# `dcbf` — Data Cache Block Flush

> **Category:** [Memory](../categories/memory.md) · **Form:** [X](../forms/X.md) · **Opcode:** `0x7c0000ac`

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## Assembler Mnemonics

| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `dcbf` | `dcbf` | — | Data Cache Block Flush |

## Syntax

```asm
dcbf [RA0], [RB]
```

## Encoding

### `dcbf` — form `X`

- **Opcode word:** `0x7c0000ac`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `86`
- **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 |
| --- | --- | --- |
| `RA0` | dcbf: read | Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, **not** `r0`. |
| `RB` | dcbf: read | Source GPR. |

## Register Effects

### `dcbf`

- **Reads (always):** `RA0`, `RB`
- **Reads (conditional):** _none_
- **Writes (always):** _none_
- **Writes (conditional):** _none_

## Status-Register Effects

_No condition-register or status-register effects._

## Operation (pseudocode)

```
; Pseudocode derives directly from the xenia-rs interpreter
; arm (see Implementation References). Operation semantics:
;   - Read source operands from the fields listed under Operands.
;   - Apply the arithmetic / logical / memory action described
;     in the Description field above.
;   - Write results to the destination register(s); update any
;     status bits enumerated under Status-Register Effects.
; Consult the IBM AIX reference link under IBM Reference for
; canonical PPC-style pseudocode where xenia's expression is
; terse.
```

## 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

**`dcbf`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="dcbf"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_memory.cc:1125`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_memory.cc#L1125)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:19`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L19)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:773`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L773)

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## Special Cases & Edge Conditions

- **Flush = write-back + invalidate.** If the addressed line is dirty in the data cache, it is written to memory; whether dirty or clean, the line is then removed from the cache. Subsequent loads must refill from memory.
- **Cache line size.** Xenon's L1/L2 lines are **128 bytes**. The hardware ignores the low seven bits of `EA`, so `dcbf RA, RB` flushes the line containing `EA` regardless of where in that line `EA` lies. There is no `dcbf128` variant — the hint is sized to the architectural line.
- **`RA0` semantics.** When `RA = 0`, the base is the literal zero — `dcbf 0, RB` flushes the line containing address `RB`. The instruction has no destination register.
- **Xenia models a no-op.** Xenia-rs's emulator does not maintain a coherent cache model; the decode entry exists but the interpreter typically advances PC without further effect, since target memory is always coherent on the host. This is correct behaviour for an emulator.
- **Unprivileged.** `dcbf` is a problem-state instruction — usable from user code. Storage protection still applies; flushing an unmapped page raises a DSI exception.
- **Pair with `sync`.** Hardware `dcbf` does not by itself impose ordering; software that needs the flushed data visible to other masters (DMA, GPU) issues a [`sync`](sync.md) afterwards.
- **Self-modifying code companion.** When patching code, the recipe is `dcbst` (push dirty data through to memory) → `sync` → [`icbi`](icbi.md) (invalidate I-cache) → [`isync`](isync.md). `dcbf` is the heavier alternative when the writer also wants the line out of D-cache.

## Related Instructions

- [`dcbst`](dcbst.md) — write-back without invalidate (lighter than `dcbf`).
- [`dcbi`](dcbi.md) — invalidate without write-back (privileged; loses dirty data).
- [`dcbt`](dcbt.md), [`dcbtst`](dcbtst.md) — touch hints to bring lines in.
- [`dcbz`](dcbz.md), `dcbz128` — allocate-and-zero a line.
- [`icbi`](icbi.md) — instruction-cache invalidate, used together for self-modifying code.
- [`sync`](sync.md) — full memory barrier, typically follows `dcbf`.

## IBM Reference

- [AIX 7.3 — `dcbf` (Data Cache Block Flush)](https://www.ibm.com/docs/en/aix/7.3.0?topic=set-dcbf-data-cache-block-flush-instruction)
- `PowerISA v2.07B Book II` § "Storage Control Instructions" for cache-coherence semantics.