In addition to registers, CPU also contains a special 32–bit EFLAGS register where certain bits are set or cleared in response to arithmetic and other operations. Some bit values can be manipulated by separate machine instructions and their values affect code execution.
For example, DF bit (Direction Flag) determines the direction of memory copy operations and can be set by STD and cleared by CLD instructions. It has the default value of 0 and its location is shown on Picture 12.1.
Figure 12.1. Picture 12.1
This is done by STOSD instruction that stores a dword value from ...