A routine whose job is to set everything in the computer -
registers, memory, flags - to zero, including that portion
of memory where it is running; its last act is to stomp on its
own "store zero" instruction. Death code isn't very useful,
but writing it is an interesting hacking challenge on
architectures where the instruction set makes it possible,
such as the PDP-8 or the
Data General Nova.
Perhaps the ultimate death code is on the TI 990 series,
where all registers are actually in
RAM, and the
instruction "store immediate 0" has the opcode 0. The
program counter will immediately wrap around core as many
times as it can until a user hits HALT. Any empty memory
location is death code. Worse, the manufacturer recommended
use of this instruction in startup code (which would be in
ROM and therefore survive).