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The example code below will create and write text into a new file. In order to use WriteFile in flat assembler you either have to include ‘WIN32AX.INC’ or handle the imports from kernel32.dll manually. I’ve used the shorter version and included win32ax.in. This sample code should work on all operating systems using the Win32 API:

format pe console 4.0
include 'WIN32AX.INC'

.data
 buf db 'TEST'
 bufsize = $ - buf
 byteswritten dd ?
.code
 main:
 invoke  CreateFile, 'test.txt', GENERIC_WRITE, 0, 0, 4, FILE_ATTRIBUTE_NORMAL, 0
 invoke  WriteFile, eax, buf, bufsize, byteswritten, 0
 invoke  ExitProcess, 0

.end main

If the file already exists, it won’t get deleted, but it will be simply overwritten.