# `stb` — Store Byte
> **Category:** [Memory](../categories/memory.md) · **Form:** [D](../forms/D.md) · **Opcode:** `0x98000000`
## Assembler Mnemonics
| Mnemonic | XML entry | Flags | Description |
| --- | --- | --- | --- |
| `stb` | `stb` | — | Store Byte |
| `stbu` | `stbu` | — | Store Byte with Update |
| `stbux` | `stbux` | — | Store Byte with Update Indexed |
| `stbx` | `stbx` | — | Store Byte Indexed |
## Syntax
```asm
stb [RS], [d]([RA0])
stbu [RS], [d]([RA])
stbux [RS], [RA], [RB]
stbx [RS], [RA0], [RB]
```
## Encoding
### `stb` — form `D`
- **Opcode word:** `0x98000000`
- **Primary opcode (bits 0–5):** `38`
- **Extended opcode:** —
- **Synchronising:** no
| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode |
| 6–10 | `RT` | destination GPR (or RS when storing) |
| 11–15 | `RA` | source GPR (0 ⇒ literal 0 for RA0 forms) |
| 16–31 | `D/SI/UI` | 16-bit signed or unsigned immediate |
### `stbu` — form `D`
- **Opcode word:** `0x9c000000`
- **Primary opcode (bits 0–5):** `39`
- **Extended opcode:** —
- **Synchronising:** no
| Bits | Field | Meaning |
| --- | --- | --- |
| 0–5 | `OPCD` | primary opcode |
| 6–10 | `RT` | destination GPR (or RS when storing) |
| 11–15 | `RA` | source GPR (0 ⇒ literal 0 for RA0 forms) |
| 16–31 | `D/SI/UI` | 16-bit signed or unsigned immediate |
### `stbux` — form `X`
- **Opcode word:** `0x7c0001ee`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `247`
- **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 |
### `stbx` — form `X`
- **Opcode word:** `0x7c0001ae`
- **Primary opcode (bits 0–5):** `31`
- **Extended opcode:** `215`
- **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 |
| --- | --- | --- |
| `RS` | stb: read; stbu: read; stbux: read; stbx: read | Source GPR (alias for RD in some stores). |
| `RA0` | stb: read; stbx: read | Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, **not** `r0`. |
| `d` | stb: read; stbu: read | 16-bit signed displacement (`d`) added to the base address register. |
| `RA` | stbu: read; stbu: write; stbux: read; stbux: write | Source GPR (`r0`–`r31`). |
| `RB` | stbux: read; stbx: read | Source GPR. |
## Register Effects
### `stb`
- **Reads (always):** `RS`, `RA0`, `d`
- **Reads (conditional):** _none_
- **Writes (always):** _none_
- **Writes (conditional):** _none_
### `stbu`
- **Reads (always):** `RS`, `RA`, `d`
- **Reads (conditional):** _none_
- **Writes (always):** `RA`
- **Writes (conditional):** _none_
### `stbux`
- **Reads (always):** `RS`, `RA`, `RB`
- **Reads (conditional):** _none_
- **Writes (always):** `RA`
- **Writes (conditional):** _none_
### `stbx`
- **Reads (always):** `RS`, `RA0`, `RB`
- **Reads (conditional):** _none_
- **Writes (always):** _none_
- **Writes (conditional):** _none_
## Status-Register Effects
_No condition-register or status-register effects._
## Operation (pseudocode)
```
EA <- (RA|0) + EXTS(d)
MEM(EA, 1) <- (RS)[56:63]
```
## 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
**`stb`**
- xenia-canary XML: [`tools/ppc-instructions.xml` — search for `mnem="stb"`](../../xenia-canary/tools/ppc-instructions.xml)
- xenia-canary emit: [`src/xenia/cpu/ppc/ppc_emit_memory.cc:404`](../../xenia-canary/src/xenia/cpu/ppc/ppc_emit_memory.cc#L404)
- xenia-rs opcode: [`crates/xenia-cpu/src/opcode.rs:67`](../../xenia-rs/crates/xenia-cpu/src/opcode.rs#L67)
- xenia-rs decoder: [`crates/xenia-cpu/src/decoder.rs:361`](../../xenia-rs/crates/xenia-cpu/src/decoder.rs#L361)
- xenia-rs interpreter: [`crates/xenia-cpu/src/interpreter.rs:1327-1335`](../../xenia-rs/crates/xenia-cpu/src/interpreter.rs#L1327-L1335)
xenia-rs interpreter body (frozen snapshot)
```rust
PpcOpcode::stb => {
let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
let ea = ea.wrapping_add(instr.d() as i64 as u64) as u32;
if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
if t.has_active_reservers() { t.invalidate_for_write(ea); }
}
mem.write_u8(ea, ctx.gpr[instr.rs()] as u8);
ctx.pc += 4;
}
```
xenia-rs interpreter body (frozen snapshot)
```rust
PpcOpcode::stbu => {
let ea = ctx.gpr[instr.ra()].wrapping_add(instr.d() as i64 as u64) as u32;
if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
if t.has_active_reservers() { t.invalidate_for_write(ea); }
}
mem.write_u8(ea, ctx.gpr[instr.rs()] as u8);
ctx.gpr[instr.ra()] = ea as u64;
ctx.pc += 4;
}
```
xenia-rs interpreter body (frozen snapshot)
```rust
PpcOpcode::stbux => {
let ea = ctx.gpr[instr.ra()].wrapping_add(ctx.gpr[instr.rb()]) as u32;
if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
if t.has_active_reservers() { t.invalidate_for_write(ea); }
}
mem.write_u8(ea, ctx.gpr[instr.rs()] as u8);
ctx.gpr[instr.ra()] = ea as u64;
ctx.pc += 4;
}
```
xenia-rs interpreter body (frozen snapshot)
```rust
PpcOpcode::stbx => {
let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
let ea = ea.wrapping_add(ctx.gpr[instr.rb()]) as u32;
if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
if t.has_active_reservers() { t.invalidate_for_write(ea); }
}
mem.write_u8(ea, ctx.gpr[instr.rs()] as u8);
ctx.pc += 4;
}
```