What meaning, if any, does the Mod R/M byte carry for the unconditional jump instruction 0xFF?

Question

Consider the following code, compiled on a 32-bit Ubuntu 14.04.2 with gcc 4.8.2

#include 

int main(){
    _exit(0)         


        

Answer 1

Actually, those last 4 bytes are not the target address. The instruction ff 25 0c a0 04 08 is an instance of jmp rm32 (jump absolute indirect), the last 4 bytes are actually the address from which the jump target will be read.

The ModRM byte 25h, as you can see in this table, has a /digit part of 4 (that makes it a jmp rm32, other instructions that have opcode byte ff are inc rm32, dec rm32, call rm32, call m16:32, jmp m16:m32, push rm32, all distinguished by the ModRM byte). The rest of the ModRM byte 25h means that the operand is a memory address of the form [sdword].