f3c512f2ab
The hand-written parts of the manual still described how the retired xenia-rs interpreter behaved: its snapshots, Rust casts and helpers. Each of those 490 statements is now either restated as what Canary's emitters and x64 backend actually do (at the pinned canary_experimental commit), or dropped where it only made sense for xenia-rs. Checking them turned up claims that were wrong, not just outdated: - VSCR[SAT] is never modelled in Canary (DID_SATURATE is a stub and mfvscr cannot see it); the pages said saturating ops set it stickily. - Canary does not implement lswi/lswx/stswi/stswx, dcbi, mtfsb0/mtfsb1, vmsum*, vmhaddshs, vupkhpx/vupklpx, and most SPRs; pages described them as working. - Traps evaluate TO in Canary; stvebx/stvehx/stvewx store one element, not 16 bytes; mtmsrd writes only EE; fres/frsqrte/vrsqrtefp precision claims and the stfs "rounds under RN / sets FPSCR" claim contradicted the spec. - Reservations are a 64 KiB block bitmap plus a value compare, not per-address tracking. Claims that neither Canary's source nor a public spec settles are marked unverified (NI at boot, vmaddcfp128 operand order, estimate bit-exactness). Generated regions are untouched; re-running the generator changes nothing. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
133 lines
5.3 KiB
Markdown
133 lines
5.3 KiB
Markdown
# `lwbrx` — Load Word Byte-Reverse Indexed
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> **Category:** [Memory](../categories/memory.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c00042c`
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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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| `lwbrx` | `lwbrx` | — | Load Word Byte-Reverse Indexed |
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## Syntax
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```asm
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lwbrx [RD], [RA0], [RB]
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```
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## Encoding
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### `lwbrx` — form `X`
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- **Opcode word:** `0x7c00042c`
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- **Primary opcode (bits 0–5):** `31`
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- **Extended opcode:** `534`
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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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| `RA0` | lwbrx: read | Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, **not** `r0`. |
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| `RB` | lwbrx: read | Source GPR. |
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| `RD` | lwbrx: write | Destination GPR. |
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## Register Effects
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### `lwbrx`
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- **Reads (always):** `RA0`, `RB`
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- **Reads (conditional):** _none_
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- **Writes (always):** `RD`
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- **Writes (conditional):** _none_
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## Status-Register Effects
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_No condition-register or status-register effects._
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## Operation (pseudocode)
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```
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; No hand-written pseudocode for this instruction yet.
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; The authoritative semantics are the Canary emitter snapshot under
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; Implementation References; about half of Canary's emitters open
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; with the PPC-style definition as a comment (`RD <- (RA) + (RB)`).
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; Every side effect is also enumerated in the Register Effects and
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; Status-Register Effects tables above.
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```
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## C Translation Example
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```c
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/* No hand-written C yet. Translate the Canary emitter snapshot */
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/* under Implementation References; its HIR maps directly: */
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/* f.LoadGPR(n) / f.StoreGPR(n, v) -> r[n] / r[n] = v */
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/* f.LoadFPR / StoreFPR, f.LoadVR / StoreVR -> f[n], v[n] */
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/* f.Load(ea, T), f.Store(ea, v) -> raw read / write; emitters */
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/* wrap them in f.ByteSwap for the big-endian guest value */
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/* f.UpdateCR(n, v) -> CR field n from v's LOW 32 BITS vs 0 */
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/* f.LoadCA / f.StoreCA -> xer.CA; f.StoreSAT -> vscr.SAT */
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/* i.XO.RA, i.D.DS, ... -> the bit-fields listed under Operands */
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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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**`lwbrx`**
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- Canary XML: [`tools/ppc-instructions.xml` — search for `mnem="lwbrx"`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/tools/ppc-instructions.xml)
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- Canary emitter: [`src/xenia/cpu/ppc/ppc_emit_memory.cc:641`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/src/xenia/cpu/ppc/ppc_emit_memory.cc#L641)
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- Sylpheed opcode: [`crates/sylpheed-ppc/src/opcode.rs:161`](../../../crates/sylpheed-ppc/src/opcode.rs#L161)
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- Sylpheed decoder: [`crates/sylpheed-ppc/src/decoder.rs:933`](../../../crates/sylpheed-ppc/src/decoder.rs#L933)
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<details><summary>Canary emitter (frozen snapshot @ <code>f21ebd49e9</code>)</summary>
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```cpp
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int InstrEmit_lwbrx(PPCHIRBuilder& f, const InstrData& i) {
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// if RA = 0 then
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// b <- 0
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// else
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// b <- (RA)
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// EA <- b + (RB)
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// RT <- i32.0 || bswap(MEM(EA, 4))
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Value* ea = CalculateEA_0(f, i.X.RA, i.X.RB);
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Value* rt = f.ZeroExtend(f.Load(ea, INT32_TYPE), INT64_TYPE);
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f.StoreGPR(i.X.RT, rt);
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return 0;
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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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- **Reads little-endian word.** Loads 4 bytes and reverses byte order. With Xenon's big-endian world view, the architectural effect is "load a little-endian word as if it were big-endian" — the standard parser instruction for PNG/ZIP/RIFF/TGA chunk fields, network protocol fields, and PC-side-formatted data.
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- **Implementation detail.** Canary loads the word without the byte swap `lwz` applies (and zero-extends it), which on its little-endian host gives the little-endian interpretation directly.
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- **X-form only — no update form.** Only the indexed form exists. `EA = (RA|0) + RB`. Pointer-bumping requires a separate `addi`.
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- **`RA0` semantics.** When `RA = 0`, base is literal zero; `lwbrx RT, 0, RB` reads at exact `RB`.
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- **Zero-extension to 64 bits.** Result occupies the full 64-bit GPR; high 32 bits zero. There is no sign-extending byte-reverse load — combine with `extsw` if needed.
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- **Alignment.** Hardware tolerates unaligned 4-byte reads. Cache-inhibited storage may raise alignment exceptions on real Xenon.
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- **Pair with [`stwbrx`](stwbrx.md).** Symmetric byte-reverse store.
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## Related Instructions
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- [`lhbrx`](lhbrx.md), [`ldbrx`](ldbrx.md) — narrower / wider byte-reverse loads.
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- [`stwbrx`](stwbrx.md) — store-word byte-reverse counterpart.
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- [`lwz`](lwz.md), [`lwzx`](lwz.md) — non-reversing zero-extending word loads.
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- [`lwa`](lwa.md), [`lwax`](lwa.md) — non-reversing sign-extending word loads.
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## IBM Reference
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- [AIX 7.3 — `lwbrx` (Load Word Byte-Reverse Indexed)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-lwbrx-load-word-byte-reverse-indexed-instruction)
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- `PowerISA v2.07B Book II` § "Byte-Reverse Storage Access".
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