function dna_strand4(dna as const zstring ptr) as zstring ptr

  const iLen = TestLen

  static as zstring*(TestLen+1) pOut
  asm
    mov esi, [dna]    'input
    lea edi, [pOut]   'output
    mov ecx, iLen     'length
    sub edi, 4
    0:
      mov edx, [esi]      'read 4 chars
      add esi, 4          'point to next char
      mov eax, edx        'make a copy of those 4 chars
      and edx, 0x02020202 'isolate bit 1... (2)
      xor edx, 0x02020202 'invert so... 2 = CG , 0 = AT
      lea ebx, [edx*8]    'ebx = N*8
      shr edx, 1          'edx = N\2
      add edx, ebx        'edx = N*8+N\2
      or edx, &h04040404  'edx ^ 4 (21 or 4)
      add edi, 4          'point to next out char
      xor eax, edx        'chars ^ edx
      sub ecx, 4          'decrement counter
      mov [edi], eax      'store char
    jnz 0b
  end asm
  return @pOut
end function