fpu100-bugfix

Unmerged patches for opencores.org/projects/fpu100 — two bugs from the upstream tracker that had never been addressed.

The first commit is fpu100 trunk rev 21 imported verbatim, so git log -p is an auditable diff between upstream and this fork.

Fixed bugs

Bugtracker #4 / #5 — denormal multiply

A multiplication whose result lands in the binary32 subnormal range comes back with the wrong mantissa. Reproducer from the report:

0xc1800300  *  0x00034000
  upstream         -> 0x80068027   (wrong)
  soft-float ref.  -> 0x80340138
  after patch      -> 0x80340138

Fix: one-line shift-amount correction at core/post_norm_mul.vhd:154.

Bugtracker #2 — ready_o premature

ready_o rises two clocks before output_o is actually valid, so any master sampling on the rising edge reads stale data.

Fix: MUL_COUNT raised from 11 to 13 at core/fpu.vhd:107 so that ready_o lines up with the first clock at which output_o is correct.

Verification

Simulation (requires GHDL, tested 4.1):

cd sim
./run.sh            # patched   — all cases PASS
./run.sh unpatched  # reverts   — bug #4/#5 case FAILs

Hardware (Intel Cyclone V, SH-2 at 50 MHz, 2026-04-22): an 8-case regression covering add / sub / mul / div / sqrt plus the bugtracker #4/#5 reproducer was driven through the Wishbone wrapper below. All passed, using both the bus-stall RESULT read path and the FPU_OP & 0x100 polling path.

Bonus: rtl/fpu_wb.v

Not part of upstream. A small Wishbone-slave wrapper around the raw fpu entity, included as a convenience:

+0x00  OPA     (R/W)
+0x04  OPB     (R/W)
+0x08  FPU_OP  (W: op + rmode, triggers start; R: status + flags)
+0x0C  RESULT  (R stalls until ready, W = re-start)

License

fpu100 is © 2006 Jidan Al-eryani <jidan@gmx.net>, distributed under:

This source file may be used and distributed without restriction provided that this copyright statement is not removed from the file and that any derivative work contains the original copyright notice and the associated disclaimer. (…) You can use this code academically, commercially, etc. for free; just acknowledge the author.

Source-file copyright notices are preserved. Patches and wrapper are contributed under the same terms.

About the modifications in this fork

kinopochan makes no claim of rights over the bug-fix diffs or the wrapper in this fork. When using this code, please follow the original upstream license (above) and retain the original author attribution (Jidan Al-eryani).

このフォークの修正差分・wrapper について、きのぽは特に権利主張を しません。利用時はオリジナル (上記) のライセンスに従い、オリジナル 作者 (Jidan Al-eryani) のクレジットを忘れずに記載してください。