I used 0 as the example because the 8bit chip I was thinking of (the Z80) used that. But the exact address isn't too relevant, the basic idea is that the CPU ha

I used 0 as the example because the 8bit chip I was thinking of (the Z80) used that. But the exact address isn't too relevant, the basic idea is that the CPU has a default state for the instruction pointer, and can get things going from there. http://lateblt.tripod.com/z80proj1.htm http://lateblt.tripod.com/z80proj1.htm "What's Happening? The computer begins with the CPU. When the CPU starts running (which basically happens as soon as it receives power and is reset with the RESET pin), it begins to pull instructions from memory and execute them. Where it starts in memory depends on the individual CPU, but in the case of the Z80, it simply begins at memory address 0, instead of using a reset vector like many other CPUs do. This means that any computer using the Z80 must have a ROM chip at memory address 0 which gives the initial start-up instructions to the Z80." Also - the memory map during boot time is broadly unrelated to the per-process memory map during OS runtime. Also, if you branch to unexpected memory addresses, you deserve all you get. Also, on a processor with privilege isolation, your ring3 user process can't just cause a reboot.