IRET - x86-64 instruction reference

iretInterrupt return, default size 
Returns from an interrupt by popping the instruction pointer, cs and the flags, and in 64-bit mode the stack pointer and ss as well, then continuing at the popped address.
8086 and later
Intel's reference: IRET/IRETD/IRETQ 
CFPFAFZFSFTFIFDFOF ●●●●●●●●●
● set from the result
CF Loaded from the stack.
PF Loaded from the stack.
AF Loaded from the stack.
ZF Loaded from the stack.
SF Loaded from the stack.
TF Loaded from the stack.
IF Loaded from the stack only with the privilege to change it, which a Linux program lacks.
DF Loaded from the stack.
OF Loaded from the stack.
Forms  iret  
Only the flags the privilege level allows are restored.
Under BITS 64, NASM assembles iret with the 32-bit operand size, as iretd: it pops 4-byte slots and loads only eip and esp. A 64-bit handler returns with iretq. A Linux program has no interrupt handlers of its own to return from.