It is not necessary to add more ROM just to get graphics characters on the screen. For example, in Fig. 2-10 a simple modification to a 2513 character generator ROM results in each text character having an equivalent pseudo-block graphics character. The four NOR gates in the drawing will pass the row dots from the ROM without change if the control inputs to the gates are grounded. If the O1 output of the ROM is fed to the gate inputs, we will get a block graphics character whose configuration depends on the ASCII code sent to the ROM. However, the resolution of such a display is not as good as the previous circuit that used an extended ROM graphics set.
The flip-flop in Fig. 2-10 is used to toggle the display between graphics and text. Obviously with this kind of display we cannot have mixed graphics and text.
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