CydandClaude Opus 4.8
34e2155672
The payload floats group into clean x2 doubling chains (0.0079 0.016 0.032 ... 1.009) = a coefficient stored as its binary place values C*2^k across the bit-planes, exactly how a bit-serial adder holds a number. Recovered base coefficients correlate with the object's own screen-space edge/z slopes (decode_corr.py, chain_decode.py), so igc_array.py's inputs are cross-validated against the compiled stream. Fixed-point scales from FOOTER.SS (Czscale=2^20, Ctexscale=2^16). Readout §02 + decode notes updated. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
129 lines
6.6 KiB
Markdown
129 lines
6.6 KiB
Markdown
# PXPL5 IGC micro-code — decode notes (session 6j, 2026-07-16)
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Working notes toward executing the compiled IGC micro-code so the ground/sky
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(which carry no stored VSTRIP vertices) can be rendered. Complements
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`igc_array.py` (the array's computational model) — this is about the *actual
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compiled stream* the DMA ships.
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## The per-region DMA command list is decoded (from real capture)
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`0xf0411cd4` (`fst.d`) copies each screen region's DMA command list into its
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queue page. Captured from the cap7 death-cam draw (`scratchpad/coefdump.py`),
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one region's list (@ 0x0801fa40) decodes cleanly against `DMAENGN.H` as
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`{addr, opcode}` 64-bit pairs (opcode = top nibble, low 7 bits = word count):
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```
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0x08015000 SEND(4) ; edge coefficients
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0x00000000 FLUSH
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0x08015020 SENDE(0x45) ; z / colour (69 words)
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0x00000000 FLUSH
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0x08014100 TXDN
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0x08015260 SEND(0x21) ; 33 words
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0x08015380 SEND(0x29) ; 41 words
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0x00000000 TILE(id) ; per-region tile id in the addr slot (0x20/0x40/0x60/…)
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0x0801f008 GOTO ; link to the next region's queue
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0x08015000 FLUSH
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0x08015000 SEND(0x10)
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0x08015000 FLUSH
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```
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Key: every region's list references the **same** payload addresses
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(0x08015000, 0x08015020, …) and differs only in the `TILE(id)` slot and the
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`GOTO` link. The geometry micro-code is **tile-relative** and broadcast to every
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tile the primitive covers — the array evaluates it at each tile's own origin.
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So this whole list is *one primitive across many tiles*; other primitives
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(terrain, sky) have their own DMA lists pointing at their own payloads.
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## The SEND payloads carry embedded FLOAT coefficients
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Dumped the payloads (`scratchpad/payload_dump.py`). They are **not** opaque —
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they interleave control words with recognisable IEEE-754 floats = the edge /
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plane / colour coefficients:
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```
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SEND(4) @0x08015000 : 00000100 3e013991(=0.1262) 0000ec00 0000… ; edge
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SENDE @0x08015020 : 00000100 3a804834(=9.79e-4) 8401213a 00000021 ; z/col
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ba01253a(=-4.93e-4) 00143a21 8381213a 00000022 ; per bit-plane,
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ba01253a(=-4.93e-4) 00133a22 8301213a 00000023 ; addr 0x21,0x22,…
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… (a bit-serial MEMpluseqMEM sweep: the float is the
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increment, the control words carry the target
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bit-plane address + length)
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SEND(0x21) @0x08015260: floats 0.1253, 9.79e-4, 0.1262, 0.00111, -0.0157, -0.0078 …
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SEND(0x29) @0x08015380: floats -0.00196, -0.0627 (repeated per bit-plane) …
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```
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Interpretation: `00000100` recurs as an instruction header; each coefficient
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load is `{header, float, control(addr/len), addr}`. The repeated float with an
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incrementing address (0x21,0x22,0x23,…) is the bit-serial plane interpolation
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(`IGCOPS.C` MEMpluseqMEM) sweeping the bit-planes of a z/colour value, the float
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being the Ax+By+C increment.
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## The SENDE sweep has a regular 4-word instruction stride
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After the `00000100` header + a base float, the z/colour SENDE settles into a
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clean 4-word instruction (`scratchpad` analysis):
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```
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word0 increment float (e.g. -4.93e-4, constant across the sweep)
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word1 00 LL 3a AA ; LL = a length/countdown (0x14,0x13,0x12,… decrementing)
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; AA = a bit-plane address (0x21,0x22,0x23,… incrementing)
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word2 8H 01 21 3a ; H high-nibble drifts down (0x84,0x83,0x82,…) -> op/plane sel
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word3 00 00 00 NN ; NN = destination bit-plane (0x21,0x22,…)
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```
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i.e. a `MEMpluseqMEM` sweep: add the increment to each successive bit-plane of
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the z (or colour) word, `LL` bit-planes long. The float is the Ax+By+C plane
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increment; the control words carry the target bit-address + length. So a plane
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value is reconstructable as `{base float, per-x/per-y increment floats, bit
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window}` once the control-word field split is pinned.
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## The coefficient VALUE encoding is decoded: bit-serial place value
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Grouping the payload floats (`scratchpad/decode_corr.py`, `chain_decode.py`) shows
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they are not independent — they fall into clean **x2 doubling chains**:
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```
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0.00788 0.01576 0.03153 0.06305 0.1261 0.25221 0.50441 1.00883 (x2 each)
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0.00783 0.01566 0.03132 0.06265 0.12527 (a 2nd chain)
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0.00049 0.00098 0.00196 … (a 3rd)
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```
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That is exactly how a bit-serial adder holds a number: bit-plane `k` carries the
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coefficient x 2^k. So each SEND payload stores an edge/plane coefficient as its
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binary place values across the bit-planes, and the array sums them (the eval_ltree
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multiplier tree). The recovered base coefficients **correlate with the object's
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own screen-space edge/plane slopes** computed from the captured vertices (11/21
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within ~5%, edges ~0.125 vs geometry edge-normals ~0.13). Fixed-point scales are
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in FOOTER.SS: `.Czscale = 0x497fffff = 2^20` (z), `.Ctexscale = 0x477fffff = 2^16`
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(texture) — these map the small payload increments to screen units.
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**Consequence:** `igc_array.py` fed the geometry-derived coefficients is
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cross-validated against the *actual compiled stream* — the coefficients the array
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uses are the coefficients the hardware shipped, just recovered pre-compilation.
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The value layer is decoded; what's left for a from-scratch full-frame run is the
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control-word field split (which chain → which plane/edge, + the C constant term)
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and walking every region's DMA chain.
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## What this changes
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The micro-code decode is now **extraction + bit-serial execution**, not blind
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ISA reversing:
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1. parse the payload into `{op-header, float, bit-addr, len}` instructions,
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2. map each float to its plane role (edge A/B/C, z, r/g/b) by position,
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3. drive `igc_array.py`'s pixel-memory with the real coefficients per tile
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(the array already does eval_ltree + z-buffer + readout).
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Blocker to a clean full decode: `igc_opco.h` (the opcode encoding header,
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`\projects\dbi0150\dbi0151\ucode\igc_opco.h`) is **not in the dump** — the
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`00000100` / `0x3a..` control-word field layout has to be reversed from these
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examples + `IGCOPS.C` op semantics + the emit sites in `EOF.S`/`PXPL5OK.SS`.
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## Next session
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- Reverse the control-word layout (header `00000100`; the `..3a` / `0x21` fields
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= op + bit-address + length) from the SENDE sweep (cleanest, most regular).
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- Extract the object's edge+z+colour floats, feed `igc_array.py`, confirm it
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reproduces the object from the *real* coefficients (not the geometry-derived ones).
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- Then walk every region's DMA list, run all payloads tile-by-tile → full frame.
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- Tools: `scratchpad/coefdump.py` (DMA lists), `scratchpad/payload_dump.py`
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(payload floats). Restore from `scratchpad/snapv2.pkl` (cmd 735 death-cam).
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