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

mtcrf — Move to Condition Register Fields

Category: Control / CR / SPR · Form: XFX · Opcode: 0x7c000120

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
mtcrf	mtcrf	—	Move to Condition Register Fields

Syntax

mtcrf [CRM], [RS]

Encoding

mtcrf — form XFX

• Opcode word: 0x7c000120
• Primary opcode (bits 0–5): 31
• Extended opcode: 144
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (31)
6–10	RT	destination / source GPR
11–20	spr/tbr/FXM	SPR/TBR number (byte-swapped halves) or CR field mask
21–30	XO	extended opcode
31	—	reserved

Operands

Field	Role	Description
RS	mtcrf: read	Source GPR (alias for RD in some stores).
CRM	mtcrf: write	8-bit CR field mask used by mtcrf — one bit per CR field.

Register Effects

mtcrf

• Reads (always): RS
• Reads (conditional): none
• Writes (always): CRM
• Writes (conditional): none

Status-Register Effects

No condition-register or status-register effects.

Operation (pseudocode)

for i in 0..7:
    if CRM[i] then CR[i] <- (RS)[32+i*4 : 35+i*4]

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

mtcrf

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="mtcrf"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_control.cc:732
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:55
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:779
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1631-1644

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::mtcrf => {
            let crm = instr.crm();
            let val = ctx.gpr[instr.rs()] as u32;
            let old = ctx.cr();
            let mut new = old;
            for i in 0..8u32 {
                if crm & (1 << (7 - i)) != 0 {
                    let mask = 0xF << (28 - i * 4);
                    new = (new & !mask) | (val & mask);
                }
            }
            ctx.set_cr(new);
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• CRM is an 8-bit field-mask, MSB-first. Each bit of CRM corresponds to one CR field: CRM[0] (mask bit 0x80) selects CR0, CRM[1] (0x40) selects CR1, …, CRM[7] (0x01) selects CR7. Each set mask bit causes the corresponding 4-bit slice of RS[32:63] to overwrite that CR field; clear mask bits leave the field untouched.
• Slice positions inside RS. Big-endian: bits 32..35 of RS map to CR0, bits 36..39 to CR1, …, bits 60..63 to CR7. The high 32 bits of RS are ignored.
• mtcr RS simplified mnemonic. When CRM = 0xFF, all eight CR fields are written; assemblers fold this into mtcr RS. This is the dominant form (function epilogue restoring the saved CR).
• mtocrf variant. PowerISA defines mtocrf as the single-field variant — encoded with the high bit of FXM set and exactly one CRM bit set. xenia-rs treats both as the same opcode and processes whatever CRM mask is present, so mtocrf works correctly without special handling.
• Use case in ABI. Save/restore non-volatile CR fields (CR2, CR3, CR4 on the Xbox 360 ABI). The standard restore is lwz r12, 8(r1); mtcrf 0x38, r12 — 0x38 = bits for CR2|CR3|CR4 — preserving the volatile fields the callee may have already updated.
• No CR0 / XER side effects. mtcrf does not record into CR0; XER is untouched.
• xenia exact match. xenia-rs decomposes the CR into a u32, applies a per-field mask, and reassembles via set_cr. The 8-bit CRM walk matches the spec exactly.
• Not synchronising. Reorderable.

Related Instructions

• mfcr — read the entire CR into a GPR.
• mcrf — copy one CR field to another (no GPR involved).
• mcrxr, mcrfs — narrower CR-field moves from XER / FPSCR.
• crand, cror, … — bit-level CR manipulation.

Simplified Mnemonics

Simplified	Expansion	Notes
mtcr RS	mtcrf 0xFF, RS	write all eight CR fields from low half of RS

mtocrf RS, FXM is a related encoding handled by the same xenia-rs slot.

IBM Reference

• AIX 7.3 — mtcrf (Move to Condition Register Fields)
• AIX 7.3 — Condition register simplified mnemonics