Apple II: HPLOT crashes system. Why?

Question

While I was writing a program on an Apple IIe, I set up access the graphics page using softswitches, and then tried to draw on it, but the system crashed so hard I had to do a Control - Open Apple - Reset reboot to get out of it. What happened?

Boot DOS 3.3

10 POKE -16304,0: POKE -16301,0: POKE -16297,0: POKE -16300,0
20 HCOLOR = 3
30 HPLOT 0,0 TO 279,159

RUN
** CRASH to monitor **
Control-C to return to BASIC

Each time RETURN is pressed, it says ?SYNTAX ERROR

Press Control-Reset
Doesn't help, still says ?SYNTAX ERROR when RETURN is pressed

Answer 1

In order for the drawing commands to work, they need to know what graphics page to use. Since HGR or HGR2 was not used, this setting (address $00E6) is left undefined, and is at the default boot-time value of zero.

So when HPLOT is used in this manner, it draws on "graphics page zero" from $0000-1FFF, and writes a string of bytes across the zero page and the stack, which then totally corrupts the system state.

If you use HPLOT like this without showing the hi-res graphics page 1, you will also see a slanted line of characters appear across the text screen, as it proceeds to corrupt everything in the first 8 kilobytes of memory.

If you want to draw on Hi-Res page 1 without clearing it first, use POKE 230, 32

To draw on Hi-Res page 2 without clearing it first, use POKE 230, 64

It is possible to use a hidden Hi-Res page 3 to store an image, at $6000-7FFF using POKE 230,96 ... any higher than this is not usable since DOS 3.3 normally starts at $9600.