test$ ~/proj/nasmtest/rc/nasm -v
NASM version 3.02rc0 compiled on Oct 13 2025
test$ cat test.asm
segment .text code use16
test:
BITS 16
        o32     ret
BITS 32
        o16     ret
.end:
test$ ~/proj/nasmtest/rc/nasm test.asm -fobj
test.asm:4: warning: invalid operand size prefix o32, must be o16 [-w+prefix-opsize]
test.asm:6: warning: invalid operand size prefix o16, must be o32 [-w+prefix-opsize]
test$ ~/proj/nasmtest/rc/nasm test.asm -felf
test.asm:1: warning: unknown section attribute 'code' ignored on declaration of section `.text' [-w+other]
test.asm:1: warning: unknown section attribute 'use16' ignored on declaration of section `.text' [-w+other]
test.asm:4: warning: invalid operand size prefix o32, must be o16 [-w+prefix-opsize]
test.asm:6: warning: invalid operand size prefix o16, must be o32 [-w+prefix-opsize]
test$ ~/proj/nasmtest/rc/nasm test.asm -fbin
test.asm:1: warning: ignoring unknown section attribute: "code" [-w+other]
test.asm:1: warning: ignoring unknown section attribute: "use16" [-w+other]
test.asm:4: warning: invalid operand size prefix o32, must be o16 [-w+prefix-opsize]
test.asm:6: warning: invalid operand size prefix o16, must be o32 [-w+prefix-opsize]
test$