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

mftb — Move from Time Base

Category: Control / CR / SPR · Form: XFX · Opcode: 0x7c0002e6

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
mftb	mftb	—	Move from Time Base

Syntax

mftb [RD], [TBR]

Encoding

mftb — form XFX

• Opcode word: 0x7c0002e6
• Primary opcode (bits 0–5): 31
• Extended opcode: 371
• 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
TBR	mftb: read	Time-Base Register selector for mftb.
RD	mftb: write	Destination GPR.

Register Effects

mftb

• Reads (always): TBR
• Reads (conditional): none
• Writes (always): RD
• 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

mftb

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="mftb"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_control.cc:719
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:53
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:803
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1664-1672

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::mftb => {
            let tbr = instr.spr();
            ctx.gpr[instr.rd()] = match tbr {
                268 => ctx.timebase & 0xFFFF_FFFF,
                269 => ctx.timebase >> 32,
                _ => 0,
            };
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Time-base register selectors. The 10-bit tbr field encodes the same way as mfspr's spr field (two halves swapped). The two values defined for the Xenon:

  Decoded	Name	Meaning
  268	TBL	Time Base, lower 32 bits
  269	TBU	Time Base, upper 32 bits

  Other selectors return 0 in xenia and are not used by titles.

• Atomic 64-bit read pattern. Because mftb reads only 32 bits at a time, software performs the canonical retry loop to avoid TBL→TBU rollover skew:

  retry:
      mftbu rH        ; read upper
      mftb  rL        ; read lower (TBR=268)
      mftbu rH2       ; read upper again
      cmpw  rH, rH2
      bne   retry
  

• Xenon clock rate. Real hardware ticks the time base at ~3.2 GHz (one tick per CPU clock divided by the architectural ratio). The PVR signature the kernel exposes (0x00710800) and the kernel-reported tick rate jointly let titles convert TB ticks to seconds.

• xenia behaviour. xenia-rs stores ctx.timebase as a u64 and increments it once per interpreted instruction, not per real-time wall clock. This guarantees deterministic replay (same trace ⇒ same TB readings) at the cost of decoupling guest time from host time. Games that rely on TB for real-time sync will run faster or slower depending on host throughput.

• mftb RT (no operand) is the simplified mnemonic for mftb RT, 268 — read the lower half. mftbu RT ≡ mftb RT, 269.

• Deprecated alternative. mfspr RT, 268/269 works on the Xenon (xenia accepts both) but post-PowerISA v2.06 deprecated reading TB through mfspr. Prefer mftb.

Related Instructions

• mfspr — generic SPR read; can also read TBL/TBU on Xenon (deprecated).
• mtspr — TBL/TBU writes are privileged; not user-accessible.
• isync — context-synchronising fence sometimes paired with mftb for tight measurement loops.

Simplified Mnemonics

Simplified	Expansion	Notes
mftb RT	mftb RT, 268	read TBL
mftbu RT	mftb RT, 269	read TBU

IBM Reference

• AIX 7.3 — mftb (Move from Time Base)
• PowerISA v2.07B, Book II §6.1 — Time Base description and the canonical 64-bit read sequence.