Question

Is there any one who can explain this code to me?
This is an old exam question and i can’t find an answer on the net.

 void (*x)();
 x=0xFFFF0000;
 printf("1"); x();
 printf("2"); x();
 printf("3"); x();

Answer 1

On classic real-mode x86 architectures, you could trigger a system boot by doing an unconditional JMP to address 0xFFFF0000.

The code is basically defining x() as a function, and setting the function’s in-memory address as 0xFFFF0000. When you invoke the function, the system would do CALL 0xFFFF000 or similar at the assembler level, and jump directly to the BIOS boot code, triggering a system reboot.