retroDE_ps2/docs/ch282_closeout.md

# Ch282 closeout — PAND; next blocker is PCPYUD (the first "upper-half" MMI op)

**Status:** Closed. **Verdict from re-running qbert.elf:**
`elf_first_unsupported_opcode (pc=0x00112CA0 instr=0x704923A9)` —
opcode `0x1C` (MMI) + funct `0x29` (MMI3) + sa `0x0E` =
**PCPYUD** (Parallel Copy **Upper** Doubleword). This is the
first MMI op that reads from the architectural **upper 64
bits** of a source register — a place our 32-bit-GPR model has
never been able to represent.

## Numbers

| Chapter | Blocker | qbert retire_count |
|---------|---------|---------------------|
| Post-Ch280 (PSUBB) | PNOR at 0x00112C94 | 27,021 |
| Post-Ch281 (PNOR) | PAND at 0x00112C98 | 27,022 |
| **Post-Ch282 (PAND)** | **PCPYUD at 0x00112CA0** | **27,024** |

2-retire delta — PAND retired plus one instruction at PC
0x00112C9C (probably another byte-broadcast or comparison),
then PCPYUD traps.

## What landed

### RTL — 5 surgical edits in `ee_core_stub.sv`

1. `localparam MMI2_PAND = 5'h12` alongside MMI2_PCPYLD.
2. `is_pand = is_mmi && (func == FUNC_MMI2) && (shamt ==
   MMI2_PAND)`.
3. Added `is_pand` to `is_rtype_alu`.
4. **Reused** the existing AND writeback: `if (is_and ||
   is_pand) rtype_alu_wb = rs_val & rt_val`.
5. `!is_pand` added to `is_nop_class`.

Highest-reuse chapter yet — MMI narrow-decode + AND writeback
arm both already in place from prior chapters.

### Focused TB — `tb_ee_core_pand.sv`

Three cases:

1. **Exact qbert encoding**: `pand $v0, $v0, $v1` (rs=2, rt=3,
   rd=2, sa=0x12, funct=0x09). Encoder asserted `0x70431489`.
   `$v0 = 0xFFFFFFFF & 0xAAAAAAAA = 0xAAAAAAAA`.
2. **Disjoint masks**: `0xF0F0F0F0 & 0x0F0F0F0F = 0` (proves
   pure bitwise AND).
3. **Zero-mask**: `0xDEADBEEF & 0 = 0`.

Result: `retired=24 halt=1 trap=0 pc=0xbfc00154 errors=0 PASS`.

### Makefile + regression

- `tb_ee_core_pand` target.
- Added to both regression lists.
- Regression: 169 → **170**.

## Ch283 framing — PCPYUD: a fork in the road

**Decoded**: `pcpyud $a0, $v0, $t1` (rs=$v0, rt=$t1, rd=$a0).
- Architectural: `$rd[127:64] = $rs[127:64]; $rd[63:0] =
  $rt[127:64]`. Extracts the upper-64 of both source operands;
  the upper-64 of rt becomes the lower-64 of rd.

**The fundamental problem**: every prior chapter has lived
inside a "low 32 bits only" approximation. The upper 96 bits
of every GPR are silently 0 in our model — never written by
SQ/SD/PCPYLD/PSUBB/PNOR/PAND. PCPYUD is the first op that
**reads** from that upper half, so the question becomes
unavoidable:

- **Option A — preserve the approximation**: implement PCPYUD
  as `$rd = 0` always. Honest "this op reads from a region we
  don't model, which is always zero by construction." qbert
  will see all-zero PCPYUD results and **may falsely conclude
  it found a sentinel byte every iteration** of the
  byte-walker. Silent divergence; the next 5-10 chapters of
  blockers might be illusory (cascading from the wrong PCPYUD
  result) rather than real qbert needs.
- **Option B — NOP-class PCPYUD (do not allow)**: leave it
  trapping; surface this as the "model boundary" that warrants
  a real-128-bit-GPR pivot in a future chapter. qbert wouldn't
  continue past 27,024 until that pivot happens.
- **Option C — implement 128-bit GPRs**: faithful but a big
  cross-cutting change (regfile width, every ALU arm, every
  load/store writeback). Multiple chapters of work. Real
  semantic correctness, but breaks the "one op per chapter"
  cadence we've held since Ch271.

**My read**: at minimum, do NOT silently NOP-class to 0. The
qbert byte-walker's correctness depends on the upper 8 bytes
of every LQ. Even if we land "Option B" first (keep the trap),
the next chapter genuinely should be the 128-bit GPR pivot.

This is the right moment to step back and frame the broader
question with Codex. The MMI-narrow-decode cadence has worked
beautifully for ops where low-32-bit semantics happen to
suffice (PCPYLD, PSUBB, PNOR, PAND). It hits a wall at
upper-half ops. Either:

1. Bite the 128-bit GPR bullet now (Ch283 = "expand regfile
   to 128 bits + propagate through every LQ/SQ/SD/PCPYLD/...
   writeback").
2. Accept that qbert is "as far as we can get" without 128-bit
   GPRs and pivot to a different ELF (homebrew that's
   32-bit-clean) or to hardware-facing deliverables.

I'd recommend (1) is the right next move — qbert has been a
productive test vector, and the SIMD byte-walker shape is
universal across PS2 stdlib code. Future game ELFs will hit
the same wall.

## Files changed

- `rtl/ee/ee_core_stub.sv` — 5 surgical edits.
- `sim/tb/integration/tb_ee_core_pand.sv` — new focused TB.
- `sim/Makefile` — target + both regression lists.

## Regression

In flight; expected **170/170**.

## Pattern review (12 chapters)

| Ch | Blocker | Edits | Pattern |
|----|---------|-------|---------|
| 271 SQ | first qbert | 5 | NEW 4-beat write |
| 272 DADDU | | 4 | NEW ALU-low-32 |
| 273 SYSCALL HLE | | 2 | NEW gated dispatcher |
| 274 BEQL | | 6 | NEW branch+squash |
| 275 SD | | 7 | REUSE SQ counter |
| 276 DSLL | | 4 | REUSE DADDU |
| 277 BNEL | | 6 | REUSE BEQL squash |
| 278 PCPYLD | | 4 | NEW MMI narrow-decode |
| 279 LQ | | 5 | REUSE LW path |
| 280 PSUBB | | 5 | REUSE MMI narrow (byte-SIMD new) |
| 281 PNOR | | 5 | REUSE MMI narrow + NOR arm |
| **282 PAND** | | **5** | **REUSE MMI narrow + AND arm** |

5 NEW patterns + 7 REUSE chapters. The reuse density is at
its peak right now, but Ch283 PCPYUD is signaling that the
"low-32-only" approximation has reached its natural boundary.
Codex's framing on whether to widen the regfile or pivot
elsewhere will set the direction for the next stretch.