[otbn] Clarify write behaviour for read-only CSR/WSR
Signed-off-by: Greg Chadwick <gac@lowrisc.org>
diff --git a/hw/ip/otbn/doc/_index.md b/hw/ip/otbn/doc/_index.md
index d3133d7..8d4777c 100644
--- a/hw/ip/otbn/doc/_index.md
+++ b/hw/ip/otbn/doc/_index.md
@@ -103,6 +103,7 @@
Control and Status Registers (CSRs) are 32b wide registers used for "special" purposes, as detailed in their description;
they are not related to the GPRs.
CSRs can be accessed through dedicated instructions, `CSRRS` and `CSRRW`.
+Writes to read-only registers are ignored; they do not signal an error.
All RW CSRs are set to 0 when OTBN starts (when 1 is written to {{< regref "CMD.start" >}}).
<table>
@@ -286,6 +287,7 @@
OTBN has 256b Wide Special purpose Registers (WSRs).
These are analogous to the 32b CSRs, but are used by big number instructions.
They can be accessed with the [`BN.WSRRS`]({{< relref "isa#bnwsrrs" >}}) and [`BN.WSRRW`]({{< relref "isa#bnwsrrw" >}}) instructions.
+Writes to read-only registers are ignored; they do not signal an error.
All RW WSRs are set to 0 when OTBN starts (when 1 is written to {{< regref "CMD.start" >}}).
<table>
