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

mfcr — Move from Condition Register

Category: Control / CR / SPR · Form: X · Opcode: 0x7c000026

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
mfcr	mfcr	—	Move from Condition Register

Syntax

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

/* 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"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_control.cc:627
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:171
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:868

Canary emitter (frozen snapshot @ f21ebd49e9)

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;
}

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.
• 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 — inverse: write selected CR fields from a GPR.
• mcrf, mcrxr, mcrfs — narrower CR-field moves.
• mfspr, mtspr — 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)