1.8 KiB
Apple ][ hires
Structure of the hires screen in RAM
The Apple ][ has 2 hires pages. One in $2000-$3FFF. The second one in $4000-$5FFF. Each page is thus 8192 bytes long.
The dimensions of one hires page is 40 bytes wide and 192 lines high. 40x192 = 7680 bytes. 512 bytes are "missing" and in fact not used/displayed.
The hires screen is divided in 3 sections of 64 lines. Each section is then divided in 8 sub-sections of 8 lines, each itself divided in 8 sub-sub-sections representing the lines themselves.
To better understand this division, it's easier to POKE bytes into RAM and see what happens.
A POKE 8192,255will 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
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 as stored 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
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).
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.