Update apple2_hires.md

tools/bitmap editor/apple2_hires.md

@@ -8,4 +8,30 @@ The hires screen is divided in 3 sections of 64 lines. Each section is then divi
 
 To better understand this division, it's easier to POKE bytes into RAM and see what happens.
 
-A POKE 8192,
+A `POKE 8192,255`will plot 7 pixels on the top left corner of the hires screen (page 1). Poking the next memory address (8193), will plot 7 more pixels on line 0 of the hires screen.
+
+So to draw the entire line 0 we could `RUN` this code
+
+    10 HGR
+    20 FOR I = 0 TO 39: POKE 8192+I, 255: NEXT
+
+![screenshot](img/apple2_hires_line0.png)
+
+8192 + 40 = 8232 ($2028) is the next byte in memory. But
+
+    POKE 8232,255
+
+will not plot 7 pixels on line 1 but on line 64 !
+If we slightly modify the above code to POKE the first 3 lines in memory, we have
+
+    10 HGR
+    20 A = 8192: REM $2000
+    30 FOR J = 0 TO 2
+    40 FOR I = 0 TO 39
+    50 POKE A, 255
+    60 A = A+ 1
+    70 NEXT I,J
+    80 PRINT A
+
+The result is this
+![screenshot](img/apple2_hires_lines0-64-128.png)