480 scans, 8-point font)
141 0x0f08 Graphics hack (see the VIDEO_GFX_HACK paragraph below)
142
143 0x1000 to 0x7fff - modes specified by resolution. The code has a "0xRRCC"
144 form where RR is a number of rows and CC is a number of columns.
145 E.g., 0x1950 corresponds to a 80x25 mode, 0x2b84 to 132x43 etc.
146 This is the only fully portable way to refer to a non-standard mode,
147 but it relies on the mode being found and displayed on the menu
148 (remember that mode scanning is not done automatically).
149
150 0xff00 to 0xffff - aliases for backward compatibility:
151 0xffff equivalent to 0x0f00 (standard 80x25)
152 0xfffe equivalent to 0x0f01 (EGA 80x43 or VGA 80x50)
153
154 If you add 0x8000 to the mode ID, the program will try to recalculate
155 vertical display timing according to mode parameters, which can be used to
156 eliminate some annoying bugs of certain VGA BIOSes (usually those used for
157 cards with S3 chipsets and old Cirrus Logic BIOSes) -- mainly extra lines at the
158 end of the display.
159
160 Options
161 ~~~~~~~
162
163 Build options for arch/x86/boot/* are selected by the kernel kconfig
164 utility and the kernel .config file.
165
166 VIDEO_GFX_HACK - includes special hack for setting of graphics modes
167 to be used later by special drivers.
168 Allows to set _any_ BIOS mode including graphic ones and forcing specific
169 text screen resolution instead of peeking it from BIOS variables. Don't use
170 unless you think you know what you're doing. To activate this setup, use
171 mode number 0x0f08 (see the Mode IDs section above).
172
173 Still doesn't work?
174 ~~~~~~~~~~~~~~~~~~~
175
176 When the mode detection doesn't work (e.g., the mode list is incorrect or
177 the machine hangs instead of displaying the menu), try to switch off some of
178 the configuration options listed under "Options". If it fails, you can still use
179 your kernel with the video mode set directly via the kernel parameter.
180
181 In either case, please send me a bug report containing what _exactly_
182 happens and how do the configuration switches affect the behaviour of the bug.
183
184 If you start Linux from M$-DOS, you might also use some DOS tools for
185 video mode setting. In this case, you must specify the 0x0f04 mode ("leave
186 current settings") to Linux, because if you don't and you use any non-standard
187 mode, Linux will switch to 80x25 automatically.
188
189 If you set some extended mode and there's one or more extra lines on the
190 bottom of the display containing already scrolled-out text, your VGA BIOS
191 contains the most common video BIOS bug called "incorrect vertical display
192 end setting". Adding 0x8000 to the mode ID might fix the problem. Unfortunately,
193 this must be done manually -- no autodetection mechanisms are available.
194
195 History
196 ~~~~~~~
197
198 =============== ================================================================
199 1.0 (??-Nov-95) First version supporting all adapters supported by the old
200 setup.S + Cirrus Logic 54XX. Present in some 1.3.4? kernels
201 and then removed due to instability on some machines.
202 2.0 (28-Jan-96) Rewritten from scratch. Cirrus Logic 64XX support added, almost
203 everything is configurable, the VESA support should be much more
204 stable, explicit mode numbering allowed, "scan" implemented etc.
205 2.1 (30-Jan-96) VESA modes moved to 0x200-0x3ff. Mode selection by resolution
206 supported. Few bugs fixed. VESA modes are listed prior to
207 modes supplied by SVGA autodetection as they are more reliable.
208 CLGD autodetect works better. Doesn't depend on 80x25 being
209 active when started. Scanning fixed. 80x43 (any VGA) added.
210 Code cleaned up.
211 2.2 (01-Feb-96) EGA 80x43 fixed. VESA extended to 0x200-0x4ff (non-standard 02XX
212 VESA modes work now). Display end bug workaround supported.
213 Special modes renumbered to allow adding of the "recalculate"
214 flag, 0xffff and 0xfffe became aliases instead of real IDs.
215 Screen contents retained during mode changes.
216 2.3 (15-Mar-96) Changed to work with 1.3.74 kernel.
217 2.4 (18-Mar-96) Added patches by Hans Lermen fixing a memory overwrite problem
218 with some boot loaders. Memory management rewritten to reflect
219 these changes. Unfortunately, screen contents retaining works
220 only with some loaders now.
221 Added a Tseng 132x60 mode.
222 2.5 (19-Mar-96) Fixed a VESA mode scanning bug introduced in 2.4.
223 2.6 (25-Mar-96) Some VESA BIOS errors not reported -- it fixes error reports on
224 several cards with broken VESA code (e.g., ATI VGA).
225 2.7 (09-Apr-96) - Accepted all VESA modes in range 0x100 to 0x7ff, because some
226 cards use very strange mode numbers.
227 - Added Realtek VGA modes (thanks to Gonzalo Tornaria).
228 - Hardware testing order slightly changed, tests based on ROM
229 contents done as first.
230 - Added support for special Video7 mode switching functions
231 (thanks to Tom Vander Aa).
232 - Added 480-scanline modes (especially useful for notebooks,
233 original version written by hhanemaa@cs.ruu.nl, patched by
234 Jeff Chua, rewritten by me).
235 - Screen store/restore fixed.
236 2.8 (14-Apr-96) - Previous release was not compilable without CONFIG_VIDEO_SVGA.
237 - Better recognition of text modes during mode scan.
238 2.9 (12-May-96) - Ignored VESA modes 0x80 - 0xff (more VESA BIOS bugs!)
239 2.10(11-Nov-96) - The whole thing made optional.
240 - Added the CONFIG_VIDEO_400_HACK switch.
241 - Added the CONFIG_VIDEO_GFX_HACK switch.
242 - Code cleanup.
243 2.11(03-May-97) - Yet another cleanup, now including also the documentation.
244 - Direct testing of SVGA adapters turned off by default, ``scan``
245 offered explicitly on the prompt line.
246 - Removed the doc section describing adding of new probing
247 functions as I try to get rid of _all_ hardware probing here.
248 2.12(25-May-98) Added support for VESA frame buffer graphics.
249 2.13(14-May-99) Minor documentation fixes.
250 =============== ================================================================
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