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base:horizontal_raster_split [2018-12-18 16:54] (current)
ftc Moved contents to a separate page.
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 +====== Horizontal Rastersplits ======
  
 +By Monte Carlos
 +
 +Very basic rastersplitting can be done by waiting for a certain rasterline an then switch some VIC register.
 +The display has 312 rasterlines at max but the VIC rasterline register ($d012) has only a range of 0 to 255.
 +Therefore bit 7 of $d011 indicates if the rasterline is <256 or >255. Now the questions arises how to wait for a certain rasterline in the whole range without the preassumption of beeing in rasterline x when the wait routine is called.
 +Here is a diagram which examples the situation:
 +
 +{{{{:​base:​rect11618.png?​400| waiting for a rasterline}}
 +
 +Dependent of which rasterline you're currently in, you can simply compare $d012 with the desired rasterline or you must first wait for bit 7 of $d011 to attain the right value and then wait again for the desired line. Even more, retrace has to be taken into account, too.
 +
 +In total you have to consider 6 different cases:
 +
 +//being in rasterline 0-255 and//
 +  * waiting for a rasterline < $d012 -> wait for bit 7 of $d011 to switch to 1 and back to 0 and then wait for lowbyte
 +  * waiting for rasterline > $d012 but < 256 -> simply wait on lowbyte to match
 +  * waiting for a rasterline > $d012 but > 255 -> wait for set bit 7 of $d011 and then wait for lowbyte to match
 +
 +//being in rasterline 256+ and//
 +  * waiting for a rasterline < $d012 and < 256 -> wait for unset bit 7 of $d011 and then wait for lowb
 +  * waiting for rasterline < $d012 but > 256 -> wait for unset bit 7 of $d011, then set bit7 of $d011 and then on lowbyte
 +  * waiting for a rasterline > $d012 -> simply wait for lowbyte to match
 +
 +<​code>​
 +
 +waitrasterline:​
 +    cpx #0
 +    beq wait0To255
 +    ;from here on we wait for a rasterline > 255
 +    bit $d011
 +    bpl *-3
 +    ;​inRasterLineGT255
 +    cmp $d012
 +    bcs waitMatchingD012
 +    bit $d011
 +    bmi *-3
 +    bit $d011
 +    bpl *-3
 +    bmi waitMatchingD012
 +wait0To255:
 +    bit $d011
 +    bmi *-3
 +    ;​inRasterlineLT256
 +    cmp $d012
 +    bcs waitMatchingD012
 +    bit $d011
 +    bpl *-3
 +    bit $d011
 +    bmi *-3
 +waitMatchingD012:​
 +    cmp $d012
 +    bne waitMatchingD012
 +    rts
 +</​code>​
base/horizontal_raster_split.txt ยท Last modified: 2018-12-18 16:54 by ftc