What version of Go are you using (go version)?

$ go version
1.15.8 windows/amd64

Does this issue reproduce with the latest release?

Checked on play.golang.org, yes.

What operating system and processor architecture are you using (go env)?

$ go env

What did you do?

package main

import (
	"log"
	"golang.org/x/arch/x86/x86asm"

)

func main() {
	// rep movsw [rdi], [rsi]
	bs := []byte{ 0x66, 0xf3, 0xa5 }
	
	inst, err := x86asm.Decode(bs, 64)
	if err != nil {
		log.Fatal(err)
	}
	
	log.Println(inst.MemBytes)
}

https://play.golang.org/p/PzsK1XpOgWZ

What did you expect to see?

2

What did you see instead?

0