Editing X86 assembly language (section)

===Using the flags register===
Flags are heavily used for comparisons in the x86 architecture. When a comparison is made between two data, the CPU sets the relevant flag or flags. Following this, conditional jump instructions can be used to check the flags and branch to code that should run, e.g.:
<syntaxhighlight lang="nasm">
	cmp	eax, ebx
	jne	do_something
	; ...
do_something:
	; do something here
</syntaxhighlight>
Aside, from compare instructions, there are a great many arithmetic and other instructions that set bits in the flags register. Other examples are the instructions sub, test and add and there are many more. Common combinations such as cmp + conditional jump are internally ‘fused’ (‘[[macro fusion]]’) into one single [[micro-instruction]] (μ-op) and are fast provided the processor can guess which way the conditional jump will go, jump vs continue.

The flags register are also used in the x86 architecture to turn on and off certain features or execution modes. For example, to disable all maskable interrupts, you can use the instruction:
<syntaxhighlight lang="asm">
	cli
</syntaxhighlight>

The flags register can also be directly accessed. The low 8 bits of the flag register can be loaded into <code>ah</code> using the <code>lahf</code> instruction. The entire flags register can also be moved on and off the stack using the instructions <code>pushfd/pushfq</code>, <code>popfd/popfq</code>, <code>int</code> (including <code>into</code>) and <code>iret</code>.

The x87 floating point maths subsystem also has its own independent ‘flags’-type register the fp status word. In the 1990s it was an awkward and slow procedure to access the flag bits in this register, but on modern processors there are ‘compare two floating point values’ instructions that can be used with the normal conditional jump/branch instructions directly without any intervening steps.