1.. SPDX-License-Identifier: GPL-2.0
2
3RISC-V Linux User ABI
4=====================
5
6ISA string ordering in /proc/cpuinfo
7------------------------------------
8
9The canonical order of ISA extension names in the ISA string is defined in
10chapter 27 of the unprivileged specification.
11The specification uses vague wording, such as should, when it comes to ordering,
12so for our purposes the following rules apply:
13
14#. Single-letter extensions come first, in canonical order.
15   The canonical order is "IMAFDQLCBKJTPVH".
16
17#. All multi-letter extensions will be separated from other extensions by an
18   underscore.
19
20#. Additional standard extensions (starting with 'Z') will be sorted after
21   single-letter extensions and before any higher-privileged extensions.
22
23#. For additional standard extensions, the first letter following the 'Z'
24   conventionally indicates the most closely related alphabetical
25   extension category. If multiple 'Z' extensions are named, they will be
26   ordered first by category, in canonical order, as listed above, then
27   alphabetically within a category.
28
29#. Standard supervisor-level extensions (starting with 'S') will be listed
30   after standard unprivileged extensions.  If multiple supervisor-level
31   extensions are listed, they will be ordered alphabetically.
32
33#. Standard machine-level extensions (starting with 'Zxm') will be listed
34   after any lower-privileged, standard extensions. If multiple machine-level
35   extensions are listed, they will be ordered alphabetically.
36
37#. Non-standard extensions (starting with 'X') will be listed after all standard
38   extensions. If multiple non-standard extensions are listed, they will be
39   ordered alphabetically.
40
41An example string following the order is::
42
43   rv64imadc_zifoo_zigoo_zafoo_sbar_scar_zxmbaz_xqux_xrux
44
45Misaligned accesses
46-------------------
47
48Misaligned accesses are supported in userspace, but they may perform poorly.
49