Sylpheed/tools/ppc-manual/control/mfcr.md

# `mfcr` — Move from Condition Register

> **Category:** [Control / CR / SPR](../categories/control.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c000026`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `mfcr` | `mfcr` | — | Move from Condition Register |

## Syntax

```asm
mfcr [RD]
```

## Encoding

### `mfcr` — form `X`

- **Opcode word:** `0x7c000026`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `19`
- **Synchronising:** no

| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode |
| 6–10 | `RT/FRT/VRT` | destination |
| 11–15 | `RA/FRA/VRA` | source A |
| 16–20 | `RB/FRB/VRB` | source B |
| 21–30 | `XO` | extended opcode (10 bits) |
| 31 | `Rc` | record-form flag |

## Operands

| Field | Role | Description |
| --- | --- | --- |
| `CR` | mfcr: read | Condition-register update. When `Rc=1`, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result. |
| `RD` | mfcr: write | Destination GPR. |

## Register Effects

### `mfcr`

- **Reads (always):** `CR`
- **Reads (conditional):** _none_
- **Writes (always):** `RD`
- **Writes (conditional):** _none_

## Status-Register Effects

_No condition-register or status-register effects._

## Operation (pseudocode)

```
RT <- 0x00000000 || CR
```

## C Translation Example

```c
/* No hand-written C yet. Translate the Canary emitter snapshot   */
/* under Implementation References; its HIR maps directly:        */
/*   f.LoadGPR(n) / f.StoreGPR(n, v)  -> r[n] / r[n] = v          */
/*   f.LoadFPR / StoreFPR, f.LoadVR / StoreVR -> f[n], v[n]        */
/*   f.Load(ea, T), f.Store(ea, v) -> raw read / write; emitters   */
/*     wrap them in f.ByteSwap for the big-endian guest value      */
/*   f.UpdateCR(n, v)  -> CR field n from v's LOW 32 BITS vs 0     */
/*   f.LoadCA / f.StoreCA -> xer.CA;  f.StoreSAT -> vscr.SAT       */
/*   i.XO.RA, i.D.DS, ... -> the bit-fields listed under Operands  */
/* The Register Effects and Status-Register Effects tables above  */
/* enumerate every side effect a faithful translation must emit.  */
```

## Implementation References

**`mfcr`**
- Canary XML: [`tools/ppc-instructions.xml` — search for `mnem="mfcr"`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/tools/ppc-instructions.xml)
- Canary emitter: [`src/xenia/cpu/ppc/ppc_emit_control.cc:627`](https://github.com/xenia-canary/xenia-canary/blob/f21ebd49e979e44f081f474df78c3fbfee9cb3f2/src/xenia/cpu/ppc/ppc_emit_control.cc#L627)
- Sylpheed opcode: [`crates/sylpheed-ppc/src/opcode.rs:171`](../../../crates/sylpheed-ppc/src/opcode.rs#L171)
- Sylpheed decoder: [`crates/sylpheed-ppc/src/decoder.rs:868`](../../../crates/sylpheed-ppc/src/decoder.rs#L868)
<details><summary>Canary emitter (frozen snapshot @ <code>f21ebd49e9</code>)</summary>

```cpp
int InstrEmit_mfcr(PPCHIRBuilder& f, const InstrData& i) {
  // mfocrf RT,FXM
  // RT <- undefined
  // count <- 0
  // do i = 0 to 7
  // if FXMi = 1 then
  //   n <- i
  //   count <- count + 1
  // if count = 1 then
  //   RT4un + 32:4un + 35 <- CR4un + 32 : 4un + 35

  // TODO(benvanik): optimize mfcr sequences.
  // Often look something like this:
  //   mfocrf  r11, cr6
  //   not r10, r11
  //   extrwi    r3, r10, 1, 26
  // Could recognize this and only load the appropriate CR bit.

  Value* v;
  if (i.XFX.spr & (1 << 9)) {
    uint32_t bits = (i.XFX.spr & 0x1FF) >> 1;
    int count = 0;
    int cri = 0;
    for (int b = 0; b <= 7; ++b) {
      if (bits & (1 << b)) {
        cri = 7 - b;
        ++count;
      }
    }
    if (count == 1) {
      v = f.LoadCR(cri);
    } else {
      v = f.LoadZeroInt64();
    }
  } else {
    v = f.LoadCR();
  }
  f.StoreGPR(i.XFX.RT, v);
  return 0;
}
```
</details>

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## Special Cases & Edge Conditions

- **Operation.** Packs all 32 CR bits into the low half of `RD`; the upper 32 bits of `RD` are zeroed. CR field 0 ends up in bits 32..35 of `RD` (i.e. bits 0..3 of the 32-bit packed value), CR field 7 in bits 60..63 (bits 28..31).
- **No CR side effect.** `mfcr` is a read; CR is unmodified. The XL-form's nominal `Rc` bit is unused on this opcode.
- **Saving CR across calls.** The Xbox 360 / SysV ABI requires non-volatile CR fields (CR2..CR4) to be preserved across calls. Standard prologue: `mfcr r12; stw r12, 8(r1)`. Epilogue restores via [`mtcrf`](mtcrf.md).
- **Bit ordering.** PowerPC numbers bits big-endian (bit 0 = MSB). The encoding into the GPR follows the same convention: CR0.LT lands in bit 32 of the doubleword (the MSB of the low word). C-side translations should mask with `0xFFFFFFFFu` before consuming.
- **`mfocrf` variant.** PowerISA defines `mfocrf` (one CR field), encoded as `mfcr` with the high bit of FXM set. Canary honours it: with exactly one FXM bit set it returns just that field, with any other count it returns 0. The spec leaves the other bits undefined, so this is compliant.
- **Not synchronising.** Reorderable.
- **Canary exact match.** Plain `mfcr` returns the whole CR (`LoadCR()`), mirroring spec semantics.

## Related Instructions

- [`mtcrf`](mtcrf.md) — inverse: write selected CR fields from a GPR.
- [`mcrf`](mcrf.md), [`mcrxr`](mcrxr.md), [`mcrfs`](mcrfs.md) — narrower CR-field moves.
- [`mfspr`](mfspr.md), [`mtspr`](mtspr.md) — generic SPR moves; CR is *not* an SPR (it has its own opcode).

`mfcr` has no simplified mnemonics. `mfocrf RT, FXM` is a related encoding handled by the same Canary emitter (`InstrEmit_mfcr`).

## IBM Reference

- [AIX 7.3 — `mfcr` (Move from Condition Register)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-mfcr-move-from-condition-register-instruction)