xenia-rs/migration/project-root/ppc-manual/alu/divdux.md

divdux — Divide Doubleword Unsigned

Category: Integer ALU · Form: XO · Opcode: 0x7c000392

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
divdu	divdux	—	Divide Doubleword Unsigned
divduo	divdux	OE=1	Divide Doubleword Unsigned
divdu.	divdux	Rc=1	Divide Doubleword Unsigned
divduo.	divdux	OE=1, Rc=1	Divide Doubleword Unsigned

Syntax

divdu[OE][Rc] [RD], [RA], [RB]

Encoding

divdux — form XO

• Opcode word: 0x7c000392
• Primary opcode (bits 0–5): 31
• Extended opcode: 457
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (31)
6–10	RT	destination GPR
11–15	RA	source A
16–20	RB	source B
21	OE	overflow-enable flag
22–30	XO	extended opcode (9 bits)
31	Rc	record-form flag

Operands

Field	Role	Description
RA	divdux: read	Source GPR (r0–r31).
RB	divdux: read	Source GPR.
RD	divdux: write	Destination GPR.
OE	divdux: write (conditional)	Overflow-enable bit. When 1, the instruction updates XER[OV] and stickies XER[SO] on signed overflow.
CR	divdux: write (conditional)	Condition-register update. When Rc=1, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result.

Register Effects

divdux

• Reads (always): RA, RB
• Reads (conditional): none
• Writes (always): RD
• Writes (conditional): OE, CR

Status-Register Effects

• divdux: CR0 ← signed-compare(result, 0) with SO ← XER[SO], when Rc=1.; XER[OV] ← signed-overflow(result); XER[SO] stickies, when OE=1.

Operation (pseudocode)

RT <- (RA) /u (RB)                       ; undefined if RB=0

C Translation Example

/* 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

divdux

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="divdux"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_alu.cc:217
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:21
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:876
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:480-496

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::divdux => {
            let ra = ctx.gpr[instr.ra()];
            let rb = ctx.gpr[instr.rb()];
            let ov = overflow::divd_ov_unsigned(rb);
            if ov {
                ctx.gpr[instr.rd()] = 0;
            } else {
                ctx.gpr[instr.rd()] = ra / rb;
            }
            if instr.oe() {
                overflow::apply(ctx, ov);
            }
            if instr.rc_bit() {
                ctx.update_cr_signed(0, ctx.gpr[instr.rd()] as i64);
            }
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Single undefined case. Division by zero (RB == 0). There is no INT_MIN/−1 overflow because both operands are unsigned. Xenia-rs returns 0 for the divide-by-zero case (interpreter.rs:306); spec leaves RT boundedly undefined.
• No trap on Xenon. As with divdx, the processor does not raise an exception; consuming code must guard RB first (typically cmpdi rb, 0; beq skip).
• OE=1 should set XER[OV] on RB == 0; xenia-rs ignores OE here.
• Rc=1 CR0 update is correctly 64-bit. interpreter.rs:311 uses as i64 directly, so the CR0 sign comparison reflects the full 64-bit unsigned quotient cast to signed. For very large unsigned quotients (> INT64_MAX) this CR0 will report LT even though the unsigned interpretation is positive — a rare but real source of CR-misuse bugs.
• Slow. Same ~70-cycle non-pipelined cost as the signed variant; consider reciprocal multiply for hot loops.
• Truncating quotient. Same C-style toward-zero rounding (trivially equal to floor for unsigned).

Related Instructions

• divdx — signed 64-bit divide.
• divwux, divwx — 32-bit unsigned/signed.
• mulldx, mulhdux — multiply pair for remainder calculation.
• cmpli, cmpl — guard the divisor.

IBM Reference

• AIX 7.3 — divdu (Divide Doubleword Unsigned)