• Cross-compiling to an other target
  
• Multithreading (see FAQ 23)
  
• Graphics in windowed mode or using OpenGL
  
• Setting ScreenRes to a size not matching any resolution supported by the graphics card
  
• Unicode isn't supported in DOS, WString will be the same as ZString, character sets other than latin aren't supported. (do it yourself)
  
• Shared libraries (DLL's) can't be created/used (at least not "easily"), amount of available static external libraries usable with DOS is limited
  
  

• Use the FB graphics library. It uses VESA (preferably linear, but also supports banked) to access the video card and supports any resolution reported by the card's VESA VBE driver, in addition to standard VGA modes.
  

• VGA mode 320x200x8bpp: very simple, maximum reliability and compatibility, but low resolution and 256 colours only, see example.
  
• VGA "ModeX" 320x240x8bpp: similar to above, less easy, good reliability and compatibility, but low resolution and 256 colours only, see example.
  
• VGA "planed" mode 640x480x4bpp: difficult to set pixels, maximum reliability and compatibility, but low resolution and 16 colours only, no public example yet (?).
  
• Some other "odd" VGA "ModeX" modes (like 360x240x8bpp) : possible, but for freaks only ;-)
  
• Write your own VESA code: More difficult, good compatibility, high-res and true color possible, there might be reliability problems if not implemented carefully.
  
• Use an external library (DUGL, Allegro, MGL, WxWidgets): Allows to create "expensive" graphics & GUI's, bloats EXE size, need to respect library license, potential loss of reliability.
  
  

(""FreeBASIC"")
'' DOS only example of inline ASM accessing low memory 
'' Run in text mode 80x25 only
''

'' Including dos/go32.bi will define "_dos_ds"
'' "pointing" into GO32 block

#include "dos/go32.bi" 

Dim As UInteger DDS

DDS=_dos_ds

? : ? "Hello world !"
? "_dos_ds=$";Hex$(DDS) 
? "This is just a tEst - abcd ABCD XYZ xyz @[`{ - press any key ..."

Do
  Sleep 1000
  If Inkey$<>"" Then Exit Do
  Asm
	mov  eax,[DDS] '' Directly using "_dos_ds" won't work here !!!
	push eax
	pop  gs        '' Just to get sure, it is usually set anyway
	Xor  ebx,ebx
	aa3:
	mov  al,[gs:0xB8000+2*ebx]
	cmp  al,65  '' "a"
	jb   aa1
	cmp  al,122 '' "z"
	ja   aa1   
	cmp  al,90  '' "Z"
	jbe  aa2
	cmp  al,97  '' "a"    
	jb   aa1 
	aa2: 
	Xor  al,32  '' Swap case
	aa1:
	mov  [gs:0xB8000+2*ebx],al
	inc  ebx
	cmp  ebx,2000
	jne  aa3
  End Asm  
Loop
? : ? "Bye"
End

(""FreeBASIC"")
#include "dos/dpmi.bi"

type RegTypeX as __dpmi_regs

#define INTERRUPTX(v,r) __dpmi_int( v, @r )

• The graphics might not work well / at all on very old PC's. If your CPU has less than cca 500 MHz, provide exact info about it, if you don't know, use RayeR's CPUID or similar program to test.
  
• Exact info about your graphics card is needed. Test on DOS using DrV's VBEDIAG (reports info only) and RayeR's VESATEST (also tries to set mode, allows visual inspection of the result). Find out what "useful" modes (640x480, 800x600) are supported and with what bitdepths (8, 16, 24, 32 bpp), and whether they can be set and look correctly.
  
• Find out and describe exactly what's wrong ("mode works with VESATEST but not with FB", "no graphics but no error either", "black screen and freezer", "graphics is messy/incomplete", ...).
  
• If some sophisticated program doesn't work, try also a minimal test like placing a circle in middle of the screen.
  
• Try without a mouse driver (this reduces the CPU "cost").
  
• Find out what modes are affected. If a mode doesn't work, reduce the resolution or bitdeph, make sure to test the "cheapest"/safest modes 640x480 with 32/24/16/8 bpp, 640x480 with 4 bpp, and 320x200 with 8bpp.
  
• For some old cards there are VESA drivers available (S3VBE/UVIVBE). Test both with and without, and include this info into your report.
  
• Remove potentially problematic content (memory managers, drivers) from DOS startup files. Nothing of such is required for FB, except a DPMI host (see also FAQ 4.).
  
• Post info about your graphics card, CPU (if old), DOS type and version, bug symptoms, and a simple example code.
  

• Set up an ISR, see "ISR_TIMER.BAS" example. This is not a "full" replacement, but sufficient in some cases.
  
• There is a pthreads library for DJGPP allowing to "emulate" Linux-like threading to some degree. It works acceptably for [P]7-ZIP DJGPP port (written in C+ + ), no tests with FB yet.
  
  

• PIT / BIOS timer (runs at 18.2 Hz by default), peek the BIOS timer or set your own, see "ISR_TIMER.BAS" example, raise PIT frequency (use with care)
  
• Poll the BIOS timer + PIT counter, method from TIMERHLP.ASM from DKRNL32, allows to enhance precision of above without raising the PIT frequency
  
• RDTSC instruction (Pentium and newer)
  
• RTC clock
  
• Delay loops
  
  

• Use logical disk access features of DOS for sector access bypassing the filesystem, see example in the forum: freebasic.net/forum/viewtopic.php?t=11830
  
• Use physical disk BIOS INT 13, bypassing DOS
  
• Use CPU ports, lowest level, bypassing both DOS and BIOS (no FB example yet, see source of IDECHECK from FAQ 27 above, FASM forum or some OS development resources)