xenia-rs/migration/project-root/ppc-manual/vmx/vmaxsb.md

vmaxsb — Vector Maximum Signed Byte

Category: VMX (Altivec) · Form: VX · Opcode: 0x10000102

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
vmaxsb	vmaxsb	—	Vector Maximum Signed Byte

Syntax

vmaxsb [VD], [VA], [VB]

Encoding

vmaxsb — form VX

• Opcode word: 0x10000102
• Primary opcode (bits 0–5): 4
• Extended opcode: 258
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (4)
6–10	VRT/VD	destination vector register
11–15	VRA/VA	source A vector register
16–20	VRB/VB	source B vector register
21–31	XO	extended opcode (11 bits)

Operands

Field	Role	Description
VA	vmaxsb: read	Source A vector register.
VB	vmaxsb: read	Source B vector register.
VD	vmaxsb: write	Destination vector register.

Register Effects

vmaxsb

• Reads (always): VA, VB
• Reads (conditional): none
• Writes (always): VD
• 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

vmaxsb

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="vmaxsb"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:838
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:101
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:454
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:4407-4414

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::vmaxsb => {
            let a = crate::vmx::as_i8x16(ctx.vr[instr.ra()]);
            let b = crate::vmx::as_i8x16(ctx.vr[instr.rb()]);
            let mut r = [0i8; 16];
            for i in 0..16 { r[i] = a[i].max(b[i]); }
            ctx.vr[instr.rd()] = crate::vmx::from_i8x16(r);
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Per-byte signed max. Sixteen byte lanes; VD[i] = max(int8(VA[i]), int8(VB[i])). Lane 0 is the most-significant byte.
• Sign-aware ordering. vmaxsb(0xFF, 0x01) = 0x01 (i.e. max(-1, 1) = 1), versus vmaxub which would return 0xFF. Pick vmaxsb deliberately when both sides are signed.
• No VSCR interaction, no XER, no exceptions. Pure compare-select.
• Common usage. Per-lane clamping with vminsb implements clamp(x, lo, hi) with no branch.
• Aliasing legal. vmaxsb v3, v3, v4 is "raise lower-bound to v4" idiom.
• No VMX128 sibling.

Related Instructions

• vminsb — the matching minimum.
• vmaxub — same width, unsigned max.
• vmaxsh, vmaxsw — signed max at half / word width.
• vcmpgtsb — separate compare-and-mask path.
• vsel — alternative selection style with arbitrary fallbacks.

IBM Reference

• AIX 7.3 — vmaxsb (Vector Maximum Signed Byte)
• IBM AltiVec Technology Programmer's Interface Manual, Chapter 6 — Min/Max