251 lines
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ReStructuredText
251 lines
12 KiB
ReStructuredText
.. include:: <isonum.txt>
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=================================
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Video Mode Selection Support 2.13
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=================================
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:Copyright: |copy| 1995--1999 Martin Mares, <mj@ucw.cz>
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Intro
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~~~~~
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This small document describes the "Video Mode Selection" feature which
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allows the use of various special video modes supported by the video BIOS. Due
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to usage of the BIOS, the selection is limited to boot time (before the
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kernel decompression starts) and works only on 80X86 machines that are
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booted through BIOS firmware (as opposed to through UEFI, kexec, etc.).
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.. note::
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Short intro for the impatient: Just use vga=ask for the first time,
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enter ``scan`` on the video mode prompt, pick the mode you want to use,
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remember its mode ID (the four-digit hexadecimal number) and then
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set the vga parameter to this number (converted to decimal first).
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The video mode to be used is selected by a kernel parameter which can be
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specified in the kernel Makefile (the SVGA_MODE=... line) or by the "vga=..."
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option of LILO (or some other boot loader you use) or by the "xrandr" utility
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(present in standard Linux utility packages). You can use the following values
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of this parameter::
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NORMAL_VGA - Standard 80x25 mode available on all display adapters.
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EXTENDED_VGA - Standard 8-pixel font mode: 80x43 on EGA, 80x50 on VGA.
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ASK_VGA - Display a video mode menu upon startup (see below).
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0..35 - Menu item number (when you have used the menu to view the list of
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modes available on your adapter, you can specify the menu item you want
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to use). 0..9 correspond to "0".."9", 10..35 to "a".."z". Warning: the
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mode list displayed may vary as the kernel version changes, because the
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modes are listed in a "first detected -- first displayed" manner. It's
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better to use absolute mode numbers instead.
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0x.... - Hexadecimal video mode ID (also displayed on the menu, see below
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for exact meaning of the ID). Warning: LILO doesn't support
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hexadecimal numbers -- you have to convert it to decimal manually.
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Menu
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~~~~
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The ASK_VGA mode causes the kernel to offer a video mode menu upon
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bootup. It displays a "Press <RETURN> to see video modes available, <SPACE>
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to continue or wait 30 secs" message. If you press <RETURN>, you enter the
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menu, if you press <SPACE> or wait 30 seconds, the kernel will boot up in
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the standard 80x25 mode.
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The menu looks like::
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Video adapter: <name-of-detected-video-adapter>
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Mode: COLSxROWS:
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0 0F00 80x25
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1 0F01 80x50
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2 0F02 80x43
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3 0F03 80x26
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....
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Enter mode number or ``scan``: <flashing-cursor-here>
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<name-of-detected-video-adapter> tells what video adapter did Linux detect
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-- it's either a generic adapter name (MDA, CGA, HGC, EGA, VGA, VESA VGA [a VGA
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with VESA-compliant BIOS]) or a chipset name (e.g., Trident). Direct detection
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of chipsets is turned off by default as it's inherently unreliable due to
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absolutely insane PC design.
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"0 0F00 80x25" means that the first menu item (the menu items are numbered
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from "0" to "9" and from "a" to "z") is a 80x25 mode with ID=0x0f00 (see the
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next section for a description of mode IDs).
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<flashing-cursor-here> encourages you to enter the item number or mode ID
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you wish to set and press <RETURN>. If the computer complains something about
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"Unknown mode ID", it is trying to tell you that it isn't possible to set such
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a mode. It's also possible to press only <RETURN> which leaves the current mode.
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The mode list usually contains a few basic modes and some VESA modes. In
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case your chipset has been detected, some chipset-specific modes are shown as
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well (some of these might be missing or unusable on your machine as different
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BIOSes are often shipped with the same card and the mode numbers depend purely
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on the VGA BIOS).
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The modes displayed on the menu are partially sorted: The list starts with
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the standard modes (80x25 and 80x50) followed by "special" modes (80x28 and
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80x43), local modes (if the local modes feature is enabled), VESA modes and
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finally SVGA modes for the auto-detected adapter.
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If you are not happy with the mode list offered (e.g., if you think your card
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is able to do more), you can enter "scan" instead of item number / mode ID. The
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program will try to ask the BIOS for all possible video mode numbers and test
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what happens then. The screen will be probably flashing wildly for some time and
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strange noises will be heard from inside the monitor and so on and then, really
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all consistent video modes supported by your BIOS will appear (plus maybe some
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``ghost modes``). If you are afraid this could damage your monitor, don't use
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this function.
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After scanning, the mode ordering is a bit different: the auto-detected SVGA
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modes are not listed at all and the modes revealed by ``scan`` are shown before
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all VESA modes.
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Mode IDs
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~~~~~~~~
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Because of the complexity of all the video stuff, the video mode IDs
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used here are also a bit complex. A video mode ID is a 16-bit number usually
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expressed in a hexadecimal notation (starting with "0x"). You can set a mode
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by entering its mode directly if you know it even if it isn't shown on the menu.
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The ID numbers can be divided to those regions::
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0x0000 to 0x00ff - menu item references. 0x0000 is the first item. Don't use
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outside the menu as this can change from boot to boot (especially if you
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have used the ``scan`` feature).
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0x0100 to 0x017f - standard BIOS modes. The ID is a BIOS video mode number
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(as presented to INT 10, function 00) increased by 0x0100.
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0x0200 to 0x08ff - VESA BIOS modes. The ID is a VESA mode ID increased by
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0x0100. All VESA modes should be autodetected and shown on the menu.
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0x0900 to 0x09ff - Video7 special modes. Set by calling INT 0x10, AX=0x6f05.
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(Usually 940=80x43, 941=132x25, 942=132x44, 943=80x60, 944=100x60,
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945=132x28 for the standard Video7 BIOS)
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0x0f00 to 0x0fff - special modes (they are set by various tricks -- usually
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by modifying one of the standard modes). Currently available:
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0x0f00 standard 80x25, don't reset mode if already set (=FFFF)
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0x0f01 standard with 8-point font: 80x43 on EGA, 80x50 on VGA
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0x0f02 VGA 80x43 (VGA switched to 350 scanlines with a 8-point font)
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0x0f03 VGA 80x28 (standard VGA scans, but 14-point font)
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0x0f04 leave current video mode
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0x0f05 VGA 80x30 (480 scans, 16-point font)
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0x0f06 VGA 80x34 (480 scans, 14-point font)
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0x0f07 VGA 80x60 (480 scans, 8-point font)
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0x0f08 Graphics hack (see the VIDEO_GFX_HACK paragraph below)
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0x1000 to 0x7fff - modes specified by resolution. The code has a "0xRRCC"
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form where RR is a number of rows and CC is a number of columns.
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E.g., 0x1950 corresponds to a 80x25 mode, 0x2b84 to 132x43 etc.
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This is the only fully portable way to refer to a non-standard mode,
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but it relies on the mode being found and displayed on the menu
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(remember that mode scanning is not done automatically).
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0xff00 to 0xffff - aliases for backward compatibility:
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0xffff equivalent to 0x0f00 (standard 80x25)
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0xfffe equivalent to 0x0f01 (EGA 80x43 or VGA 80x50)
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If you add 0x8000 to the mode ID, the program will try to recalculate
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vertical display timing according to mode parameters, which can be used to
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eliminate some annoying bugs of certain VGA BIOSes (usually those used for
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cards with S3 chipsets and old Cirrus Logic BIOSes) -- mainly extra lines at the
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end of the display.
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Options
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~~~~~~~
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Build options for arch/x86/boot/* are selected by the kernel kconfig
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utility and the kernel .config file.
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VIDEO_GFX_HACK - includes special hack for setting of graphics modes
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to be used later by special drivers.
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Allows to set _any_ BIOS mode including graphic ones and forcing specific
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text screen resolution instead of peeking it from BIOS variables. Don't use
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unless you think you know what you're doing. To activate this setup, use
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mode number 0x0f08 (see the Mode IDs section above).
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Still doesn't work?
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~~~~~~~~~~~~~~~~~~~
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When the mode detection doesn't work (e.g., the mode list is incorrect or
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the machine hangs instead of displaying the menu), try to switch off some of
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the configuration options listed under "Options". If it fails, you can still use
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your kernel with the video mode set directly via the kernel parameter.
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In either case, please send me a bug report containing what _exactly_
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happens and how do the configuration switches affect the behaviour of the bug.
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If you start Linux from M$-DOS, you might also use some DOS tools for
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video mode setting. In this case, you must specify the 0x0f04 mode ("leave
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current settings") to Linux, because if you don't and you use any non-standard
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mode, Linux will switch to 80x25 automatically.
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If you set some extended mode and there's one or more extra lines on the
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bottom of the display containing already scrolled-out text, your VGA BIOS
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contains the most common video BIOS bug called "incorrect vertical display
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end setting". Adding 0x8000 to the mode ID might fix the problem. Unfortunately,
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this must be done manually -- no autodetection mechanisms are available.
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History
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~~~~~~~
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=============== ================================================================
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1.0 (??-Nov-95) First version supporting all adapters supported by the old
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setup.S + Cirrus Logic 54XX. Present in some 1.3.4? kernels
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and then removed due to instability on some machines.
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2.0 (28-Jan-96) Rewritten from scratch. Cirrus Logic 64XX support added, almost
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everything is configurable, the VESA support should be much more
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stable, explicit mode numbering allowed, "scan" implemented etc.
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2.1 (30-Jan-96) VESA modes moved to 0x200-0x3ff. Mode selection by resolution
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supported. Few bugs fixed. VESA modes are listed prior to
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modes supplied by SVGA autodetection as they are more reliable.
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CLGD autodetect works better. Doesn't depend on 80x25 being
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active when started. Scanning fixed. 80x43 (any VGA) added.
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Code cleaned up.
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2.2 (01-Feb-96) EGA 80x43 fixed. VESA extended to 0x200-0x4ff (non-standard 02XX
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VESA modes work now). Display end bug workaround supported.
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Special modes renumbered to allow adding of the "recalculate"
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flag, 0xffff and 0xfffe became aliases instead of real IDs.
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Screen contents retained during mode changes.
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2.3 (15-Mar-96) Changed to work with 1.3.74 kernel.
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2.4 (18-Mar-96) Added patches by Hans Lermen fixing a memory overwrite problem
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with some boot loaders. Memory management rewritten to reflect
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these changes. Unfortunately, screen contents retaining works
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only with some loaders now.
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Added a Tseng 132x60 mode.
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2.5 (19-Mar-96) Fixed a VESA mode scanning bug introduced in 2.4.
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2.6 (25-Mar-96) Some VESA BIOS errors not reported -- it fixes error reports on
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several cards with broken VESA code (e.g., ATI VGA).
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2.7 (09-Apr-96) - Accepted all VESA modes in range 0x100 to 0x7ff, because some
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cards use very strange mode numbers.
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- Added Realtek VGA modes (thanks to Gonzalo Tornaria).
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- Hardware testing order slightly changed, tests based on ROM
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contents done as first.
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- Added support for special Video7 mode switching functions
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(thanks to Tom Vander Aa).
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- Added 480-scanline modes (especially useful for notebooks,
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original version written by hhanemaa@cs.ruu.nl, patched by
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Jeff Chua, rewritten by me).
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- Screen store/restore fixed.
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2.8 (14-Apr-96) - Previous release was not compilable without CONFIG_VIDEO_SVGA.
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- Better recognition of text modes during mode scan.
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2.9 (12-May-96) - Ignored VESA modes 0x80 - 0xff (more VESA BIOS bugs!)
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2.10(11-Nov-96) - The whole thing made optional.
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- Added the CONFIG_VIDEO_400_HACK switch.
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- Added the CONFIG_VIDEO_GFX_HACK switch.
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- Code cleanup.
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2.11(03-May-97) - Yet another cleanup, now including also the documentation.
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- Direct testing of SVGA adapters turned off by default, ``scan``
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offered explicitly on the prompt line.
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- Removed the doc section describing adding of new probing
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functions as I try to get rid of _all_ hardware probing here.
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2.12(25-May-98) Added support for VESA frame buffer graphics.
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2.13(14-May-99) Minor documentation fixes.
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=============== ================================================================
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