The MOS 6567/6569 video controller (VIC-II) and its application in the Commodore 64 |
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Contents 3. Description of the VIC 3.7. Text/bitmap display |
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The text/bitmap display logic in the VIC is in one of two states at any time: The idle state and the display state.
The transition from idle to display state occurs as soon as there is a Bad Line Condition (see section 3.5.). The transition from display to idle state occurs in cycle 58 of a line if the RC (see next section) contains the value 7 and there is no Bad Line Condition. As long as register $d011 is not modified in the middle of a frame, the display logic is in display state within the display window and in idle state outside of it. If you set a YSCROLL other than 3 in a 25 line display window and store a value not equal to zero in $3fff you can see the stripes generated by the sequencer in idle state on the upper or lower side of the window. In [4], idle accesses as well as g-accesses in idle state are called "idle bus cycle". But the two phenomena are not the same. The accesses marked with "+" in the diagrams of [4] are normal g-accesses. In this article, the term "idle access" is only used for the accesses marked with "i" in the diagrams in section 3.6.3., and not for the g-accesses during idle state. |
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