# BBA compatibility TODO Gaps between this implementation and what real GC software expects, found by reading the three drivers that actually drive a BBA: - **Swiss (in-game patches)** — `cube/patches/bba/bba.c` in [emukidid/swiss-gc](https://github.com/emukidid/swiss-gc). Note that directory holds one file per adapter (`bba.c` is the real DOL-015; `enc28j60.c`, `w5500.c`, `w6100.c`, `w6300.c` are other hardware) — check which file you are in. - **libogc2** — `lwip/arch/gc/netif/gcif.c` in [extremscorner/libogc2](https://github.com/extremscorner/libogc2). This is what Swiss's own menu networking uses, via `if_configex()`. - **stock devkitPro libogc** — same shapes as libogc2; read out of `/opt/devkitpro/libogc/lib/cube/libbba.a` (`gcif.o`) with `objdump`. ⚠️ **Games use the official Nintendo SDK driver, which none of us can read.** Everything below is derived from the three open drivers plus the register map they document. A game may exercise a mechanism no open driver touches, so "no open driver does this" is weaker evidence than "the hardware has no such register". The register map referenced throughout is the one in libogc2's `gcif.c` (`BBA_*` defines), which is the official one. --- ## 1. No hardware RX filtering — **it hung Swiss** **Status: ✅ DONE 2026-10-10** — `S0_MR` now sets the MACRAW **MF** bit, so the W5100 delivers own-MAC (`SHAR`) + broadcast only, matching what the real BBA does. `NCRB.PR` (bit 0) clears MF if the GC asks for promiscuous, sampled when socket 0 is OPENed (i.e. at init, so a later `PR` change takes effect on the next NCRA reset). **How it presented, which is worth knowing because it does not look like a filtering problem.** Swiss hung forever on "Initialising Network". libogc2's `bba_interrupt()` is ```c ir = bba_in8(BBA_IR); imr = bba_in8(BBA_IMR); status = ir & imr; while (status) { bba_out8(BBA_IR, status); ... start_rx ...; status = ir & imr; } ``` — it only exits when `IR & IMR` reaches zero. Unfiltered, every frame on the segment set `RI`, which re-asserted faster than the loop could clear it, so it **never exited**. Not slow: livelocked. Unplugging the ethernet cable let Swiss get all the way through (then fail DHCP, there being no server), which is what identified it — a one-minute experiment after a lot of theorising. **Why filtering against `SHAR` was safe after all.** The earlier worry was that MF filters on `SHAR`, which we write only during init, so a driver that WRITES PAR0–5 would leave it stale and MF would drop everything. libogc2 **reads** the MAC out of PAR0–5 rather than writing it, so for this path `SHAR` (written from `reg.par` in the same init) cannot go stale. Broadcast still passes MF, and a DHCP OFFER unicast to our MAC passes too. ⚠️ **Still to do for a driver that writes PAR0–5 after init:** refresh `SHAR` on a PAR write and re-open the socket, otherwise MF filters on the old MAC. Nothing we have seen does this, but the official SDK driver is unread. ⚠️ **The W5500 back-end was NOT changed** — it is the alternate path and unused. If it is ever revived it needs the same treatment (`Sn_MR` MF bit). Tests: `w5100_parallel_master` **T1** (the init sequence expects `S0_MR = MACRAW|MF`) and **T1b** (`promisc` high re-inits with MF clear). Mutation-tested: never setting MF fails both, with the message naming the Swiss hang. ### Original analysis, kept for the reasoning **Was: OPEN. Needs a decision, not just code.** The real BBA filters received frames. libogc2 configures `NCRB = AB|CA|2_PACKETS_PER_INT` = `0x52`, i.e. `PR=0` (not promiscuous), `PM=0` (no multicast), `AB=1` (accept broadcast) — so the device delivers **own-MAC + broadcast only**. We run the W5100 in MACRAW with the **MF (MAC filter) bit clear** (`_S0_MR_MACRAW = 0x04`, `w5100_parallel_master.py`), so **every frame on the segment enters the SPRAM ring.** Observed consequence, already seen on hardware: the test app's ring filled to page 15 from what looked like "ambient broadcast traffic" but was in fact all unicast traffic on the segment too. Ring pressure makes RBFI drop frames we DO want, and a game's lean driver may not expect foreign unicast at all. ⚠️ **Setting MF is not a one-line fix.** MF filters against the W5100's `SHAR`, and we write `SHAR` only during the init sequence. libogc2 *reads* the MAC out of PAR0–5 and never writes it, so SHAR never diverges there — but a game that WRITES PAR0–5 would leave SHAR stale and MF would then drop everything. Doing it properly needs: 1. decode `NCRB` (see item 3) so `PR`/`PM` can steer the filter; 2. refresh `SHAR` when PAR0–5 changes, not only at init; 3. re-open socket 0 when the filter mode changes (`Sn_MR` is sampled at OPEN). **The decision:** filter correctly (matches real hardware, fixes the ring pressure) at the cost of that machinery, or stay a permissive superset where nothing is ever lost but the ring fills with other hosts' traffic. ## 2. Ring geometry is hardcoded — silent corruption risk **Status: ✅ DONE 2026-10-10.** `RXFrameAssembler` takes the ring bounds from `bp`/`rhbp`, wired from the GC's `BP` (0x0A/0B) and `RHBP` (0x1A/1B) registers in `BBATop`. The wrap and the free-space calculation both use them, so a driver's smaller ring is respected instead of ignored. **Garbage is rejected, not obeyed.** The bounds are used only when `bp >= 1` (page 0 is the TX buffer), `rhbp <= 0x0F` and `rhbp > bp`; otherwise the hardcoded pages 1–15 stand and `geom_honoured` goes low. Without that check, `BP = 0` or an `RHBP` below `BP` gives a zero or negative ring size and the assembler writes outside it. ⚠️ **`geom_honoured` is NOT yet observable from outside the FPGA.** It is declared on `RXFrameAssembler` and asserted correctly (T7/T7b check it), but `BBATop` does not wire it to anything — no status register, no LED, no UART line. So a rejected geometry is currently silent to the GC and to the bench. Wiring it to the `StatusPanel` or the UART console is a small loose end worth closing; until then "rejected rather than obeyed" is the guarantee, not "reported". ⚠️ **A ring must be LARGER than `_MAX_FRAME_PAGES` (6) or nothing is ever stored.** A frame's length is unknown until EOF, so the assembler reserves the worst case (1518 + 4 bytes = 6 pages) before accepting one. With a 4-page ring `free` can never reach 6, so RBFI drops every frame — correct, conservative, and useless. This was found by writing the test with `RHBP = 4` first, where it passed **vacuously** because nothing was stored at all. If a real driver ever configures a ring that small, the reservation has to become length-aware (accept a frame whose actual length fits), which is a bigger change than this one. Tests: `rx_frame_assembler` **T7** (an 8-page ring is honoured, frames land only in pages 1–8) and **T7b** (all three garbage cases rejected, and the fallback layout still stores). Mutation-tested: wrapping at the hardcoded page 15 writes pages 9–11 and fails T7; removing the sanity check fails all four T7b assertions. ⚠️ **This change cost a 0/8 timing failure before it was tuned, and the shape of that is worth remembering.** The first version computed the sanitised bounds combinationally, which put ``` bp/rhbp -> comparators -> Mux -> subtract -> pg_total -> add -> raw -> compare -> free -> compare -> have_room ``` in ONE combinational path where it had previously been constant-folded. `clk` fmax collapsed **29 → 17 MHz and all eight seeds failed** the 24 MHz target. Two things were needed: 1. **Register the bounds** (`pg_first`/`pg_last`/`pg_total`, and `rrp_s`). They change only when the GC writes `BP`/`RHBP`/`RRP`, so a cycle of staleness costs nothing, and `free` already reaches the FSM through the registered `have_room`. That alone got it to ~20 MHz — still failing. 2. **Narrow them to 4 bits.** Ring pages are 1–15. An 8-bit variable-vs-variable comparison is a carry chain, where variable-vs-CONSTANT folds into a couple of LUTs — which is exactly what was lost by making the bounds dynamic. `geom_ok` guarantees `bp < rhbp <= 0x0F`, which is what makes the narrow slice safe. Result: 26.9–28.5 MHz on the two sampled seeds, against 28.0–29.2 for the build without this change. **Always sweep after touching the assembler's ring arithmetic** — and note the register-space decode (item 4) was measured separately and cost nothing, so attributing a regression needs the A/B. For the record, libogc2 writes `TLBP = 0x0000`, `BP = 0x0001`, `RHBP = 0x000F` — exactly the old hardcoded layout, which is why this never showed up in practice. ## Throughput: measured 850 KB/s, and it is NOT the adapter **Measured 2026-10-10**, Swiss dumping a disc over the adapter, captured on the PC and analysed with `scripts/analyse-pcap.py`: ``` GC -> PC 18395 frames, 19.83 MB, 850 KB/s (6.97 Mbit/s, 25.8% of the EXI ceiling) 807 frames/s, mean frame 1078 B, retransmissions 0 PC -> GC 9200 frames, mean 54 B (pure ACKs) GC advertised window 2920 B; gaps >5 ms total 56 ms (0.2% of 22.78 s) ``` **The flow is window × RTT bound and nothing else:** | derived | value | |---|---| | data frames per ACK | 2.0 | | payload per round trip | 2048 B | | implied RTT | 2.48 ms | | 2048 B / 2.48 ms | **826 KB/s** ≈ the measured 850 | | payload per frame | **1024 B** (wire 1078 − 54 of headers) | | window ÷ segment | 2920 / 1024 → **2 segments in flight** | So the sender can have 2048 bytes outstanding and each round trip costs 2.48 ms. **Zero retransmissions** means the 15-page ring never overflowed and we drop nothing; **0.2% stall time** means nothing is waiting on the disc. Our W5100 bus engine runs at roughly 17% utilisation at this rate. The adapter is not the constraint. The levers, in order of effect, and **none of the first two are ours**: 1. **A larger TCP send buffer on the GC** (lwIP `TCP_SND_BUF`). Throughput scales with it almost linearly while the flow is window-bound. 2. **Full-MSS segments.** Swiss writes 1024 bytes at a time, so a 2920 B window carries 2048 B of payload. At 1460 B segments the same window and RTT would give ~**1150 KB/s, a 1.43× gain** for free. 3. **A shorter round trip** — throughput scales 1/RTT. The 2.48 ms is GC-side interrupt dispatch and lwIP, plus EXI lock contention: libogc2's `bba_event_handler` bails out if `EXI_Lock` fails and defers until the TX path unlocks, so RX handling queues behind TX on the single channel. ⚠️ **RX interrupt coalescing would make this WORSE, and that reverses an earlier recommendation in this file.** `NCRB.RXINTC` asks for 2 packets per interrupt and `RXINTT` adds a timer; both were listed as throughput levers. For a **window-bound** flow they are not: delaying the ACK's interrupt lengthens the round trip, and throughput is bytes-per-round-trip ÷ RTT. The measurement is what showed this — the guess was backwards. ### ⚠️ The 850 KB/s figure does NOT bound ROM streaming — different stack The dump was Swiss's **menu** path: libogc2 + lwIP + TCP, 1024-byte application writes, a 2920-byte window. **ROM streaming uses a completely different code path** — Swiss's in-game patch driver and its own stack in `cube/patches/bba/tcpip.c`, which has **no TCP in it at all**: - **UDP + FSP** (File Service Protocol), with ICMP and ARP. No TCP, no lwIP, no congestion control, no window. - Reads are sized to the path MTU, not 1024: `_fsp.data_length = MIN(length, env->pmtu - (fsp->data - eth->data))` — about 1446 B of payload per datagram at a 1500 MTU. - `QUEUE_SIZE 2` requests outstanding. - `udp->checksum = 0x0000` — no payload checksum, which is real CPU saved versus TCP. - It runs in the game's context via the patch driver, which uses the leaner 4-byte command-space form (see the command-space note in CLAUDE.md). So the TCP window × RTT limit is not the operative one there, and the measured 850 KB/s says nothing about it. What the dump DID establish, and which carries over, is that the adapter has headroom: **zero retransmissions**, ~17% W5100 bus utilisation, 26% of the EXI ceiling. **Not yet measured.** `analyse-pcap.py` now counts UDP payload and picks the bulk direction automatically (streaming flows PC → GC, the opposite of a dump), so `--bulk to_gc` on a streaming capture gives the real number. Until that is run, "can it stream?" is open — but it is open on the GC's side, not on the adapter's. The hard ceiling remains **3375 KB/s** (27 MHz × 1 bit/clock), and the GC's own disc delivers roughly 2000–3100 KB/s, so there is no headroom to spare if a game wants full disc rate — but games read in bursts, not continuously at peak. ## 3. `NCRB` (0x01) is not decoded at all **Status: ✅ DECODED 2026-10-10** — stored, readable, and exported as `BBARegisterFile.ncrb`. Nothing consumes the bits yet; that is item 1. Bits, from libogc2: | bit | name | meaning | |---|---|---| | 0 | `PR` | promiscuous mode | | 1 | `CA` | capture effect mode | | 2 | `PM` | pass multicast | | 3 | `PB` | pass bad frame | | 4 | `AB` | accept broadcast | | 6–7 | `RXINTC` | RX interrupt counter: 1 / 2 / … packets per interrupt | We interrupt per frame where libogc2 asks for every 2 packets, which is benign (more interrupts than requested). `PR`/`PM`/`AB` feed item 1. ⚠️ **Do NOT implement `RXINTC`/`RXINTT` coalescing as a throughput optimisation.** Measurement showed the bulk flow is window × RTT bound (see "Throughput" above), so delaying an interrupt lengthens the round trip and *lowers* throughput. Implement it for fidelity if a driver depends on the behaviour, not for speed. `bba_register_file` **T15** asserts the export is `0x52` with `PR=0`, `PM=0`, `AB=1` — i.e. the real BBA would deliver own-MAC + broadcast only — which is the fact item 1 has to act on. ## 4. Registers that read `0x00` but have non-zero hardware defaults **Status: ✅ DONE 2026-10-10** for every register libogc2 writes. They now store writes and read back, with the documented hardware defaults at reset. `bba_register_file` **T15** checks the defaults, replays libogc2's twelve init writes and reads every one back; mutation-tested (dropping `NWAYC`'s `0x84` default fails it, as does dropping `NCRB` writes). Still reading `0x00`, deliberately: `LTPS` (0x04), `LRPS` (0x05), `TWP` (0x0E/0F), `TRP` (0x12/13) and `TXFIFOCNT` (0x3E/3F). These are **status** registers, not configuration — a driver reads them to learn what the device did, so storing what was written to them would be a lie rather than a default. Drivers read `LRPS`/`LTPS` only on error bits, and `0x00` reads as "no error", which is the truthful answer for a device that does not detect those errors. `TWP`/`TRP`/`TXFIFOCNT` would need the TX engine to actually report its page pointers and FIFO depth; worth doing if a driver is ever seen to poll them. The original problem for reference — a driver doing read-modify-write, or sanity-checking a default, saw zero. libogc2 *does* read `NWAYC` (0x30) before writing it. | reg | name | hardware default | |---|---|---| | 0x01 | `NCRB` | — | | 0x04 | `LTPS` | last transmitted packet status, RO | | 0x05 | `LRPS` | last received packet status, RO | | 0x0A/0B | `BP` | boundary page pointer | | 0x0C/0D | `TLBP` | TX low boundary page | | 0x0E/0F | `TWP` | TX buffer write page | | 0x12/13 | `TRP` | TX buffer read page | | 0x14/15 | `RXINTT` | RX interrupt timer | | 0x1A/1B | `RHBP` | RX high boundary page | | 0x30 | `NWAYC` | **0x84** | | 0x32 | `GCA` | 0x00 | | 0x3D | `MISC` | **0x3C** | | 0x3E/3F | `TXFIFOCNT` | TX FIFO counter, RO | | 0x50 | `MISC2` | 0x00 | | 0x58–0x5F | SI regs | libogc2 writes `80 00 03 83 32 FE 1F 1F` | | 0x60 | `SI_ACTRL2` | libogc2 writes `0x00` then `0x74` | ## 5. Two of our registers do not exist on real hardware **Status: KNOWN, low risk, do not "fix" blindly.** `0x34`/`0x35` (`TWD_LO`/`TWD_HI`, our TX length latch) and `0x3A` (`HIPR`, returning `0x01`) are **not in the official register map** — `0x3A` is unassigned there and `MISC` is `0x3D`. They are our own inventions, and it is now confirmed that **no driver writes TWD** (the TX length is counted at TXDATA instead — see CLAUDE.md). They are harmless unless a game writes those addresses for some other purpose and trips our TX-length latch. Removing them would break `gc_bba_test`, which does write TWD, so keep them but do not add more inventions. ## 6. Lower risk / no action - **Challenge/response** (command regs 0x08/0x09/0x0B) is **device-initiated** — gated on command status bit `0x10`, which we never raise, so the path cannot be entered. Do NOT implement `bba_calc_response` emulation. - **No EEPROM**: the MAC is a fixed reset value in PAR0–5. Since libogc2 reads the MAC from there rather than writing it, that value is what the console uses. Games keying anything off the MAC see ours. - **`RXINTT`** (RX interrupt timer) is interrupt coalescing; ignoring it means we interrupt earlier than asked, which is safe. - **Command register `0x0F`** may be a reset strobe rather than a register: `bba_cmd_in8_slow(0x0F)` holds CS for 200 µs between two 10 ms delays and discards the result. Implemented as storage; the NCRA reset follows immediately in that sequence so ignoring any side effect costs nothing today.