Comment by AnimalMuppet

The 68000 series used 0 as the initial (boot) program counter, and 4 as the initial stack pointer. (I might have those two backwards; it's been a long time.) That meant that they had to be in ROM, which meant that they were not writable. But addresses 8 through 1K were the interrupt vector table, and they did have to be writable.

This led to strange hardware implementations like "0 and 4 point to 0x800000 and 0x800004 (or wherever the ROM is) until a latch is cleared, then they point to 0" - with the latch being cleared fairly early in the boot process. This let you create a different entry point for soft and hard boot, if you wanted.

In that implementation, you could read and write to 0, once the latch was cleared.

Or you could have an implementation where 0 and 4 pointed to ROM always, and you could not have a different entry point for soft boot, and you could not write to 0, ever.