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Update apple2_hires.md

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@ -29,7 +29,7 @@ If we slightly modify the above code to POKE the first 3 lines as stored in memo
30 FOR J = 0 TO 2 30 FOR J = 0 TO 2
40 FOR I = 0 TO 39 40 FOR I = 0 TO 39
50 POKE A, 255 50 POKE A, 255
60 A = A+ 1 60 A = A + 1
70 NEXT I,J 70 NEXT I,J
80 PRINT A 80 PRINT A
@ -41,4 +41,51 @@ We have drawn line 0, line 64 and line 128 !
Now the next address to `POKE` seems to be 8312 ($2078 in hex -- the resulting value in our variable `A`). Now the next address to `POKE` seems to be 8312 ($2078 in hex -- the resulting value in our variable `A`).
But if we do `POKE 8312, 255` we don't see any change on the screen ! This is because we have reached one the hires screen holes ! But if we do `POKE 8312, 255` we don't see any change on the screen ! This is because we have reached one the hires screen holes !
In fact, all lines between 128 and 191 in RAM have 8 unused bytes at their end. Those 8x64 lines represent 512 bytes. Those are the missing bytes in first computation. In fact, all lines between 128 and 191 in RAM have 8 unused bytes at their end. Those 8x64 lines represent 512 bytes. Those are the missing bytes in first computation.
Now that we now that, we could slightly modify the above code so that after having drawn 3 lines, we add 8 to `A`so that it points to the next line location in memory. Let's do it and plot 3 times 3 lines.
10 HGR
20 A = 8192: REM $2000
30 FOR K = 0 TO 2
40 FOR J = 0 TO 2
50 FOR I = 0 TO 39
60 POKE A, 255
70 A = A + 1
80 NEXT I,J
90 A = A + 8
100 NEXT K
110 PRINT A
We end up with
![screenshot](img/apple2_hires_lines_triplets.png)
As you watch how the lines are filled, you better understand the hires screen structure:
* the first 3 lines of 40 bytes delimit the three 64-lines sections and represent lines 0, 64 and 128 of the screen which are line 0 of each of the sections
* then 8 bytes are wasted
* the next 3 lines of 40 bytes represent line 8 of each of the 64 lines section (that is the base line of each section + 8, so we have lines 0+8, 64+8 and 128+8)
* then 8 bytes are wasted
* the next 3 lines of 40 bytes represent line 16 of each of the 64 lines section
* the 8 bytes are wasted
* This continues until we've arrived at line 56 relative to each section. That is line 56, line 64+56=120 and line 128+56=184.
The following code will do it
10 HGR
20 A = 8192: REM $2000
30 FOR K = 0 TO 7
40 FOR J = 0 TO 2
50 FOR I = 0 TO 39
60 POKE A, 255
70 A = A + 1
80 NEXT I,J
90 A = A + 8
100 NEXT K
110 PRINT A
![screenshot](img/apple2_hires_lines_8.png)
So what happens next ?