</p>

<p>
In this program:
</p>

<pre>
var a, b int

func f() {
	a = 1
	b = 2
}

func g() {
	print(b)
	print(a)
}

func main() {
	go f()
	g()
}
</pre>

<p>
it can happen that <code>g</code> prints <code>2</code> and then <code>0</code>.
</p>

<p>
This fact invalidates a few common idioms.
</p>

<p>

The assembler uses them to hold temporary values when assembling some instructions.
</p>

<p>
<code>R13</code> points to the <code>g</code> (goroutine) structure.
This register must be referred to as <code>g</code>; the name <code>R13</code> is not recognized.
</p>

<p>
<code>R15</code> points to the stack frame and should typically only be accessed using the
virtual registers <code>SP</code> and <code>FP</code>.
</p>