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

mtspr — Move to Special-Purpose Register

Category: Control / CR / SPR · Form: XFX · Opcode: 0x7c0003a6

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
mtspr	mtspr	—	Move to Special-Purpose Register

Syntax

mtspr [SPR], [RS]

Encoding

mtspr — form XFX

• Opcode word: 0x7c0003a6
• Primary opcode (bits 0–5): 31
• Extended opcode: 467
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (31)
6–10	RT	destination / source GPR
11–20	spr/tbr/FXM	SPR/TBR number (byte-swapped halves) or CR field mask
21–30	XO	extended opcode
31	—	reserved

Operands

Field	Role	Description
RS	mtspr: read	Source GPR (alias for RD in some stores).
SPR	mtspr: write	Special-Purpose-Register number. Encoded with the two 5-bit halves swapped (bits 11-15 become the high half, bits 16-20 the low half).

Register Effects

mtspr

• Reads (always): RS
• Reads (conditional): none
• Writes (always): SPR
• Writes (conditional): none

Status-Register Effects

No condition-register or status-register effects.

Operation (pseudocode)

n <- spr_number(SPR)
SPR(n) <- (RS)

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

mtspr

• Canary XML: tools/ppc-instructions.xml — search for mnem="mtspr"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_control.cc:773
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:185
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:925

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_mtspr(PPCHIRBuilder& f, const InstrData& i) {
  // n <- spr[5:9] || spr[0:4]
  // if length(SPR(n)) = 64 then
  //   SPR(n) <- (RS)
  // else
  //   SPR(n) <- (RS)[32:63]

  Value* rt = f.LoadGPR(i.XFX.RT);

  const uint32_t n = ((i.XFX.spr & 0x1F) << 5) | ((i.XFX.spr >> 5) & 0x1F);
  switch (n) {
    case 1:
      // XER
      f.StoreXER(rt);
      break;
    case 8:
      // LR
      f.StoreLR(rt);
      break;
    case 9:
      // CTR
      f.StoreCTR(rt);
      break;
    case 256:

      f.StoreContext(offsetof(PPCContext, vrsave), f.Truncate(rt, INT32_TYPE));
      // VRSAVE
      break;
    default:
      XEINSTRNOTIMPLEMENTED();
      return 1;
  }

  return 0;
}

Special Cases & Edge Conditions

• SPR halves are swapped in the encoding. As with mfspr, the 10-bit spr field stores the two 5-bit halves transposed. Software always names the logical SPR number; assemblers handle the swap. Decoded number n = ((field & 0x1F) << 5) | ((field >> 5) & 0x1F).

• SPRs writable from userspace (Xenon, modelled by xenia).

  Decoded #	Name	Effect
  1	XER	unpacked into ctx.xer_so/xer_ov/xer_ca and length field
  8	LR	ctx.lr ← RS
  9	CTR	ctx.ctr ← RS
  256	VRSAVE	ctx.vrsave ← RS & 0xFFFFFFFF

• SPRs xenia silently swallows (no observable effect). SPRG0..3, HID0, HID1, DAR, DSISR — these are kernel/diagnostic registers; xenia accepts the write to avoid spurious "unimplemented SPR" warnings, but the value is discarded.

• Privileged SPRs. On real hardware, writes to MSR-visible kernel SPRs (SPRG0..3, HID0/1, DSISR, DAR, PIR, etc.) require supervisor mode and trap from problem state. xenia does not enforce privilege.

• Time-base writes are privileged. mtspr 268/269 (TBL/TBU) only works in supervisor mode on real hardware. xenia will warn mtspr: unimplemented SPR for these — do not assume the time base can be guest-written.

• Simplified mnemonics. mtxer RS ≡ mtspr 1, RS, mtlr RS ≡ mtspr 8, RS, mtctr RS ≡ mtspr 9, RS. These dominate Xbox 360 disassembly.

• No CR / XER side effects. mtspr itself doesn't record (the target SPR may itself be XER, in which case XER is being directly overwritten).

• Not synchronising. xenia's XML omits the sync flag; PowerISA does require some mtspr cases (e.g. SDR1, MMU regs) to be context-synchronising — none of them appear in title binaries.

Related Instructions

• mfspr — inverse: read an SPR into a GPR.
• mftb — read time-base (preferred over mfspr TBL/TBU).
• mtmsr, mtmsrd — write MSR (separate opcode).
• mcrxr — sample-and-clear XER's overflow/carry bits.

Simplified Mnemonics

Simplified	Expansion
mtxer RS	mtspr 1, RS
mtlr RS	mtspr 8, RS
mtctr RS	mtspr 9, RS

IBM Reference

• AIX 7.3 — mtspr (Move to Special Purpose Register)
• PowerISA v2.07B, Book III §4 — SPR number table and privilege rules.