Provided by: svgatextmode_1.9-20_i386
grabmode - a Textmode/XWindows mode grabber
clockprobe - a Textmode/XWindows pixel clock probe
grabmode [ -ndrhXTbcpv ] [ -x <width> ] [ -y <height> ]
clockprobe [ -ndrhv ] [ -x <width> ] [ -y <height> ]
clockprobe tries to measure the pixel clock which is currently used for
the display that is visible while the probe is running, be it a
textmode display or a graphical one. Its default behaviour is to
display the pixel clock value in MegaHerz, together with the horizontal
and vertical refresh frequencies.
Grabmode basically does the same, but also grabs the entire state of
the VGA controller, and shows it on the screen in a format compatible
with the XF86Config file, the XFree86 X-server configuration file, or
alternatively in the TextConfig file format, which is the config file
format used by SVGATextMode.
Both programs are in fact the same, but depending on the name with
which it is called, it does a clockprobe or a grabmode (clockprobe is a
link to grabmode).
It does not change anything to the mode currently being probed. VGA
registers are read only. This means it does NOT actually change pixel
clocks like the X-server does when it probes for the clocks. This also
implies that it should work on ANY VGA card, even if it is unsupported
by either XFree86 or SVGATextMode. This is because it only uses
standard VGA registers. No extended registers are used.
That also uncovers the biggest problem with grabmode: since it does not
know about extended registers, it doesn’t know about special chipset-
dependent modes like 16/24/32-bit mode, or interlaced modes. See the
BUGS section for more details. The mode grabber tries to be as clever
as possible about interpreting the data retrieved from the VGA card,
but it doesn’t always get it right. Using grabmode requires some common
sense from the user to interpret the data.
Grabbing a standard VGA graphics mode (from svgalib or dosemu, or even
from DOS itself, using grabmode.exe ) for inclusion in your own
TextConfig (or XF86Config) file is probably the most attractive one.
"vgaset", included in most X-distributions, can give you that, but it
does NOT tell you the pixel clock currently used, neither does it tell
you about doublescan modes, sync polarities, etc. The output of
grabmode can be pasted right into your XF86Config file (if it’s
Of course, you could also grab the default Linux (and/or DOS) text
modes to include them in your TextConfig file. So you can show your
friends how the old modes looked, and how the SVGATextMode-"enhanced"
modes look in contrast ;-)
Debugging XFree86 or SVGATextMode setups is another one. E.g. if
SVGATextMode or XFree86 fails to show you the kind of screen you’d
expect, and/or your monitor doesn’t seem to like what you send it, then
the most common reason for that is that the pixel clock is not
programmed correctly, or not at all. Clockprobe or grabmode (even when
typed blindly, redirected to a file for later viewing) should be able
to tell you what clock you are ACTUALLY using. It’s not because your
"clocks" line says "50 MHz" for some clock, that that clock index
actually IS 50 MHz. You could be using the wrong clocks line, or there
could be a bug in the clock setting code. In either case, the
clockprobe can tell you the ACTUAL clock.
The first group of options are valid for both grabmode and clockprobe.
-n don’t touch VGA registers, not even to read them. Of course,
nothing useful will be done either. This option is more for
completeness, and for debugging.
-d debugging mode. grabmode/clockprobe will output lots of
debugging information about all the things it attempts to do.
This is mostly for "internal purposes" only, but it could help
you discover why grabmode doesn’t react as you expect it to.
This could also be useful when reporting a problem.
-h prints out a short help and the version number.
-r "raw" mode. All smartwork will be skipped. The program will not
attempt to be smart about displaying the mode line and/or the
clock calue. Grabmode prints values as they are in the VGA
registers, clockprobe shows the actual clock used inside the
VGA, not the per-pixel clock. See the SMART BOMB section below
for more information on how smart/stupid grabmode can be.
-v verbose mode. Report all special mode flags. Only useful to VGA
-y <height> Specify width of the screen mode to be grabbed. In
most cases, the screen width and height are known to you in
advance. E.g. you know you’ll be grabbing a 1024x768 mode, and
all you want are the exact timings.
Specifying the width and/or height for the mode grabber or the
clock probe will bypass all the guessing (see "bugs" and "smart
bombs" section) it would normally have to do in order to detect
special modes (15/16/24 BPP, interlace, ...). This will result
in a much more reliable mode line and/or clock frequency report.
This option is for the clockprobe only:
-p print only the clock value (in MHz), without ANY further
comment. This is probably useful in scripts and pipes.
These options are for grabmode only:
-X grabmode normally checks to see if you are grabbing a text mode
or a graphics mode. It then adapts its output so it complies
with a TextConfig mode line (for text modes) or with an
XF86Config mode line (for graphics modes). This option forces it
to output an XF86Config compatible mode line.
-T The opposite of the -X option above: output a TextConfig mode
line, even if you’re grabbing a graphics mode.
Since graphical screens have nothing to do with hardware
assisted font rendering (as from VGA text mode - let’s forget
about accelerators for a while), they set the "character height"
to 1 pixel (in fact they have to, otherwise each line gets
repeated 16 times, if they leave the "16" from the 9x16
character mode in there!). So font size given will of course be
wrong. In most cases you will be presented with am 8x1 font
(8x2 in some DoubleScan modes), and with a mode label equally
distorted. grabbing a graphical 1024x768 mode this way would
result in a mode line labelled "128x768" with a font size of
8x1. You’ll have to change the font size (and the label) if you
want to turn that into a useful text mode.
This could be used to create a text mode completely compatible
with an X-Windows mode you already have. switching from one to
the other would be much faster, since the monitor wouldn’t have
-b show video blanking information in addition to the normal mode
line timing information. Not very useful, unless you want to
know everything about a VGA mode. XF86Config nor TextConfig
allow you to specify the blanking. Both automatically program a
suitable (though different) blanking. If you are using the VGA
overscan color for some reason, then this option will tell you
how far the overscan color will reach.
-c Do not probe for the pixel clock, just print the mode line with
the clock replaced by "***". Not probing for the pixel clock
makes grabmode a lot faster.
clockprobe: Estimated vertical scanrate = 69.754 Hz.
clockprobe: Estimated horizontal scanrate = 55.803 kHz.
clockprobe: Estimated pixel clock = 74.83 MHz
Note that the clockprobe (both in clockprobe and grabmode) only
measures the vertical refresh rate. The rest is calculated from the VGA
register contents and the vertical refresh. Extended and special modes
which could cause grabmode to mis-detect a mode, will also cause the
horizontal frequency and/or the pixel clock to be wrong.
(the output comes on one line)
"116x48" 74.832 928 968 1104 1192 768 775 776 800 +Hsync
+Vsync font 9x16 # 55.803kHz/69.75Hz
adds one more line to that:
# Active special VGA register flags: WORDMODE
Grabmode, and to a lesser extent, clockprobe, have some intelligence
built-in, so they can cope with non-standard VGA configurations. As
said before, both programs use standard VGA registers only to get their
information. Only, nobody has a standard VGA card nowadays.
To remain chipset-independent, grabmode and clockprobe try to guess
from what they can read in the standard VGA registers what mode you
could actually be using.
A program can only be as intelligent as its creator (except by
mistake). Well, grabmode is extremely stupid on some occasions.
Because of that, it will report all non-trivial changes that it made to
the original timings (as read from the VGA) before printing it out, and
also WHY it did that.
That’s why this section is called "smart bombs": When one of those gets
The built-in smart (hah!) features are:
This is a rather trivial feature, and is (for as far as I know)
reliable enough to go without warning. Doublescan is a way to display
modes with very few lines on a screen that was made to display many
more. The most common application is the CGA-emulation mode 320x200.
Displaying only 200 lines on a VGA screen would look awful (you’d see
large black lines in between the 200 video lines).
Doublescan displays each line twice, so there are actually 400 lines on
screen, but only 200 different ones. In this mode, the VGA is
programmed for 400 lines, but with Doublescan enabled, the ACTUAL line
count is half that. Grabmode will thus divide all vertical timings by
2 before displaying them.
vertical timing wrap detection
The standard VGA register set only allows for a maximum of 2048 pixels
horizontally and 1024 lines vertically (this is the FULL size,
including sync and blanking, so the actual visible size is even
smaller). Modern SVGA cards can do modes with 1024 active lines and
more, requiring more than 1024 lines in total. A 1280x1024 mode for
example needs 1024 active lines. Add sync and blanking time to that,
and the grand total is over 1024 lines.
Although standard VGA has a "vertical double" mode bit, which
multiplies all vertical values by 2, most SVGA drivers prefer to use
extended (chipset dependent) registers to accomplish that. Grabmode
cannot read these extended registers (we want to keep it chipset-
independent, remember?), so it will show wrapped-around timings for
those modes with around 1024 lines.
Depending on how exactly a mode is implemented on your specific
VGA card, a 1280x1024 mode at 60 Hz, which should be detected
"1280x1024" 110.34 1280 1328 1512 1712 1024 1025 1028 1054
will be detected (in "raw" mode) as:
"1280x1024" 3.131 1280 1328 1512 1712 1024 1 4 30
Grabmode will see that the sequence of vertical timings is wrong, and
will assume that this was an extended mode, using extended registers.
It will correct the sequence to make it look like the correct one.
This feature only works when the vertical timings happen to cross over
the 10-bit standard VGA limit, which means it will only work for
1280x1024 modes. Far from perfect, but better than not at all...
Golden ratio assumption
The "golden ratio" rule suggests that most displays have a physical
aspect ratio of 4x3 (width/height = 4/3). Most applications assume
pixels are square (same width and height), so the normal case would be
to use the same ratio in programming the VGA card.
640x480 modes have an aspect ratio of 640/480 = 1.333 (=4/3). 800x600,
1024x768, 1280x1024, 1600x1200: all those modes use an aspect ratio of
4/3 (or a close match) (anyone have a 16/9 computer display -- they
If grabmode detects a mode that looks like double or triple that ratio,
it assumes this is a special mode (see below), and divides the
horizontal timings by 2 or 3. In most common cases, this is the correct
result. An appropriate warning is given.
The golden ratio assumption is then used by the CGA emulation detector,
the Hi-TrueColor mode detector and the interlace detector to adjust the
Grabmode (and clockprobe) report the _actual_ pixel clock used at which
data is being transferred from the VGA chip to the RAMDAC. This means
that any special data transfer scheme will mislead the clock probe,
causing a faulty report.
HighColor and TrueColor modes on most cheap cards are one such example:
the VGA registers for e.g. a 640x480x16M mode on a normal VGA card with
a standard 8-bit RAMDAC (like most cheap VGA cards, e.g. using the S3
GenDAC or ATT20C490/1/2) are set up as if the mode was actually 2x (16
bpp) or 3x (24 bpp) or 4x (32 bpp) the actual horizontal width, and the
pixel clock is multiplied by the same value. So in that case a 24-bit
640x480 mode at 25 MHz pixel clock is programmed in the VGA chip as a
1920x480 mode at 75 MHz! For this mode, the RAMDAC is programmed to
assemble three consecutive bytes into one giant 24-bit color pixel.
This ramdac programming is very RAMDAC-dependent, so it is impossible
to detect what is going on without knowing the RAMDAC type. RAMDAC’s
with 16-bit interfaces (like the S3 SDAC) use this scheme for 24/32-bit
modes: they transfer one 24-bit pixel in 2 clock periods, or, worse
even, for 24-bit packed pixel modes, in 3/2 clock periods...
To make things even worse, grabmode will not even correctly report
simple 256-color (8-bit) modes on some cards. Especially high-end cards
(like #9, Diamond, ...) using special VGA DAC’s (Bt485,...) with pixel
multiplexing. They will report completely wrong clocks and refresh
frequencies things on higher (>67.5 MHz, for example) pixel clocks.
Such RAMDAC’s can transfer 2 or more entire pixels in one clock period,
the opposite of simple RAMDAC’s in 16/24/32-bit modes.
On many (if not all) VGA cards, interlaced modes can be used to allow a
high resolution mode to be displayed on a low-end display (= with a low
maximum horizontal scan rate) without having to resort to ridiculously
low vertical refresh rates.
In order to do this, they program the VGA chip with HALF the amount of
actual lines on the screen, and set a special bit that tells the chip
this is an interlaced mode.
This "interlace-bit" is, again, in the extended (non-standard) VGA
registers, and can thus not be detected by the mode grabber. What _can_
be detected is that the amount of lines programmed in the VGA registers
is only half of the height of the screen. This would be easy if there
was a way to know how much lines the screen actually has...
But since there isn’t, grabmode must resort to some assumptions:
assuming that most graphics modes use one the standard screen sizes
the mode grabber can then check if the current mode has exactly HALF
the amount of lines of one of these modes: 240, 300, ..., 600 lines. If
so, it ASSUMES (!) that this is an interlaced mode, and changes the
timings accordingly before displaying them.
This immediately shows that the mode grabber will not be able to detect
non-standard interlaced modes.
CGA emulation detection
A very popular graphics mode is the 320x200 mode with 256 colors. This
is CGA resolution, but with 256 colors (BIOS mode 13h): the only
standard VGA 256 color mode. In this mode, the VGA reads 4 bits per
pixel clock, assembles each 2 groups of 4 bits into one 8-bit pixel and
sends this to the VGA. The actual VGA registers are thus programmed for
a 640x400 mode: 400 lines with DoubleScan is 200 real lines, and 640
pixels with each 2 pixels output as one is 320 real pixels. Why make it
This pixel joining scheme is enabled by a certain bit in the VGA
registers, which is detected by grabmode to trigger it to transform the
mode into a "CGA" emulated mode. Easy.
BUT. Extended VGA modes use extended VGA registers, and those have
priority over the standard VGA registers. Hence, even if this special
CGA-emulation bit is set ON, it doesn’t necessarily mean pixels are
All in all, even if this special mode is set in the standard VGA
registers, the extended registers could override that. Detecting this
mode is thus not trivial. This is why the "golden ratio" detection is
used for this: after doublescan has been accounted for, the mode
timings come out as 640x200, which triggers the golden ratio detector
to transform this into 320x200, with an appropriate warning.
Vertical retrace timeout detection
The clock probe measures the average time between two vertical retraces
(busy-waiting. Yuck!). This means it will wait forever when there is no
vertical retrace signal. This can be caused either by a program
actually wrecking the current video mode (SVGATextMode has been known
to do that :-) or when the display system is in power-saving mode.
When there is no vertical retrace detected within a few seconds, the
probe assumes it will never get one, and bail out with a message to
that extent. This avoids that the probe hangs forever.
grabmode.exe & scanmode.exe (DOS port of grabmode)
There is a DOS port of grabmode in the SVGATextMode distribution, which
allows grabbing modes from dos.
There’s one caveat to this. Previous versions of the DOS grabmode were
compiled using a commercial DOS C compiler, and required a lot of
special code. The latest version was compiled using DJGPP version 2.0,
the GNU C compiler for DOS. This required almost no special code, and
thus it is a cleaner port. BUT... It needs DPMI (DOS Protected Mode
Interface). A DMPI environment can be found in MS-Windows (DOS boxes),
OS/2, Windows-NT, qdpmi, 386max, etc. A free DMPI program is included
in the distribution (The DJGPP DPMI program CWSDPMI).
Scanmode.exe scans all VGA modes from the DOS BIOS, including the VESA
modes, and outputs two files in the current directory: tconfig and
xconfig. The text modes are stored in the tconfig file, and the
graphics modes are stored in xconfig.
The DOS port of all SVGATextMode programs is UNSUPPORTED. This
means I am mostly unable and, together with the people who
helped me with this, rather unwilling to put much effort in
improving the DOS side of these tools. Consider the DOS port a
proof-of-concept. It was just plain fun to see the DOS GCC
compiler was so incredibly compatible with it’s UNIX parent.
Is there an entry in the Guinness book of Records for amount-of-
bugs/code-size ratio? This program would come in second (just behind
clock probing accuracy
The clock probe (used by clockprobe and grabmode) works best when the
system is not severely loaded. Especially disk access seems to have a
bad influence. DOSEMU is even worse. Very slow machines (i386) might
run out of steam while measuring. On 486 machines, the probe works
However, even busy machines should give a reasonable result, unlike the
X clock probe. Even heavily loaded (swapping!) machines should give a
rather close estimate (maybe a few MHz off, but that should be all).
System loads to up to 3 or 4 will probably still yield results which
are only a few MHz wrong.
The clockprobe first performs 100 measurements of the time between two
vertical retraces, then calculates a histogram of those measurements.
If the histogram does not have a nice peak (mostly due to heavy system
load, causing many measurements to be wrong), the measurement is done
all over again. After three failures, the clock probe will stop
retrying, using the last measurement, but also warning you about the
unreliability of the result.
In most cases, the clock probe will measure a value slightly higher or
slightly lower than you expected. It might be for example that you
programmed a mode line that specified a 70.000 MHz pixel clock, but the
clock probe insists that it is 75.175 MHz. This is due to the fact that
any clock generator cannot produce just _any_ clock you ask for, but
must pick the closest available one, either from a list (the `Clocks´
line in SVGATextMode or XFree86), or from the possible settings on a
programmable clock chip.
If the clockprobe insists on the same value over and over, it means
that that is the actual clock. If it insists on something far from the
one you expected, then it’s either a bug in the probe, or a
misconception on your side.
Some special modes like CGA emulation, HiCilor, TrueColor, pixel
multiplexing, ..., will cause grabmode to fail to detect the correct
number of pixels per line. And because the horizontal sync and clock
speed are all calculated from the H and V timings, they too are wrong.
The vertical sync speed is ALWAYS correct, since it is MEASURED,
independently from ANY timing register.
The DOS grabmode/clockprobe programs will report heavy system load when
they are ran from a Windows DOS box. Even when you are not running any
other program. This is typical Windows behaviour: wasting time while
When executed in an MSWindows DOS box, you will ALWAYS probe the VGA
parameters from the full-screen DOS version, even when you are running
the DOS box in a window and not full-screen. This is because MSWindows
hides its VGA registers from normal programs (probably through the MMU
address mapping), so the mode grabber gets fooled into believing this
is still a text mode box.
For the same reason, doing a grabmode in a DOS window will even lie
about the font size. Windows will give you an "appropriate" font size,
which doesn’t have to be the real one.
A Full-screen DOS box WILL give a correct probe, although there could
be a warning about serious system-load and the resulting timing-
Clock probing is a DOS window is a hoax. You will get extremely high
clock rates, or even floating point overflows. Use the "-c" option
(don’t probe clock rate) to avoid this.
Conclusion: Probing a DOS window is TOTALLY USELESS. You get the wrong
VGA register contents (those from the text mode, not those from the
Windows mode), and the wrong pixel clock.
Too many to count. Or even more...
Most bugs not yet mentioned above result from the various assumptions
the mode grabber has to make along the way, which in turn is caused by
the wealth of different methods VGA chip makers come up with to make
special mode programming non-standard.
As mentioned above, the following special modes could cause grabmode
and/or clockprobe to get it all wrong:
Pixel multiplexed modes
Extended VGA modes ( > 1280x1024)
The reasons for this are explained in one of the sections above.
If you don’t trust the result, use the "-r" (raw probing), and add some
common sense. Then determine what the real mode parameters should be.
In most cases, you know what the actual mode is (e.g. 1024x768), so you
can use that knowledge to interpret the raw timing report.
Some IBM 8514 compatible cards (like ATI Mach) use a completely
different set of registers than the normal VGA set when they are in
graphics mode. Because of the chipset-independence of grabmode,
grabmode will not notice this, and report the values which are in the
now unused VGA register set. In most cases, this is the text mode from
which the graphics program (XFree86, for example) was started.
The main reason for all these problems are the author’s stubbornness
(combined with a fair amount of laziness) to keep grabmode chipset-
Grabmode is only reliable when used to grab a pure text mode, or when
the mode sizes are given on the command line. I just HATE "intelligent"
software, don’t you?
clockprobe nor grabmode require any external files. They do NOT use the
Koen Gadeyne <email@example.com> (original Linux version)
See the CREDITS file in the distribution for a full list of all those
SVGATextMode(8) - Textmode manipulation/enhancement tool
TextConfig(5) - Configuration file for SVGATextMode
XF86Config(5) - Configuration file for XFree86
The ‘doc’ directory in the SVGATextMode distribution contains a lot of
miscellaneous documentation on a range of topics related to configuring
and using SVGATextMode. The monitor timing tutorial in the SVGATextMode
distribution could be a handy reference for understanding mode timings.