16-, 32-, and 64-bit byteswap implementation for win32 with no CRT

win32_bswap.asm

;; win32_bswap.asm
;;
;; This is intended as a replacement to the MS _byteswap_xxxx functions
;; This is unneeded if not build with /NODEFAULTLIB
;;

.code

_byteswap_ulong proc
    bswap   ecx
    mov     eax, ecx
    ret
_byteswap_ulong endp

_byteswap_uint64 proc
    bswap   rcx
    mov     rax, rcx
    ret
_byteswap_uint64 endp

_byteswap_ushort proc
    xchg    cl, ch
    movzx   eax, cx
    ret
_byteswap_ushort endp

end