Editing Spec.isa (section)

==== Constructing a mask ====
Using hexits or even bits to describe a mask is easy enough if you're working with fairly small constant values (a full 64-bit mask is 16 hexits!  That's a lot to write out already, never mind trying to write it in binary!)  If you're working with non-constant values however, or if you just don't want to manually work out such large numbers, you can create your own mask with shifts and ORs.  For example, if you want the 38th bit, you can simply use:
<pre>
%mask = 1 << 38
</pre>
{{note|type=info|This shift operation counts bits starting at 0, so your LSB will be <code>1&nbsp;&lt;&lt;&nbsp;0</code>, not <code>1&nbsp;&lt;&lt;&nbsp;1</code>, despite it often being referred to as the "first" bit.}}

If you want the three bits from the 36th through the 38, you can OR them together like so:
<pre>
%bit36 = 1 << 36
%bit37 = 1 << 37
%bit38 = 1 << 38
%mask = %bit36 | %bit37 | %bit38
</pre>

That is obviously going to get very cumbersome if you want to extract 8 or 16 or more bits.  We can use a nice property of binary numbers to simplify this however:  <code>1&nbsp;&lt;&lt;&nbsp;n</code> has a single bit set at position <code>n</code>, so <code>(1&nbsp;&lt;&lt;&nbsp;n)-1</code> has that single bit cleared and all lowest <code>n</code> bits set:
<pre>
%mask = (1<<39)-1
</pre>
will return a mask with the lowest 39 bits enabled.
{{note|type=info|We want to include %bit38 from the prior example, so we need to go one higher before subtracting!}}

But we can do better!  If we want to recreate the 3-bit mask from above, we can construct it by using a second mask to "turn off" some of the lower bits from the first mask:
<pre>
%hi_mask = (1<<39)-1
%lo_mask = (1<<36)-1
%mask = %hi_mask ^ %lo_mask
</pre>
This initially creates <code>%hi_mask</code> with the lowest 39 bits enabled, as before.  It then creates <code>%lo_mask</code> with only the lowest 36 bits enabled.  The XOR operation performs the task of "turning off" the lower 36 bits:
* bits 0..35 are turned off because <code>1^1&nbsp;=&nbsp;0</code>
* bits 36..38 remain on because <code>1^0&nbsp;=&nbsp;1</code>
* bits 39..63 remain off because <code>0^0&nbsp;=&nbsp;0</code>

Finally, we can perform a shift to fully encapsulate the bit slicing behavior.  In fact, we don't even need to "turn off" the lower bits this time, as we'll be shifting them out:
<pre>
%mask = (1<<39)-1
%value = 0x123456789ABCDEF
%extracted = (%value & %mask) >> 36
</pre>
Description	What you type	What you get
Italic	''Italic text''	Italic text
Bold	'''Bold text'''	Bold text
Bold & italic	'''''Bold & italic text'''''	*Bold & italic text*