On the VIC20 you had to make the video chip change the address of the character ROM so part of it pointed to RAM. Then you alter that memory to change the pixel

On the VIC20 you had to make the video chip change the address of the character ROM so part of it pointed to RAM. Then you alter that memory to change the pixels in a character (8x8). Then anywhere that character was drawn on screen the altered graphics would show. In order to draw graphics like a bitmap screen you had to use multiple characters in a grid like. ABCDEF GHIJKL MNOPQR Then you had to calculate the which character held which pixels and then alter the bytes inside that character to alter the exact pixel you wanted. So a pixel at 7,7 would be inside the A but a pixel at 10,10 would be inside the H.