| commit | 9a4f47b060130dc0b4b2107b37a5b64e528762dd | [log] [tgz] |
|---|---|---|
| author | Rupert Swarbrick <rswarbrick@lowrisc.org> | Mon Jan 04 15:48:24 2021 +0000 |
| committer | Rupert Swarbrick <rswarbrick@gmail.com> | Wed Jan 13 09:06:38 2021 +0000 |
| tree | 4a64e9850a5d60e88b6fc730b5db6af9a2d4ee6d | |
| parent | 51ec3894c39b0f48e798e3f370f30d6c0552cbdc [diff] |
[otbn] Fix wrapping for grs1 increment with bn.lid/bn.sid
Because grs1 has a 14-bit signed immediate added to it, we can have a
valid address even when grs1 is really big. For example, suppose that
x21 is 0x1000 in
bn.lid x10, -2144(x21++)
Then we should write x21 = 0x1020 (rather than the 0x20 that we were
writing).
Similarly, if x21 is 0xffffffc0 (2^32 - 64) then
bn.lid x10, 64(x21++)
loads from the valid address of 0 and should increment x21 to
0xffffffe0.
Getting this right implies a 27-bit adder, plus extra muxing for the
top bits of increment_out. To avoid that, we could e.g. change the
spec to only operate on the lower 16 bits of the base address (which
would then need an 11-bit adder and no muxing on most of the upper
bits: much more manageable!)
Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
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