The EFlags register

It would have been impossible to write programs in any language for a given platform if the processor had no means to report its status and/or the status of the last operation. More than that, the processor itself needs this information from time to time. Try to imagine a processor unable to conditionally control the execution flow of a program--sounds like a nightmare, doesn't it?

The most common way for a program to obtain information on the last operation or on a certain configuration of an Intel-based processor is through the EFlags register (E stands for extended). This register is referred to as Flags in real mode, EFlags in protected mode, or RFlags in long mode.

Let's take a look at the meaning of the individual bits (also referred to as flags) of this register and its usage.