> A 32-bit assembler (like gas) will turn `mov %esp,%ebp` into `89 e5` while `mov %sp,%bp` becomes `66 89 e5` -- the former being correct when actually in 16-bit.

He is using nasm with "bits 16" directive, so 66 prefix will be emitted for "mov ebp,esp". gas with 32-bit target is totally unrelated to this discussion.

> http://wiki.osdev.org/Unreal_Mode

So what? MBR runs in 16-bit real mode.

> He is using nasm with "bits 16" directive

So he is!

I had to download nasm to check, but that sounds useful.

geocar, you're wrong, mkup is right. I've checked a few disassembled

https://onlinedisassembler.com/odaweb/z1mMaYSk/0

locations of his binary file (which is identical to the binary I can produce with the nasm):

      
    :0000001f    6683c402 add $0x2,%esp	      
            
    :00000025    6655     push %ebp	      
      
    :00000027    6689e5   mov %esp,%ebp

There's a 66h "Operand-size override" prefix present in the binaries which is not needed in the 16-bit code. The proper instructions would be "push bp" etc.