xenia-rs/migration/project-root/ppc-manual/control/mcrf.md

mcrf — Move Condition Register Field

Category: Control / CR / SPR · Form: XL · Opcode: 0x4c000000

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
mcrf	mcrf	—	Move Condition Register Field

Syntax

mcrf [CRFD], [CRFS]

Encoding

mcrf — form XL

• Opcode word: 0x4c000000
• Primary opcode (bits 0–5): 19
• Extended opcode: 0
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (19)
6–10	BT/BO	target / branch options
11–15	BA/BI	source A / CR bit to test
16–20	BB	source B
21–30	XO	extended opcode (10 bits)
31	LK	link flag

Operands

Field	Role	Description
CRFS	mcrf: read	CR source field.
CRFD	mcrf: write	CR destination field (crf, 0–7).

Register Effects

mcrf

• Reads (always): CRFS
• Reads (conditional): none
• Writes (always): CRFD
• 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 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

mcrf

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="mcrf"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_control.cc:424
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:51
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:710
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1683-1686

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::mcrf => {
            ctx.cr[instr.crfd()] = ctx.cr[instr.crfs()];
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Field-level (4-bit) move. Unlike the bit-level CR-logical family (crand, …, crxor), mcrf copies all four bits of a CR field (LT, GT, EQ, SO) in one instruction. CRFD and CRFS are 3-bit field indices (0..7), each naming a 4-bit slice of the 32-bit CR.
• No source-field clobber. The source field is read, not modified — mcrf 0, 1 copies CR1 into CR0 leaving CR1 intact.
• Same-field is a NOP. mcrf cr0, cr0 reads-then-writes the same field; xenia's interpreter still does the assignment but the architectural state is unchanged.
• Use case. Promote a non-default compare result into cr0 so a default-cr0 simplified branch (beq label) can consume it without spelling out cr1/cr2/etc. The alternative — crmove — would require four cror instructions to move all four bits.
• No CR0/XER side effects. Pure CR-field dataflow.
• Not synchronising. Reorderable.
• xenia exact match. xenia-rs models the CR as an array of eight 4-bit fields, so mcrf is a single struct copy (ctx.cr[crfd] = ctx.cr[crfs]). Matches PowerISA semantics exactly.

Related Instructions

• mfcr — read the entire 32-bit CR into a GPR.
• mtcrf — write selected CR fields from a GPR (uses an 8-bit field-mask).
• mcrxr — copy XER[SO..CA] into a CR field and clear them.
• mcrfs — copy an FPSCR field into a CR field.
• crand, crandc, creqv, crnand, crnor, cror, crorc, crxor — bit-level alternatives.
• bcx — typical consumer.

mcrf has no simplified mnemonics.

IBM Reference

• AIX 7.3 — mcrf (Move Condition Register Field)