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Based on kernel version 4.15. Page generated on 2018-01-29 10:01 EST.

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