Teletext Graphics, Mode 7
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Mode 7 on the BBC micro is the only mode in which the shapes of the characters are not explicitly stored in memory. Instead just one byte is used for each of the screen character locations which, in the simplest case, holds the ASCII code of the character to be displayed there. The screen display is then generated by a special micro-chip which contains the data for each character. The chip looks at the screen memory to see which character is to be displayed at that position and then includes the correct data for that character directly into the television signal. This means that the chip has to recalculate the data for the whole screen each time the television display is refreshed, 50 times a second (60 times a second in the U.S.A.).
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