The MOS 6567/6569 video controller (VIC-II) and its application in the Commodore 64
Contents 3. Description of the VIC 3.14. Effects/applications
3.14.5. Doubled text lines |
The display of a text line is normally finished after 8 raster lines, because then RC=7 and in cycle 58 of the last line the sequencer goes to idle state (see section 3.7.2.). But if you now assert a Bad Line Condition between cycles 54-57 of the last line, the sequencer stays in display state and the RC is incremented again (and thus overflows to zero). The VIC will then in the next line start again with the display of the previous text line. But as no new video matrix data has been read, the previous text line is simply displayed twice.