Editing Spec.isa (section)

== Fields ==
The <code>[fields]</code> section defines different types of fields that can be included in the instruction definitions, as well as what values they can have.  Fields are separated by blank lines, and each field starts with a field name on the first line followed by one or more allowable field values.  Values are bit patterns and all values within a field must have the same bit length.

=== Example 1 - Overture ===
The Overture specification provides a good starting example:
<pre>
register
r0 000
r1 001
r2 010
r3 011
r4 100
r5 101
in 110
out 110
</pre>
Here we can see a couple of features:
* Names are arbitrary.  <code>r0</code> through <code>r5</code> are considered registers only by convention, while <code>in</code> and <code>out</code> follow an entirely different naming scheme within the same <code>register</code> field.
* Value repetition is allowed.  <code>in</code> and <code>out</code> both have the same value of <code>110</code>.
* Completion is not required.  <code>111</code> is not assigned to any label.

=== Example 2 - aarch64 ===
Also note that the field name itself is arbitrary.  We can take an excerpt from https://github.com/Stuffe/isa_spec/blob/main/spec_lib/aarch64/aarch64.isa as an example:
<pre>
condition_code
eq 0000
ne 0001
cs 0010
hs 0010
</pre>
(In this case, all 16 values are included in the ISA itself, and has been truncated here for the sake of brevity.)

We can see a couple of other features in this example:
* The field name is not <code>register</code>, as noted.
* The field is four bits wide rather than three.  In principle, the fields can be any width desired (greater than zero).

=== Example 3 - aarch64, again ===
A final example from the same <code>aarch64.isa</code>:
<pre>
pound
"#" 0
"" 0
</pre>
The features this time are:
* <code>"literal"</code> syntax to require a literal string be included in your instructions.
* <code>""</code> to allow an empty string, effectively making this field optional.
In this instance, the <code>pound</code> field is used for immediate values, allowing the player to write either <code>#37</code> or just <code>37</code> to describe the immediate value 37.

=== Reserved field names ===
There are two reserved names:
* <code>label</code> references labels within the assembler (eg: <code>mylabel:</code>).
* <code>immediate</code> references immediate values (eg: <code>37</code>).