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
3.14.2. FLD |
When building the graphics out of text lines, the VIC orientates itself exclusively by the occurrence of Bad Lines: A Bad Line gives the "start signal" for the display of one text line. By appropriately changing YSCROLL (in register $d011) you can suppress and arbitrarily delay the Bad Line Condition (see 3.5.). So you can exactly control in which raster lines Bad Lines should occur and thus from which raster lines the VIC should start to display one text line each. The distance between two text lines can be arbitrarily increased if only you hold back the next Bad Line long enough. This effect is called "Flexible Line Distance" (FLD). E.g. if you only allow three Bad Lines on the screen at raster lines $50, $78 and $a0, the VIC will also only display three text lines at these positions. The sequencer is in idle state in the lines between. If you only delay the occurrence of the first Bad Line, you can scroll down the complete graphics display by large distances without moving a single byte in display memory.