The flat assembler is a fast and efficient self-assembling 80x86 assembler for DOS, Windows and Linux. It supports all 8086-80486/Pentium instructions with MMX, SSE, SSE2, SSE3 and 3DNow! extensions, can produce output in binary, MZ, PE or COFF format. Has the advanced but easy to use macroinstruction support and does multiple quick passes to optimize the code for size. It is self-compilable and the full source code is included.

Official distributions:
  version 1.51 for DOS [31-01-2004]
  version 1.51 for Linux [31-01-2004]
  version 1.51 for Win32 console [31-01-2004]
  version 1.51 for Win32 GUI (interface 0.91) [31-01-2004]

Additional documentation:
  Frequently Asked Questions [08-12-2003]
  Keyboard commands for Windows version [21-01-2003]