Chapter 10: Branching

Overview

The processor's instruction pointer is just that a pointer to the instruction that is about to be executed. This register is the RIP in 64-bit, EIP in Protected Mode, and IP in Real Mode. It behaves very similarly to that of a CD player. You can only read one data stream at a time. To read elsewhere, you have to move the pointer to the new location to read. (A better visualization would be a record player with its needle that cannot skip around.)

The only way to read the value of the instruction pointer is to call a function with the CALL instruction and then read the value on the stack where you had been. There is no MOV EAX,EIP instruction.

There are four primary methods that can be used to change the position of the processor's instruction pointer: jump, call, interrupt, and return. You can jump a delta, near, or far distance; call and return near or far; interrupt; and return. These instructions tend to be the most confusing to an assembler, and the exact instruction that you think you are using sometimes is not.

No 64-bit 

The concept of Real Mode memory does not exist in 64-bit mode, nor does 16-bit displacement as it is sign extended to 64 bits for the 64-bit instruction pointer (RIP).