xenia-rs/migration/project-root/ppc-manual/fpu/faddx.md

faddx — Floating Add

Category: Floating-Point · Form: A · Opcode: 0xfc00002a

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
fadd	faddx	—	Floating Add
fadd.	faddx	Rc=1	Floating Add

Syntax

fadd[Rc] [FD], [FA], [FB]

Encoding

faddx — form A

• Opcode word: 0xfc00002a
• Primary opcode (bits 0–5): 63
• Extended opcode: 21
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (59 or 63)
6–10	FRT	destination FPR
11–15	FRA	source A FPR
16–20	FRB	source B FPR
21–25	FRC	source C FPR (multiplier for madd-style ops)
26–30	XO	extended opcode (5 bits)
31	Rc	record-form flag (updates CR1)

Operands

Field	Role	Description
FA	faddx: read	Source A floating-point register (fr0–fr31).
FB	faddx: read	Source B floating-point register.
FD	faddx: write	Destination floating-point register.
CR	faddx: write (conditional)	Condition-register update. When Rc=1, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result.
FPSCR	faddx: write	Floating-Point Status and Control Register.

Register Effects

faddx

• Reads (always): FA, FB
• Reads (conditional): none
• Writes (always): FD, FPSCR
• Writes (conditional): CR

Status-Register Effects

• faddx: CR1 ← FPSCR[FX, FEX, VX, OX] when Rc=1.; FPSCR updated per IEEE-754 flags (FX, FEX, FPRF, FR, FI, exceptions).

Operation (pseudocode)

FRT <- FRA + FRB                         ; double-precision

C Translation Example

/* fadd / fadd. — IEEE-754 double-precision add (A-form)            */
f[insn.FRT] = f[insn.FRA] + f[insn.FRB];
if (insn.Rc) update_cr1_from_fpscr();
/* FPSCR[FPRF, FR, FI, FX, exceptions] implicitly updated by the FPU. */

Implementation References

faddx

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="faddx"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_fpu.cc:38
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:27
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:922
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:2555-2564

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::faddx => {
            let a = ctx.fpr[instr.ra()];
            let b = ctx.fpr[instr.rb()];
            fpscr::check_invalid_add(ctx, a, b, false);
            let result = a + b;
            ctx.fpr[instr.rd()] = result;
            fpscr::update_after_op(ctx, result, a.is_finite() && b.is_finite());
            if instr.rc_bit() { update_cr1_from_fpscr(ctx); }
            ctx.pc += 4;
        }

Extended Pseudocode

FRT <- round(FRA + FRB, FPSCR[RN])                 ; double precision, current rounding mode

; FPSCR side-effects (always)
    FPSCR[FPRF]  <- classify(FRT)                  ; sign / class bits
    FPSCR[FR,FI] <- round_info
    if overflow           then FPSCR[OX]   <- 1; FPSCR[FX] <- 1
    if underflow          then FPSCR[UX]   <- 1; FPSCR[FX] <- 1
    if inexact            then FPSCR[XX]   <- 1; FPSCR[FX] <- 1
    if NaN input or ±∞−±∞ then FPSCR[VXISI]<- 1; FPSCR[FX] <- 1
    FPSCR[FEX] <- any-enabled-exception

if Rc then
    CR1 <- FPSCR[FX, FEX, VX, OX]                  ; the four "summary" bits

Special Cases & Edge Conditions

• Double precision. fadd always operates on IEEE-754 binary64 regardless of whether either source was produced by a single-precision instruction. Single-precision adds use faddsx and automatically round the result to binary32 precision.
• No immediate / carry / OE. FPU arithmetic has no immediate forms, no carry, and no overflow-enable bit. Rc is the only modifier — it writes CR1 from the four top FPSCR bits.
• FPSCR is always updated. Even the non-record form (fadd) updates FPSCR[FPRF, FR, FI, FX, …] as a side effect of execution; xenia's interpreter currently does not model this, so translations that rely on observing FPSCR bits across a pair of FPU instructions will diverge from hardware. If your translator needs compatible FPSCR state, emit explicit updates — or accept the simplification, which matches real Xbox 360 title behaviour in practice (titles rarely read FPSCR except via mffs for exception sanity checks).
• NaN propagation. Per IEEE-754, any NaN input produces a NaN output; PowerPC specifies that the signalling bit of the result NaN is cleared (quietening a signalling input). Xenia uses host-native f64 +, which may preserve the signalling bit on some platforms — assume quietening for correctness.
• ±∞ − ±∞ is an invalid operation. Produces a quiet NaN (QNaN(VXISI)) and sets FPSCR[VXISI]. Xenia emits the host-native NaN.
• Denormal handling. Xenon's default mode flushes denormal results to zero (FPSCR[NI] / "non-IEEE mode" bit set at boot). Xenia inherits host semantics by default; if title code explicitly clears NI (rare) you'll get IEEE-compliant denormals from the host FPU.
• Rounding mode. FPSCR[RN] selects one of four rounding modes (nearest-even, toward 0, toward +∞, toward −∞). Games rarely change RN from the default nearest-even. If your translator needs faithful rounding-mode support emit fesetround around the operation.
• Register encoding. A-form: FRT, FRA, FRB, FRC, Rc — but fadd ignores FRC (the "C" multiplier operand used by fmadd-style ops). The FRC field is architecturally don't-care but typically encoded as 0.

Related Instructions

• faddsx — single-precision add; result is rounded to binary32 then stored as binary64.
• fsubx, fsubsx — double / single subtract.
• fmulx, fmulsx — double / single multiply.
• fmaddx, fmsubx, fnmaddx, fnmsubx — fused multiply-add family (single-rounding; preferred for dot products).
• mffsx, mtfsfx — read/write FPSCR.

IBM Reference

• AIX 7.3 — fadd (Floating Add)
• PowerISA v2.07B, Book I, Chapter 4 — Floating-Point Processor (complete FPSCR and NaN-propagation rules).