VGA Hardware: Difference between revisions
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@BI: "artefacts" was the correct spelling
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==== Timing Model ====
The horizontal timing registers are based on a unit called 'character' (As they match one character in text mode). Each character equals 8 ('''9/8 Dot Mode''' is set) or 9 ('''9/8 Dot Mode''' is clear) pixels. Each scanline contains '''Horizontal Total''' + 5 characters, zero based. '''Horizontal Display End''' tells us the last character that is calculated from memory (i.e. the horizontal resolution in characters minus one). '''Horizontal Blanking Start''' and '''Horizontal Retrace Start''' give us the the last character before either period is started. '''Horizontal Blanking End''' and '''Horizontal Retrace End''' need more explanation, as they only contain part of a number. When blanking or horizontal retrace is enabled the significant bits are checked against the character counter, and if these bits match the respective period will be ended. The quick solution is to calculate the appropriate values, compute the last character clock at which each period should be active, then AND it with 0x3F (Blank) or 0x1F (Retrace) to get the register's value. Note that the periods must be between 1 and 63(Blank)/31(Retrace) character clocks. To be safe, there must be at least one character of overscan on each side of the screen to avoid additional
The vertical timing is similar, apart from the fact that these registers operate on scan lines (pixels) instead of characters. The '''Vertical Retrace End''' and '''Vertical Blank End''' registers work also similar, although they are different sizes. The Retrace End is 4 bits wide (AND with 0xF, period is 1-15 scanlines), The Blank End size is at least 7 bits (some say its 8, some say its 7), so the value is computed by ANDing with 0xFF, with the period ranging from 1-127 scanlines. As with horizontal timing, at least one scan line of overscan must be present to avoid possible
The clock can be selected using '''Clock Select'''. Only two of four possible clocks are present on all VGAs. A clock of 25MHz is selected when this field is zero, 28MHz is selected when this field equals 1. Some boards have other clocks under values 2 and 3, but you should not write these values unless you know the clock that is there. Note that selecting the 28MHz clock and 9 pixels per pixel results in the same timings as selecting the 25MHz clock and 8 pixels per character, only with different resolutions.
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