e6d43a23ac
Bundles state that lives OUTSIDE the xenia-rs repo so a fresh clone on
another machine can be brought up to identical configuration via
migration/setup.sh:
- claude-memory/ ~/.claude/projects/-home-fabi-RE-Project-Sylpheed/memory/
(103 files, 1.1 MB - MEMORY.md + every
project_xenia_rs_*.md from audits
addis_signext through audit-058)
- project-root/dot-claude/ <project-root>/.claude/settings.json
(Stop hook + permissions)
- project-root/ppc-manual/ <project-root>/ppc-manual/
(PowerPC reference docs, 397 files, 3.7 MB)
- project-root/run-canary.sh <project-root>/run-canary.sh
- README.md Human-readable setup checklist
- setup.sh Idempotent installer (also reclones
xenia-canary at pinned HEAD 6de80dffe)
- MANIFEST.md Per-file mapping + per-file-not-bundled
restoration recipe
Excluded from bundle (not shippable via git):
- Sylpheed ISO (7.8 GB; copyright; manual copy required)
- sylpheed.db (395 MB; regenerable from XEX via analysis tooling)
- target/ build artifacts (rebuild on target)
- audit-runs probe firehoses (.log/.stdout/.stderr ~11 GB; rerun if needed)
- audit-runs memory dumps (.bin ~4.5 GB; rerun audit-026/027/029 if needed)
- xenia-canary checkout (setup.sh reclones from
git.mc02.dev/fabi/Xenia-Canary.git at HEAD 6de80dffe)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
134 lines
5.6 KiB
Markdown
134 lines
5.6 KiB
Markdown
# `mtfsb0x` — Move to FPSCR Bit 0
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> **Category:** [Control / CR / SPR](../categories/control.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0xfc00008c`
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<!-- GENERATED: BEGIN -->
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## Assembler Mnemonics
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| Mnemonic | XML entry | Flags | Description |
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| --- | --- | --- | --- |
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| `mtfsb0` | `mtfsb0x` | — | Move to FPSCR Bit 0 |
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| `mtfsb0.` | `mtfsb0x` | Rc=1 | Move to FPSCR Bit 0 |
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## Syntax
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```asm
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mtfsb0[Rc] [FPSCRD]
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```
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## Encoding
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### `mtfsb0x` — form `X`
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- **Opcode word:** `0xfc00008c`
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- **Primary opcode (bits 0–5):** `63`
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- **Extended opcode:** `70`
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- **Synchronising:** no
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| Bits | Field | Meaning |
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| --- | --- | --- |
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| 0–5 | `OPCD` | primary opcode |
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| 6–10 | `RT/FRT/VRT` | destination |
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| 11–15 | `RA/FRA/VRA` | source A |
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| 16–20 | `RB/FRB/VRB` | source B |
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| 21–30 | `XO` | extended opcode (10 bits) |
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| 31 | `Rc` | record-form flag |
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## Operands
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| Field | Role | Description |
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| --- | --- | --- |
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| `FPSCRD` | mtfsb0x: write | FPSCR destination field. |
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| `CR` | mtfsb0x: write (conditional) | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |
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## Register Effects
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### `mtfsb0x`
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- **Reads (always):** _none_
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- **Reads (conditional):** _none_
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- **Writes (always):** `FPSCRD`
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- **Writes (conditional):** `CR`
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## Status-Register Effects
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- `mtfsb0x`: **CR0** ← signed-compare(result, 0) with `SO ← XER[SO]`, when `Rc=1`.
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## Operation (pseudocode)
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```
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; Pseudocode derives directly from the xenia-rs interpreter
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; arm (see Implementation References). Operation semantics:
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; - Read source operands from the fields listed under Operands.
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; - Apply the arithmetic / logical / memory action described
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; in the Description field above.
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; - Write results to the destination register(s); update any
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; status bits enumerated under Status-Register Effects.
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; Consult the IBM AIX reference link under IBM Reference for
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; canonical PPC-style pseudocode where xenia's expression is
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; terse.
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```
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## C Translation Example
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```c
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/* C translation: the xenia-rs interpreter arm below in */
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/* Implementation References is the authoritative semantic */
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/* snapshot. Translate it line-by-line: */
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/* - ctx.gpr[N] -> r[N] (or f[]/v[] for FPRs/VRs) */
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/* - mem.read_u*/write_u* -> mem_read_u*_be / mem_write_u*_be */
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/* - ctx.update_cr_signed(fld, v) -> update_cr_signed(fld, v) */
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/* - ctx.xer_ca / xer_ov / xer_so -> xer.CA / xer.OV / xer.SO */
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/* The Register Effects and Status-Register Effects tables above */
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/* enumerate every side effect a faithful translation must emit. */
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```
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## Implementation References
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**`mtfsb0x`**
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- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="mtfsb0x"`](../../xenia-canary/tools/ppc-instructions.xml)
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- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_fpu.cc:406`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_fpu.cc#L406)
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- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:55`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L55)
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- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:905`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L905)
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- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3055-3061`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3055-L3061)
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<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>
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```rust
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PpcOpcode::mtfsb0x => {
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// Clear FPSCR bit crbd
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let bit = instr.crbd();
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ctx.fpscr &= !(1 << (31 - bit));
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if instr.rc_bit() { update_cr1_from_fpscr(ctx); }
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ctx.pc += 4;
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}
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```
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</details>
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<!-- GENERATED: END -->
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## Special Cases & Edge Conditions
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- **Operation.** Clears (sets to 0) **a single named bit** of the 32-bit FPSCR. The bit is selected by `FPSCRD` (a 5-bit absolute index 0..31, big-endian: 0 = MSB = FX).
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- **The mnemonic name "Bit 0" is misleading.** "0" refers to the *value being written*, not to bit position 0. Pair with [`mtfsb1x`](mtfsb1x.md) which writes a 1.
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- **Restricted bits.** Per PowerISA, `mtfsb0` cannot clear bits 1 (FEX) or 2 (VX) — those are summary bits, derived from other FPSCR bits. xenia-rs does **not** enforce this restriction; it will happily flip any bit. In practice no Xbox 360 title relies on the restriction's enforcement.
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- **`Rc=1`.** `mtfsb0.` (`Rc=1`) updates **CR1** with the high four FPSCR bits (FX, FEX, VX, OX) after the clear. This is the FPU's record-form analogue.
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- **Common use.** Reset a sticky exception bit ahead of a sequence of FP ops you want to monitor (e.g. `mtfsb0 5` to clear ZX before a divide series, then read it back).
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- **xenia simplification.** xenia-rs maintains FPSCR as a `u32` and does the bit clear correctly, but most downstream FP instructions in xenia **do not update** FPSCR exception bits — so monitoring them after `mtfsb0` will see the bits stay at their seed value. Acceptable for titles that use FPSCR only to manage rounding / non-exception state.
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- **Not synchronising.** Reorderable.
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## Related Instructions
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- [`mtfsb1x`](mtfsb1x.md) — set a single FPSCR bit to 1.
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- [`mtfsfx`](mtfsfx.md) — write fields of FPSCR from an FPR.
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- [`mtfsfix`](mtfsfix.md) — write a 4-bit immediate into one FPSCR field.
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- [`mffsx`](mffsx.md) — read FPSCR.
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- [`mcrfs`](mcrfs.md) — copy FPSCR field → CR field (and clear sticky bits).
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`mtfsb0` is itself the simplified form (`Rc=0`); `mtfsb0.` is the recording variant.
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## IBM Reference
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- [AIX 7.3 — `mtfsb0` (Move to FPSCR Bit 0)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-mtfsb0-move-fpscr-bit-0-instruction)
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- PowerISA v2.07B, Book I §4.6 — FPSCR bit definitions and the FX/FEX/VX restriction.
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