CPU Status Registers (EFLAGS64-Bit RFLAGS)

CPU Status Registers (EFLAGS/64-Bit RFLAGS)

Figure 3-6: EFLAGS register

Each 80x86 processor has standard status flags stored in an EFLAGS/ RFLAGS register, which indicate various conditions of the processor. In 64-bit mode the EFLAGS register is referred to as the 64-bit register RFLAGS, with the upper bits set to zero. As this book is written mainly for the application programmer there are only a few EFLAGS that you need to know initially. All the EFLAGS are defined in Chapter 18 but for now:

Table 3-9: RFLAG/EFLAG(s) and bit encoding

EFLAG	Code	Bit	Flag Descriptions
EFLAGS_CF	000000001h		Carry
EFLAGS_PF	000000004h	2	Parity
EFLAGS_AF	000000010h	4	Auxiliary Carry
EFLAGS_ZF	000000040h	6	Zero
EFLAGS_SF	000000080h	7	Sign
EFLAGS_OF	000000800h	11	Overflow

In this book you will find the following table with each general-purpose instruction description indicating the flags that it sets. Those flags are in reality the individual bits found in the EFLAGS register.

Flags	O.flow	Sign	Zero	Aux	Parity	Carry
	-	-	-	-	-	-

LAHF Load AH Flags

Mnemonic

P

PII

K6

3D!

3Mx+

SSE

SSE2

A64

SSE3

E64T

LAHF