test$ cat test.asm

        cpu 386
MULTIBOOT_TARGET_SEGMENT equ 200h

[bits 16]
        mov eax, cr0
        dec ax
        mov cr0, eax                            ; clear PE bit

        jmp dword MULTIBOOT_TARGET_SEGMENT:.rm  ; reload cs

.rm:
test$ nasm -v
NASM version 2.16.02rc2 compiled on Oct 12 2023
test$ nasm test.asm
test$ ~/proj/nasmtest/rc/nasm -v
NASM version 3.00rc9 compiled on Sep 19 2025
test$ ~/proj/nasmtest/rc/nasm test.asm
test.asm:6: error: no instruction for this cpu level
test.asm:8: error: no instruction for this cpu level
test.asm:10: error: invalid operand sizes for instruction
test$