VGA Hardware: Difference between revisions

The Plane Write Enable register is used to choose the plane to be written, then the memory can be written by written by accessing the corresponding address in memory.

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 artefacts.