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12. Putting it All Together

The Xconfig file Table of Video Modes contains lines of numbers, with each line being a complete specification for one mode of X-server operation. The fields are grouped into four sections, the name section, the clock frequency section, the horizontal section, and the vertical section.

The name section contains one field, the name of the video mode specified by the rest of the line. This name is referred to on the "Modes" line of the Graphics Driver Setup section of the Xconfig file. The name field may be omitted if the name of a previous line is the same as the current line.

The dot clock section contains only the dot clock (what we've called DCF) field of the video mode line. The number in this field specifies what dot clock was used to generate the numbers in the following sections.

The horizontal section consists of four fields which specify how each horizontal line on the display is to be generated. The first field of the section contains the number of dots per line which will be illuminated to form the picture (what we've called HR). The second field of the section (SH1) indicates at which dot the horizontal sync pulse will begin. The third field (SH2) indicates at which dot the horizontal sync pulse will end. The fourth field specifies the toal horzontal frame length (HFL).

The vertical section also contains four fields. The first field contains the number of visible lines which will appear on the display (VR). The second field (SV1) indicates the line number at which the vertical sync pulse will begin. The third field (SV2) specifies the line number at which the vertical sync pulse will end. The fourth field contains the total vertical frame length (VFL).


     #Modename    clock  horizontal timing  vertical timing

     "752x564"     40    752 784  944 1088  564 567 569 611
	           44.5  752 792  976 1240  564 567 570 600

(Note: stock X11R5 doesn't support fractional dot clocks.)

For Xconfig, all of the numbers just mentioned - the number of illuminated dots on the line, the number of dots separating the illuminated dots from the beginning of the sync pulse, the number of dots representing the duration of the pulse, and the number of dots after the end of the sync pulse - are added to produce the number of dots per line. The number of horizontal dots must be evenly divisible by eight.

Example horizontal numbers: 800 864 1024 1088

This sample line has the number of illuminated dots (800) followed by the number of the dot when the sync pulse starts (864), followed by the number of the dot when the sync pulse ends (1024), followed by the number of the last dot on the horizontal line (1088).

Note again that all of the horizontal numbers (800, 864, 1024, and 1088) are divisible by eight! This is not required of the vertical numbers.

The number of lines from the top of the display to the bottom form the frame. The basic timing signal for a frame is the line. A number of lines will contain the picture. After the last illuminated line has been displayed, a delay of a number of lines will occur before the vertical sync pulse is generated. Then the sync pulse will last for a few lines, and finally the last lines in the frame, the delay required after the pulse, will be generated. The numbers that specify this mode of operation are entered in a manner similar to the following example.

Example vertical numbers: 600 603 609 630

This example indicates that there are 600 visible lines on the display, that the vertical sync pulse starts with the 603rd line and ends with the 609th, and that there are 630 total lines being used.

Note that the vertical numbers don't have to be divisible by eight!

Let's return to the example we've been working. According to the above, all we need to do from now on is to write our result into Xconfig as follows:

<name>   DCF     HR  SH1 SH2   HFL   VR  SV1 SV2 VFL

where SH1 is the start tick of the horizontal sync pulse and SH2 is its end tick; similarly, SV1 is the start tick of the vertical sync pulse and SV2 is its end tick.

To place these, recall the discussion of black magic and sync pulses given above. SH1 is the dot that begins the leading edge of the horiziontal sync pulse; thus, SH1 = HR + HGT1. SH2 is the trailing edge; thus, SH2 = SH1 + HSP. Similarly, SV1 = VR + VGT (but VGT is usually zero) and SV2 = SV1 + VSP.

#name    clock   horizontal timing   vertical timing    flag
936x702  65      936 968 1200 1232   702 702 710 737

No special flag necessary; this is a non-interlaced mode. Now we are really done.

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