Safe OS

[gord campbell]

"On Ancient Computers like Commodore all the OS was stored in a Rom. You
could not change it at all. Nice thing about it was other people could 
not mess with it, and make it do things when they tried to make changes 
to it."

That wasn't completely true. I created a modified version of the ROM,
loaded it into the highest RAM, "sealed off" that RAM by changing a
pointer, and changed the other low-memory pointers to invoke my version
of the OS.

(The interrupt handler blocked interrupts for around 1200 microseconds.
I rewrote it to enable interrupts after about 100 microseconds, which
was important to a project I was doing.)

The first step was disassembling the ROM to create source code, then
spending a couple of days making it readable source code. Could you
imagine trying to do that with Windows? Or even DOS 5?

The key resource for all that: Jim Butterfield, RIP.