tests - Create routine for plotting a pixel at an (x, y) coordinate

test-programs/draw-xy.asm

@@ -0,0 +1,69 @@
+; Routine for drawing at (x, y) coordinates
+
+#zeroflag 1
+
+; *** Set your desired (x, y) here: ***
+#input_x 1
+#input_y 1
+
+; Set up some handy shortcuts
+#x $FA
+#y $FB
+#return_addr_ptr $FE
+
+
+; Main:
+
+LDA #input_x
+STO #x
+LDA #input_y
+STO #y
+LDA * 6 ; acc = current address + 6 (LDA, STO, JMP = 6)
+STO #return_addr_ptr
+JMP @getxy
+
+LDA $FF
+STO ($FD)
+END
+
+;; Convert a pair of (x, y) coords
+;; to the address of a pixel on the display
+;;
+;; Call with:
+;; - x in #x
+;; - y in #y
+;; - return address in #return_addr_ptr
+;;
+;; Returns: pixel address in $FD
+@getxy
+ #gxy_px $FD
+
+ ; stash x...
+ LDA (#x)
+ STO #gxy_px
+
+ ; check if this is row 0...
+ LDA (#y)
+ FHP #zeroflag
+  JMP @getxy_loop
+ JMP (#return_addr_ptr) ; if row 0, we're done
+
+ @getxy_loop
+  LDA (#gxy_px)
+  ADD 5           ; add 5 to get to the next row
+  STO #gxy_px
+  LDA (#y)        ; decrement y (it's acting as a loop counter)...
+  SUB 1
+  STO #y
+  FHP #zeroflag
+   JMP @getxy_loop
+  JMP (#return_addr_ptr)
+
+;; Main variables:
+;; F8
+;; F9
+;; FA - x coord
+;; FB - y coord
+;; FC - gxy temp
+;; FD - gxy temp
+;; FE - Return address for subroutine