$ nasm -v
NASM version 2.16rc0 compiled on Sep  6 2022
$ cat test.asm
xchg word [0], ax
xchg ax, word [0]
lock xchg word [0], ax
lock xchg ax, word [0]
$ nasm test.asm
test.asm:4: warning: instruction is not lockable [-w+lock]
$ nasm test.asm -l /dev/stderr
     1 00000000 87060000                xchg word [0], ax
     2 00000004 87060000                xchg ax, word [0]
     3 00000008 F087060000              lock xchg word [0], ax
test.asm:4: warning: instruction is not lockable [-w+lock]
     4 0000000D F087060000              lock xchg ax, word [0]
     4          ******************       warning: instruction is not lockable [-w+lock]
$