Refactor: Rewrite fill-display:

- make more use of constants
- improve comments

test-programs/fill-display.asm

@@ -1,27 +1,25 @@
 ;; Fill display with $FF
 ; updated for 5x5 display
 
-=pixelIndex $24
+=*px  $24
-=pixelFill $25
+=fill $25
 
-LDA $00 ; Start of display
+              ; Initialize variables...
-STO =pixelIndex
+LDA $00       ; (Address for the first px on the display)
+STO =*px      ; Pointer to current px
+LDA $FF       ; ($FF is 'on', $00 is 'off')
+STO =fill     ; Stash value to fill with
 
-LDA $FF
+@paint
-STO =pixelFill
+ LDA (=fill)  ; (A = mem[fill] = $FF)
+ STO (=*px)   ; Paint pixel (mem[mem[*px]] = A = $FF)
 
-@copy-to-display
+LDA (=*px)    ; Increment pixel pointer...
-  LDA ($25) ; A = mem[$25] = $FF
-  STO ($24) ; update pixel ... mem[mem[$24]] = A = $FF
-
-; increment pixel index
-LDA ($24)
 ADD $01
-STO =pixelIndex
+STO =*px
-
-LDA ($24) ; pixelIndex
-SUB $19   ; if pixelIndex - $19 == 0, we've reached the end
-FHP 2     ; Zero flag is #2
-  JMP @copy-to-display
 
+LDA (=*px)    ; Test whether to loop or not...
+SUB $19       ; if *px - $19 == 0, we've reached the end
+FHP 2         ; (Zero flag is #2)
+  JMP @paint
 END