sending an item acquires the semaphore, and receiving an item releases
the semaphore.
This is a common idiom for limiting concurrency.
</p>

<p>
This program starts a goroutine for every entry in the work list, but the
goroutines coordinate using the <code>limit</code> channel to ensure
that at most three are running work functions at a time.
</p>

<pre>
var limit = make(chan int, 3)

instructions in the function.
</p>

<p>
When defining a <code>TEXT</code>, specifying frame size <code>$-4</code>
tells the linker that this is a leaf function that does not need to save <code>LR</code> on entry.
</p>

<p>
The name <code>SP</code> always refers to the virtual stack pointer described earlier.
For the hardware register, use <code>R13</code>.
</p>

<p>