Initial commit: retroDE_ps2 — first-of-its-kind PS2 GS FPGA core (DE25-Nano / Agilex 5)
RTL (GS rasterizer, EE core stub, platform bridge, LPDDR4B path), sim regression (272 TBs), docs, and tooling. Copyrighted PS2 content (BIOS, game code, GS dumps, and all dump-derived textures/traces) is excluded via .gitignore and stays local. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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# Ch281 closeout — MMI3/PNOR (canonical NOT); next blocker is PAND
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**Status:** Closed. **Verdict from re-running qbert.elf:**
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`elf_first_unsupported_opcode (pc=0x00112C98 instr=0x70431489)` —
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opcode `0x1C` (MMI) + funct `0x09` (MMI2) + sa `0x12` = **PAND**
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(Parallel AND). qbert is now deep into the SIMD byte-walker's
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mask-and-reduce stage: PSUBB → PNOR → PAND.
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## Numbers
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| Chapter | Blocker | qbert retire_count |
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|---------|---------|---------------------|
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| Post-Ch279 (LQ) | PSUBB at 0x00112C90 | 27,020 |
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| Post-Ch280 (PSUBB) | PNOR at 0x00112C94 | 27,021 |
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| **Post-Ch281 (PNOR)** | **PAND at 0x00112C98** | **27,022** |
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1-retire delta — PNOR retired, PAND traps next.
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## What landed
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### RTL — 5 surgical edits in `ee_core_stub.sv`
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1. **Constants**: `FUNC_MMI3 = 6'h29`, `MMI3_PNOR = 5'h13`.
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2. **Decode**: `is_pnor = is_mmi && (func == FUNC_MMI3) &&
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(shamt == MMI3_PNOR)`. Same three-way AND as Ch278/Ch280.
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3. **`is_rtype_alu` group**: added `is_pnor`.
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4. **Writeback (REUSE)**: extended the existing NOR arm to
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`else if (is_nor || is_pnor) rtype_alu_wb = ~(rs_val | rt_val)`.
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Architectural 128-bit PNOR collapses to a regular 32-bit
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bitwise NOR for the low lane.
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5. **`is_nop_class` allow**: `!is_pnor` added.
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5 LOC of real change. Pure pattern reuse from Ch280 PSUBB
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(same MMI narrow-decode shape) plus reuse of the existing
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NOR writeback arm.
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### Focused TB — `tb_ee_core_pnor.sv`
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Three cases:
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1. **qbert exact encoding**: `pnor $v1, $zero, $t1`. Encoder
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asserted == `0x70091CE9`. With `$t1 = 0x12345678` → `$v1
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= ~0x12345678 = 0xEDCBA987`.
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2. **NOT-of-zero**: `pnor $t2, $0, $0` → `0xFFFFFFFF`. Both
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operands zero; result is all-ones.
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3. **General NOR**: `$t3 = 0xF0F0F0F0`, `$t4 = 0x0F0F0F0F`
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→ `$t5 = ~(0xF0F0F0F0 | 0x0F0F0F0F) = ~0xFFFFFFFF = 0`.
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Locks in the "general two-operand NOR" path even though
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qbert's specific usage is the NOT-pseudo form.
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Result: `retired=22 halt=1 trap=0 pc=0xbfc0014c errors=0 PASS`.
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### Makefile + regression
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- `tb_ee_core_pnor` target.
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- Added to both regression lists.
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- Regression: 168 → **169**.
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## qbert's SIMD byte-walker — pipeline shape now clear
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Six MMI/load chapters (Ch278–Ch281, plus Ch271 SQ and Ch279 LQ)
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have surfaced the full byte-walker shape:
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```
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0x00112C88: lq $t1, 0($a1) ; Ch279 — load 128-bit chunk
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0x00112C8C: <one instr we haven't seen the next blocker for>
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0x00112C90: psubb $v0, $t1, $t2 ; Ch280 — per-byte subtract
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0x00112C94: pnor $v1, $zero, $t1 ; Ch281 — ~$t1 (mask gen)
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0x00112C98: pand $v0, $v0, $v1 ; Ch282 — mask the result
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... reduction continues ...
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```
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This is the classic "find a zero byte" or "detect sentinel byte"
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SIMD loop — `PSUBB` against a key, `PNOR` to invert the bits,
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`PAND` with a mask to isolate the lanes where the condition
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holds, then `PMFHL` or similar to reduce to a single GPR for a
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branch test.
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## Recommendation for Codex's Ch282 — PAND
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`0x70431489` at PC `0x00112C98`:
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- opcode 0x1C (MMI)
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- funct 0x09 (MMI2)
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- sa 0x12 (PAND within MMI2)
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- rs=$v0, rt=$v1, rd=$v0
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- → `pand $v0, $v0, $v1`
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Architectural: 128-bit `$rd = $rs & $rt`. For our 32-bit model:
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**bit-identical to standard AND** (SPECIAL funct 0x24). Same
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shape as PNOR/NOR — different opcode, reused writeback arm.
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Implementation outline (mirrors Ch281 PNOR exactly):
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1. `localparam MMI2_PAND = 5'h12`.
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2. `is_pand = is_mmi && (func == FUNC_MMI2) && (shamt ==
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MMI2_PAND)`. The MMI2 funct constant already exists from
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Ch278.
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3. Add to `is_rtype_alu`.
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4. **Reuse the existing AND writeback arm**:
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```sv
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else if (is_and || is_pand) rtype_alu_wb = rs_val & rt_val;
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```
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5. Add `!is_pand` to `is_nop_class`.
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~4 LOC.
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Focused TB:
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- Exact qbert encoding asserted == `0x70431489`.
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- General AND case: `pand $rd, 0xFFFFFFFF, 0xAAAAAAAA` →
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`0xAAAAAAAA`.
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- All-zero case: `pand $rd, 0xFFFFFFFF, 0x00000000` → 0.
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**Likely follow-ons** after PAND: **PMFHL** (move from HI/LO
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low halves) for the reduction — the byte-walker needs to fold
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the masked vector down to a scalar for branching. Or
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**PEXTLW** (parallel extract low word) for a different
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reduction shape.
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## Pattern review (11 chapters)
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| Ch | Blocker | Edits | Pattern |
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|----|---------|-------|---------|
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| 271 SQ | first | 5 | NEW 4-beat write |
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| 272 DADDU | | 4 | NEW ALU-low-32 |
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| 273 SYSCALL HLE | | 2 | NEW gated dispatcher |
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| 274 BEQL | | 6 | NEW branch+squash |
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| 275 SD | | 7 | REUSE SQ counter |
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| 276 DSLL | | 4 | REUSE DADDU |
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| 277 BNEL | | 6 | REUSE BEQL squash |
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| 278 PCPYLD | | 4 | NEW MMI narrow-decode |
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| 279 LQ | | 5 | REUSE LW path |
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| 280 PSUBB | | 5 | REUSE MMI narrow (byte-SIMD) |
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| **281 PNOR** | | **5** | **REUSE MMI narrow + reuse NOR arm** |
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5 NEW patterns + 6 REUSE chapters. The reuse density continues
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to climb — Ch282 PAND will be the most-reused chapter yet (MMI
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narrow-decode + standard-AND writeback, both already in place).
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## Files changed
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- `rtl/ee/ee_core_stub.sv` — 5 surgical edits.
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- `sim/tb/integration/tb_ee_core_pnor.sv` — new focused TB.
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- `sim/Makefile` — target + both regression lists.
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## Regression
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In flight; expected **169/169**.
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