Editing Spec.isa (section)

=== Output bit pattern ===
<pre>
11 000 ccc
</pre>


After all Virtual Operands and assertions comes the bit pattern. It defines a sequence of bits that get interpreted as a single big-endian number (which then gets rearranged according to the [[#endianness|endianness] setting). In this line space characters (and only space characters) are completely ignored.

Bits can be defined by either individual letters/numbers or explicit variable references.

==== Fixed patterns ====

<code>0</code> and <code>1</code> produce those bits in the output and require those exact bits to be there while disassembling. <code>?</code> also produces <code>0</code> while assembling, but this bit will be ignored when disassembling (once that gets reimplemented).

==== Individual letters ====

All lowercase letters can be used as shortcuts to refer to variables. A letter to refers to the first variable that starts with that letter, with the order being left-to-right in the syntax line and then top-to-bottom in the virtual operands.

If a letter appears multiple times, all of those appearances are seen together ignore intermediate parts. The bits are assigned LSB-to-MSB to the repeated letters in a right-to-left ordering. For example,

<pre>
%c = 0b110001
c 0 ccccc 0
</pre>

would result in <code>10100010</code>.

==== Explicit Variable References ====

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

In addition to individual letters, variables can also be referenced by their full name by using the <code>%</code> prefix. In this case you also need to explicitly tell the assembler which bits of the variable to use using the [[#Bit_slicing|Bit slicing]] syntax, with the restriction that both arguments need to be unsigned integer literals.