# `mtspr` — Move to Special-Purpose Register

> **Category:** [Control / CR / SPR](../categories/control.md) · **Form:** [XFX](../forms/XFX.md) · **Opcode:** `0x7c0003a6`

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## Assembler Mnemonics

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

## Syntax

```asm
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

```c
/* C translation: the xenia-rs interpreter arm below in           */
/* Implementation References is the authoritative semantic        */
/* snapshot. Translate it line-by-line:                            */
/*   - ctx.gpr[N]  -> r[N]       (or f[]/v[] for FPRs/VRs)        */
/*   - mem.read_u*/write_u* -> mem_read_u*_be / mem_write_u*_be   */
/*   - ctx.update_cr_signed(fld, v) -> update_cr_signed(fld, v)   */
/*   - ctx.xer_ca / xer_ov / xer_so -> xer.CA / xer.OV / xer.SO   */
/* The Register Effects and Status-Register Effects tables above  */
/* enumerate every side effect a faithful translation must emit.  */
```

## Implementation References

**`mtspr`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="mtspr"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_control.cc:771`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_control.cc#L771)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:55`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L55)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:810`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L810)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:1596-1626`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L1596-L1626)
<details><summary>xenia-rs interpreter body (frozen snapshot)</summary>

```rust
        PpcOpcode::mtspr => {
            let spr = instr.spr();
            let val = ctx.gpr[instr.rs()];
            match spr {
                crate::context::spr::XER => ctx.set_xer(val as u32),
                crate::context::spr::LR => ctx.lr = val,
                crate::context::spr::CTR => ctx.ctr = val as u32 as u64,
                crate::context::spr::DEC => ctx.dec = val as u32,
                crate::context::spr::TBL_WRITE => {
                    ctx.timebase = (ctx.timebase & 0xFFFF_FFFF_0000_0000) | (val & 0xFFFF_FFFF);
                }
                crate::context::spr::TBU_WRITE => {
                    ctx.timebase = (ctx.timebase & 0x0000_0000_FFFF_FFFF) | ((val & 0xFFFF_FFFF) << 32);
                }
                crate::context::spr::VRSAVE => ctx.vrsave = val as u32,
                // Benign writes — swallow silently to avoid false Unimplemented
                // warnings on SPRs that have no observable effect in userspace.
                crate::context::spr::SPRG0
                | crate::context::spr::SPRG1
                | crate::context::spr::SPRG2
                | crate::context::spr::SPRG3
                | crate::context::spr::HID0
                | crate::context::spr::HID1
                | crate::context::spr::DAR
                | crate::context::spr::DSISR => {}
                _ => {
                    tracing::warn!("mtspr: unimplemented SPR {}", spr);
                }
            }
            ctx.pc += 4;
        }
```
</details>

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## Special Cases & Edge Conditions

- **SPR halves are swapped in the encoding.** As with [`mfspr`](mfspr.md), 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`](mfspr.md) — inverse: read an SPR into a GPR.
- [`mftb`](mftb.md) — read time-base (preferred over `mfspr TBL/TBU`).
- [`mtmsr`](mtmsr.md), [`mtmsrd`](mtmsrd.md) — write MSR (separate opcode).
- [`mcrxr`](mcrxr.md) — 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)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-mtspr-move-special-purpose-register-instruction)
- PowerISA v2.07B, Book III §4 — SPR number table and privilege rules.