Differences

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--- base:demo_programming [2018-12-18 16:53] – [Rasterbars] ftc
+++ base:demo_programming [2018-12-18 16:54] – [Horizontal Rastersplits] - Moved to separate page ftc
@@ Line 13: / Line 13: @@
 ====== Effects ======
 Many demo effects depend on precise timing. Therefore it might be useful to have a look on the [[base:interrupts|Interrupts and Timing]] page.
-==== Horizontal Rastersplits ====
-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>