I learned that assembly code doesn’t depend on the OS. For verification, I generate the assembly code of the following code snippet (named test.c) under windows and try assembling it under WSL.

int add(int a, int b){
   return a+b;
}

After I run gcc test.c -S under windows, the file test.s is generated as

        .file   "test.c"
        .text
        .globl  add
        .def    add;    .scl    2;      .type   32;     .endef
        .seh_proc       add
add:
        pushq   %rbp
        .seh_pushreg    %rbp
        movq    %rsp, %rbp
        .seh_setframe   %rbp, 0
        .seh_endprologue
        movl    %ecx, 16(%rbp)
        movl    %edx, 24(%rbp)
        movl    16(%rbp), %edx
        movl    24(%rbp), %eax
        addl    %edx, %eax
        popq    %rbp
        ret
        .seh_endproc
        .ident  "GCC: (MinGW-W64 x86_64-posix-seh, built by Brecht Sanders) 11.2.0"

If gcc test.s -c is run under windows, the object file test.o is generated without any error. However, if gcc test.s -c is run under WSL, the following errors are reported.

test.s: Assembler messages:
test.s:4: Error: unknown pseudo-op: `.def'
test.s:4: Error: unknown pseudo-op: `.scl'
test.s:4: Error: Missing symbol name in directive
test.s:4: Error: unrecognized symbol type "32"
test.s:4: Error: unknown pseudo-op: `.endef'
test.s:5: Error: unknown pseudo-op: `.seh_proc'
test.s:8: Error: unknown pseudo-op: `.seh_pushreg'
test.s:10: Error: unknown pseudo-op: `.seh_setframe'
test.s:11: Error: unknown pseudo-op: `.seh_endprologue'
test.s:19: Error: unknown pseudo-op: `.seh_endproc'

Similarly, if I run gcc test.c -S under WSL, the generated file test.s can only be assembled under WSL.

I want to know the reason of it, and how to fix the reported errors.