[otbn] Refactor get_quarter_word_unsigned in ISS/docs
Instead of passing the register and lane indices to the state, we
inline the lookup and then call a static function to extract the lane.
This means that we can easily document the extraction function, and
the documentation loses another reference to state internals.
Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/hw/ip/otbn/doc/isa.md b/hw/ip/otbn/doc/isa.md
index 5aeb890..9d18d1d 100644
--- a/hw/ip/otbn/doc/isa.md
+++ b/hw/ip/otbn/doc/isa.md
@@ -93,6 +93,12 @@
shift_bits = 8 * shift_bytes
shifted = value << shift_bits if shift_type == 0 else value >> shift_bits
return shifted & mask256
+
+def extract_quarter_word(value: int, qwsel: int) -> int:
+ '''Extract a 64-bit quarter word from a 256-bit value.'''
+ assert 0 <= value < (1 << 256)
+ assert 0 <= qwsel <= 3
+ return (value >> (qwsel * 64)) & ((1 << 64) - 1)
```
<!-- Documentation for the instructions in the ISA. Generated from ../data/insns.yml. -->
