added sys.waitrastborder() for c64

compiler/res/prog8lib/c64/syslib.p8

@@ -502,6 +502,8 @@ sys {
         ; --- busy wait till the next vsync has occurred (approximately), without depending on custom irq handling.
         ;     note: a more accurate way to wait for vsync is to set up a vsync irq handler instead.
         %asm {{
+-           lda  c64.RASTER
+            beq  -
 -           lda  c64.RASTER
             bne  -
             bit  c64.SCROLY
@@ -510,6 +512,15 @@ sys {
         }}
     }
 
+    inline asmsub waitrastborder() {
+        ; --- busy wait till the raster position has reached the bottom screen border (approximately)
+        ;     note: a more accurate way to do this is by using a raster irq handler instead.
+        %asm {{
+-           bit  c64.SCROLY
+            bpl  -
+        }}
+    }
+
     asmsub memcopy(uword source @R0, uword target @R1, uword count @AY) clobbers(A,X,Y) {
         %asm {{
             ldx  cx16.r0

docs/source/libraries.rst

@@ -105,9 +105,15 @@ sys (part of syslib)
 
 ``waitvsync()``
     busy wait till the next vsync has occurred (approximately), without depending on custom irq handling.
+    can be used to avoid screen flicker/tearing when updating screen contents.
     note: a more accurate way to wait for vsync is to set up a vsync irq handler instead.
     note for cx16: the system irq handler has to be active for this to work (this is not required on c64)
 
+``waitrastborder()`` (c64 target only)
+    busy wait till the raster position has reached the bottom screen border (approximately)
+    can be used to avoid screen flicker/tearing when updating screen contents.
+    note: a more accurate way to do this is by using a raster irq handler instead.
+
 ``reset_system()``
     Soft-reset the system back to initial power-on Basic prompt.
     (called automatically by Prog8 when the main subroutine returns and the program is not using basicsafe zeropage option)

examples/test.p8

@@ -1,10 +1,26 @@
 %import textio
 %zeropage basicsafe
+%option no_sysinit
 
 main {
 
     sub start() {
 
+        ubyte color=0
+        ubyte xx
+        uword ptr = $0400
+
+        @($02) = 0
+
+        repeat {
+            sys.waitvsync()
+            %asm {{
+                ldy  $02
+                lda  #'*'
+                sta  $0400,y
+                inc  $02
+            }}
+        }
         txt.print("hello")
 
 ;        str filename = "titlescreen.bin"