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Section: Maintenance Commands (8) Updated: October 1998
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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 correct...).

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.

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.

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.

prints out a short help and the version number.

"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.

verbose mode. Report all special mode flags. Only useful to VGA gurus.

-x <width>
-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:

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:

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.

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 to re-synchronize.

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.

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


grabmode -v

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 confused...

The built-in smart (hah!) features are:


DoubleScan detection.

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 as:
"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 exist) ?

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 mode timings.

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.


Hi/TrueColor detection

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.


Interlace detection

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 below:


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 easy?

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 being combined.

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 Microsoft).


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 fine.

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.


grabmode.exe limitations

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 doing nothing...

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-inaccuracy.

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.


grabmode caveats

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:

Interlaced modes
Hicolor modes
TrueColor modes
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-independent.



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 TextConfig file.



Koen Gadeyne <> (original Linux version)

See the CREDITS file in the distribution for a full list of all those who helped.



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.



grabmode -v
DoubleScan detection.
vertical timing wrap detection
Golden ratio assumption
Hi/TrueColor detection
Interlace detection
CGA emulation detection
Vertical retrace timeout detection
grabmode.exe & scanmode.exe (DOS port of grabmode)
clock probing accuracy
grabmode.exe limitations
grabmode caveats

This document was created by man2html, using the manual pages.
Time: 22:41:31 GMT, April 16, 2011