Editing Spec.isa (section)

==== Explicit Operand References ====

<pre>
00 %imm[5:0]
</pre>

In addition to individual letters, operands can also be referenced by their full name using the <code>%</code> prefix.  In this case you also need to explicitly tell the assembler which bits to use, using a syntax similar to [[#Bit_slicing|bit slicing]]:
<pre>
%operand[start:end]
</pre>
Unlike the arbitrary bit slicing available when creating virtual operands, the syntax here is much more restrictive:  <code>operand</code> must be a single operand name (instruction operand or virtual), and <code>start</code> and <code>end</code> must be unsigned integer literals.

As with the individual letter syntax, values shorter than the requested bits will be sign- or zero-extended appropriately:
<pre>
test_extended_slicing
%s:S3 = 0b100
%u:U3 = 0b100
%s[7:0] %u[7:0]
</pre>

will result in <code>252 4</code>.

Unlike the single letter syntax, we do not have to worry about overruns with slicing syntax as we're specifying exactly which bits we want, regardless of whether there are more bits available or not.