About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / svga.txt




Custom Search

Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 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 (see CONFIG_VIDEO_SVGA in chapter 4 to see
71	how to enable it if you really want) 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 CONFIG_VIDEO_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	Some options can be set in the source text (in arch/i386/boot/video.S).
164	All of them are simple #define's -- change them to #undef's when you want to
165	switch them off. Currently supported:
166	
167	CONFIG_VIDEO_SVGA - enables autodetection of SVGA cards. This is switched
168	off by default as it's a bit unreliable due to terribly bad PC design. If you
169	really want to have the adapter autodetected (maybe in case the ``scan`` feature
170	doesn't work on your machine), switch this on and don't cry if the results
171	are not completely sane. In case you really need this feature, please drop me
172	a mail as I think of removing it some day.
173	
174	CONFIG_VIDEO_VESA - enables autodetection of VESA modes. If it doesn't work
175	on your machine (or displays a "Error: Scanning of VESA modes failed" message),
176	you can switch it off and report as a bug.
177	
178	CONFIG_VIDEO_COMPACT - enables compacting of the video mode list. If there
179	are more modes with the same screen size, only the first one is kept (see above
180	for more info on mode ordering). However, in very strange cases it's possible
181	that the first "version" of the mode doesn't work although some of the others
182	do -- in this case turn this switch off to see the rest.
183	
184	CONFIG_VIDEO_RETAIN - enables retaining of screen contents when switching
185	video modes. Works only with some boot loaders which leave enough room for the
186	buffer. (If you have old LILO, you can adjust heap_end_ptr and loadflags
187	in setup.S, but it's better to upgrade the boot loader...)
188	
189	CONFIG_VIDEO_LOCAL - enables inclusion of "local modes" in the list. The
190	local modes are added automatically to the beginning of the list not depending
191	on hardware configuration. The local modes are listed in the source text after
192	the "local_mode_table:" line. The comment before this line describes the format
193	of the table (which also includes a video card name to be displayed on the
194	top of the menu).
195	
196	CONFIG_VIDEO_400_HACK - force setting of 400 scan lines for standard VGA
197	modes. This option is intended to be used on certain buggy BIOSes which draw
198	some useless logo using font download and then fail to reset the correct mode.
199	Don't use unless needed as it forces resetting the video card.
200	
201	CONFIG_VIDEO_GFX_HACK - includes special hack for setting of graphics modes
202	to be used later by special drivers (e.g., 800x600 on IBM ThinkPad -- see
203	ftp://ftp.phys.keio.ac.jp/pub/XFree86/800x600/XF86Configs/XF86Config.IBM_TP560).
204	Allows to set _any_ BIOS mode including graphic ones and forcing specific
205	text screen resolution instead of peeking it from BIOS variables. Don't use
206	unless you think you know what you're doing. To activate this setup, use
207	mode number 0x0f08 (see section 3).
208	
209	Still doesn't work?
210	~~~~~~~~~~~~~~~~~~~
211	
212	When the mode detection doesn't work (e.g., the mode list is incorrect or
213	the machine hangs instead of displaying the menu), try to switch off some of
214	the configuration options listed in section 4. If it fails, you can still use
215	your kernel with the video mode set directly via the kernel parameter.
216	
217	In either case, please send me a bug report containing what _exactly_
218	happens and how do the configuration switches affect the behaviour of the bug.
219	
220	If you start Linux from M$-DOS, you might also use some DOS tools for
221	video mode setting. In this case, you must specify the 0x0f04 mode ("leave
222	current settings") to Linux, because if you don't and you use any non-standard
223	mode, Linux will switch to 80x25 automatically.
224	
225	If you set some extended mode and there's one or more extra lines on the
226	bottom of the display containing already scrolled-out text, your VGA BIOS
227	contains the most common video BIOS bug called "incorrect vertical display
228	end setting". Adding 0x8000 to the mode ID might fix the problem. Unfortunately,
229	this must be done manually -- no autodetection mechanisms are available.
230	
231	If you have a VGA card and your display still looks as on EGA, your BIOS
232	is probably broken and you need to set the CONFIG_VIDEO_400_HACK switch to
233	force setting of the correct mode.
234	
235	History
236	~~~~~~~
237	
238	=============== ================================================================
239	1.0 (??-Nov-95)	First version supporting all adapters supported by the old
240			setup.S + Cirrus Logic 54XX. Present in some 1.3.4? kernels
241			and then removed due to instability on some machines.
242	2.0 (28-Jan-96)	Rewritten from scratch. Cirrus Logic 64XX support added, almost
243			everything is configurable, the VESA support should be much more
244			stable, explicit mode numbering allowed, "scan" implemented etc.
245	2.1 (30-Jan-96) VESA modes moved to 0x200-0x3ff. Mode selection by resolution
246			supported. Few bugs fixed. VESA modes are listed prior to
247			modes supplied by SVGA autodetection as they are more reliable.
248			CLGD autodetect works better. Doesn't depend on 80x25 being
249			active when started. Scanning fixed. 80x43 (any VGA) added.
250			Code cleaned up.
251	2.2 (01-Feb-96)	EGA 80x43 fixed. VESA extended to 0x200-0x4ff (non-standard 02XX
252			VESA modes work now). Display end bug workaround supported.
253			Special modes renumbered to allow adding of the "recalculate"
254			flag, 0xffff and 0xfffe became aliases instead of real IDs.
255			Screen contents retained during mode changes.
256	2.3 (15-Mar-96)	Changed to work with 1.3.74 kernel.
257	2.4 (18-Mar-96)	Added patches by Hans Lermen fixing a memory overwrite problem
258			with some boot loaders. Memory management rewritten to reflect
259			these changes. Unfortunately, screen contents retaining works
260			only with some loaders now.
261			Added a Tseng 132x60 mode.
262	2.5 (19-Mar-96)	Fixed a VESA mode scanning bug introduced in 2.4.
263	2.6 (25-Mar-96)	Some VESA BIOS errors not reported -- it fixes error reports on
264			several cards with broken VESA code (e.g., ATI VGA).
265	2.7 (09-Apr-96)	- Accepted all VESA modes in range 0x100 to 0x7ff, because some
266			  cards use very strange mode numbers.
267			- Added Realtek VGA modes (thanks to Gonzalo Tornaria).
268			- Hardware testing order slightly changed, tests based on ROM
269			  contents done as first.
270			- Added support for special Video7 mode switching functions
271			  (thanks to Tom Vander Aa).
272			- Added 480-scanline modes (especially useful for notebooks,
273			  original version written by hhanemaa@cs.ruu.nl, patched by
274			  Jeff Chua, rewritten by me).
275			- Screen store/restore fixed.
276	2.8 (14-Apr-96) - Previous release was not compilable without CONFIG_VIDEO_SVGA.
277			- Better recognition of text modes during mode scan.
278	2.9 (12-May-96)	- Ignored VESA modes 0x80 - 0xff (more VESA BIOS bugs!)
279	2.10(11-Nov-96) - The whole thing made optional.
280			- Added the CONFIG_VIDEO_400_HACK switch.
281			- Added the CONFIG_VIDEO_GFX_HACK switch.
282			- Code cleanup.
283	2.11(03-May-97) - Yet another cleanup, now including also the documentation.
284			- Direct testing of SVGA adapters turned off by default, ``scan``
285			  offered explicitly on the prompt line.
286			- Removed the doc section describing adding of new probing
287			  functions as I try to get rid of _all_ hardware probing here.
288	2.12(25-May-98) Added support for VESA frame buffer graphics.
289	2.13(14-May-99) Minor documentation fixes.
290	=============== ================================================================
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.