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>
137 lines
6.0 KiB
Markdown
137 lines
6.0 KiB
Markdown
# `vaddsbs` — Vector Add Signed Byte Saturate
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> **Category:** [VMX (Altivec)](../categories/vmx.md) · **Form:** [VX](../forms/VX.md) · **Opcode:** `0x10000300`
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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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| `vaddsbs` | `vaddsbs` | — | Vector Add Signed Byte Saturate |
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## Syntax
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```asm
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vaddsbs [VD], [VA], [VB]
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```
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## Encoding
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### `vaddsbs` — form `VX`
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- **Opcode word:** `0x10000300`
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- **Primary opcode (bits 0–5):** `4`
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- **Extended opcode:** `768`
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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 (4) |
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| 6–10 | `VRT/VD` | destination vector register |
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| 11–15 | `VRA/VA` | source A vector register |
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| 16–20 | `VRB/VB` | source B vector register |
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| 21–31 | `XO` | extended opcode (11 bits) |
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## Operands
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| Field | Role | Description |
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| --- | --- | --- |
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| `VA` | vaddsbs: read | Source A vector register. |
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| `VB` | vaddsbs: read | Source B vector register. |
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| `VD` | vaddsbs: write | Destination vector register. |
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| `VSCR` | vaddsbs: write | Vector Status and Control Register (NJ/SAT bits). |
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## Register Effects
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### `vaddsbs`
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- **Reads (always):** `VA`, `VB`
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- **Reads (conditional):** _none_
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- **Writes (always):** `VD`, `VSCR`
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- **Writes (conditional):** _none_
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## Status-Register Effects
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- `vaddsbs`: **VSCR[SAT]** may be stickied on saturating vector operations.
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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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**`vaddsbs`**
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- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="vaddsbs"`](../../xenia-canary/tools/ppc-instructions.xml)
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- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_altivec.cc:348`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_altivec.cc#L348)
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- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:89`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L89)
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- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:498`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L498)
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- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:3258-3269`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L3258-L3269)
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<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>
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```rust
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PpcOpcode::vaddsbs => {
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let a = crate::vmx::as_i8x16(ctx.vr[instr.ra()]);
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let b = crate::vmx::as_i8x16(ctx.vr[instr.rb()]);
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let mut r = [0i8; 16]; let mut sat = false;
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for i in 0..16 {
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let (v, s) = crate::vmx::sat_add_i8(a[i], b[i]);
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r[i] = v; sat |= s;
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}
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if sat { ctx.set_vscr_sat(true); }
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ctx.vr[instr.rd()] = crate::vmx::from_i8x16(r);
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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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- **Sixteen signed-byte lanes, saturating.** Each `VD[i] = clamp(VA[i] + VB[i], -128, +127)` for `i = 0..15`, with both inputs interpreted as signed `int8`. Lane 0 is the most-significant byte (the byte at the lowest address after `stvx`).
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- **`VSCR[SAT]` is sticky-set** when *any* lane saturates — either positively (overflow above `+127`) or negatively (underflow below `-128`). The SAT bit is never cleared by this op; software must use [`mtvscr`](mtvscr.md) to clear it. Xenia routes the OR of per-lane saturation flags into `ctx.set_vscr_sat(true)` exactly when at least one lane clamped (see `crate::vmx::sat_add_i8` in [`crates/xenia-cpu/src/vmx.rs`](../../xenia-rs/crates/xenia-cpu/src/vmx.rs)).
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- **Compare with the modulo sibling.** [`vaddubm`](vaddubm.md) is bit-pattern-identical to a hypothetical `vaddsbm` and silently wraps without touching `VSCR[SAT]`. Use `vaddsbs` whenever clipping is desired and you need the sticky overflow flag.
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- **Asymmetric clamp.** `+127 + 1 = +127`; `-128 + (-1) = -128`. Tests that look for "any saturation" should mask both saturation directions.
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- **No XER side effects.** Altivec never updates `XER[CA]` / `XER[OV]`. The only status bit affected is `VSCR[SAT]`.
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- **Aliasing legal.** `vaddsbs v3, v3, v4` is the standard accumulate idiom for a clamping sum.
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- **No VMX128 sibling.**
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## Related Instructions
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- [`vaddubs`](vaddubs.md) — same width, **unsigned** saturating add (clamps to `0..255`).
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- [`vaddubm`](vaddubm.md) — same width, modulo (non-saturating) add; sign-agnostic.
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- [`vaddshs`](vaddshs.md), [`vaddsws`](vaddsws.md) — signed saturating add at half / word width.
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- [`vsubsbs`](vsubsbs.md) — the matching signed saturating subtract.
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- [`mtvscr`](mtvscr.md) / [`mfvscr`](mfvscr.md) — read or clear the sticky `VSCR[SAT]` bit observed here.
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## IBM Reference
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- [AIX 7.3 — `vaddsbs` (Vector Add Signed Byte Saturate)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-vaddsbs-vector-add-signed-byte-saturate-instruction)
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- [IBM AltiVec Technology Programmer's Interface Manual, Chapter 6 — Saturating Integer Arithmetic](https://www.nxp.com/docs/en/reference-manual/ALTIVECPIM.pdf)
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