&gt; (The 68000 had a similar thing going on; all instructions starting with 0xAxxx or 0xFxxx would trap, so we used them to compress calls in the ROM on the Atari ST).<p>This brings back memories of patching A-traps in classic Mac OS. Fundamentally the same mechanism -- OxA instructions would trap, the handler would consult a table in RAM to find the location in ROM for the system call. You could dynamically replace a system call&#x27;s implementation by patching the lookup address for fun and profit.