The MOS 6567/6569 video controller (VIC-II) and its application in the Commodore 64
Contents 3. Description of the VIC 3.6. Memory access/Timing of a raster line
3.6.1. The X coordinates |
Before explaining the timing of memory accesses within a raster line, we will quickly explain how to obtain the X coordinates. This is necessary because the VIC doesn't have a counterpart to the RASTER register (which gives the current Y coordinate) to hold the X coordinates, so you cannot simply read them with the processor. But the VIC surely keeps track of the X coordinates internally as the horizontal sprite positions are based on them, and a pulse at the lightpen input LP latches the current X position in the register LPX ($d013). Determining the absolute X coordinates of events within a raster line is not trivial as you cannot e.g. simply put a sprite to a well-defined X coordinate and conclude from the text characters displayed at the same X position to the X coordinates of the memory accesses belonging to these characters. The memory access and the display are separate function units and the read graphics data is not immediately displayed on the screen (there is a delay of 12 pixels). So a different approach has been taken: The absolute position of a single X coordinate within the raster line was measured with the LPX register and the other X coordinates were determined relative to this. To do that, the IRQ output of the VIC has been connected to the LP input and the VIC has been programmed for a raster line interrupt. As the negative edge of IRQ was defined to be the start of a raster line, the absolute X position of the line start could be determined. The position of the negative edge of BA during a Bad Line was also measured with this method and the result was consistent with the relative distance of IRQ and BA to each other. Based on these two measurements, the X coordinates of all other events within a raster line have been determined (see [4]). Not until now the sprite X coordinates were used to be able to determine the moment of the display generation of the text characters. This of course implicitly assumes that the LPX coordinates are the same as the sprite X coordinates. There is, however, no indication and thus no reason to suppose that they don't (a direct correlation would also be the most simple solution in terms of circuit design).