DOS related FAQ
DOS
The FreeBASIC port to DOS is based on the DJGPP port of the GNU toolchain to 32-bit protected-mode DOS.
The current maintainer of this port is DrV.
To be written: platform-specific information, differences from Win32/Linux, differences from QB?, tutorials, etc.
WANTED TESTERS
The DOS version/target of FreeBASIC needs more testers. If you are interested in using FreeBASIC on DOS, please don't wait for future releases, give it a try now. Tests from running in DOS on both old and new PC's are welcome (graphics, file I/O, serial port, ...). If something doesn't work, please place a detailed bug report into the forum or bug Tracker. If all works well, you can write about your success as well. Make sure to test a recent version of FB (reports from FB older than 0.90 will be probably considered as obsolete and useless), and check this document before complaining about anything.
Limitations
The DOS target is fairly well working and supported by FreeBASIC, and up-to-date. A few differences compared to other platforms exist, however. The features missing are mostly those not supported by the operating system or DOS extender or C runtime:

- 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

FreeBASIC DOS related questions:
- 1. FB is a 32-bit compiler - do I need a 32-bit DOS?
- 2. What about FreeDOS-32? Does/will FB work, is/will there be a version?
- 3. When running FreeBASIC in DOS, I get a 'Error: No DPMI' message!
- 4. Is there a possibility how to get rid of this CWSDPMI.EXE and CWSDPMI.SWP?
- 5. Can I use other DOS extenders, like DOS/4GW, Causeway, DOS/32A?
- 6. Where is the nice blue screen with all the ... / where is the IDE?
- 7. How can I view the documentation in CHM or PDF format in DOS?
- 8. How can I write/edit my source code?
- 9. How can I play sound in DOS?
- 10. How can I use USB in DOS?
- 11. How can I use graphics in DOS?
- 12. DEF SEG is missing in FB! How can I workaround this in my code?
- 13. How can I rewrite QB's CALL INTERRUPT / access the DOS and BIOS interrupts?
- 14. How can I rewrite QB's XMS/EMS handling?
- 15. FBC gives me a 'cannot find lsupcxx' error!
- 16. How can I use the serial or parallel port?
- 17. How can I use a printer?
- 18. How can I make a screenshot of a FreeBASIC program running in DOS?
- 19. Graphics mode doesn't work (freeze / black screen / garbage output)!
- 20. Mouse trouble! Mouse doesn't work at all in DOS / arrow 'jumps' / etc. ...
- 21. What about the 64 KiB and 640 KiB problems / how much memory is supported by FB in DOS?
- 22. My program crashes when I try to use more than cca 1 MiB RAM! Is this a bug in FreeBASIC?
- 23. Threading functions are disallowed in DOS? Help!
- 24. Executables made with FB DOS are bloated!
- 25. Compilation is very slow with FB!
- 26. SLEEP doesn't work! How can I cause a delay?
- 27. The performance is very bad in DOS!
- 28. Can I access disk sectors with FB?
- 29. Can I use inline ASM with advanced instructions like SSE in DOS ?
See also
FreeBASIC DOS related questions
1. FB is a 32-bit compiler - do I need a 32-bit DOS?
No, the DOS version of FreeBASIC uses a DOS extender, allowing you to execute 32-bit code on top of a 16 bit DOS kernel. You can use FreeDOS (16-bit), Enhanced-Dr-DOS, old closed Dr-DOS, or even MS-DOS down to version cca 4. You need at least 80386 CPU, see also Requirements.
2. What about FreeDOS-32? Does/will FB work, is/will there be a version?
FreeDOS-32 is experimental at time of writing, but it should execute FreeBASIC and applications generated by it with no change. While FB DOS support already works on FreeDOS (16), it should be ready for FreeDOS-32 as well.
3. When running FreeBASIC in DOS, I get a 'Error: No DPMI' message!
You need a DPMI host (DPMI kernel, DPMI server), means the file "CWSDPMI.EXE" (cca 20 KiB) or HDPMI32.EXE (cca 34 KiB). See requirements, and FAQ 4 for more details.
4. Is there a possibility how to get rid of this CWSDPMI.EXE and CWSDPMI.SWP?
Yes, 2 possibilities. To get rid of CWSDPMI.EXE and create a standalone DOS executable embedding CWSDPMI, you need the CWSDPMI package and the "EXE2COFF.EXE" file. Using EXE2COFF, you remove the CWSDPMI.EXE loader (file loses 2 KiB of size, resulting in a "COFF" file without extension), and then glue the file "CWSDSTUB.EXE" before this COFF. The new executable is cca 21 KiB bigger than the original one, but it is standalone, no additional files are needed. To get rid of CWSDPMI.SWP, you can then edit your executable with CWSPARAM.EXE, and disable the swapping (occasionally also - incorrectly - referred as paging). Note, however, that this will limit the memory that can be allocated to the amount of physical memory that is installed in a system. This work can be done both with the FBC.EXE file and all executables created by FBC. The method is also described in the CWSDPMI docs in the package. Alternatively, you can use the WDOSX or D3X extender. They don't swap and create standalone executables. Since they run your executable in Ring 0, the crash handling of them is not very good and can cause freezers or reboots on bugs where other hosts exit the "civil" way with a register dump. Also, spawning might not work well / at all with WDOSX or D3X. Finally, you can use HDPMI . Download the "HXRT.ZIP" file (here: japheth.de/HX.html alternative links), extract "HDPMI32.EXE" (cca 34 KiB) and "HDPMI.TXT" (not required by the code, just for your information), and include it to your DOS startup ("HDPMI32 -r"). This will make HDPMI resident and prevent all FreeBASIC (also FreePASCAL and DJGPP) programs from both crying about missing DPMI and swapping. HDPMI can not (easily / yet) be included into your executables. Running an executable containing D3X, CWSDPMI or some DPMI host inside under HDPMI or other external host is fine - the built-in host will be simply skipped. Using DPMI is definitely required for FreeBASIC, since it can't generate 16-bit real mode code, and there is no other good way to execute 32-bit code in DOS.
5. Can I use other DOS extenders, like DOS/4GW, Causeway, DOS/32A?
Not any extender around. So-called WATCOM-like extenders can't be used because of important differences in memory management and executable structure. WDOSX and D3X do work, since they are a multi-standard extenders, not only WATCOM-like. You also can use PMODE/DJ (not "original" Tran's PMODE, nor PMODE/W (!), saves cca 5 KiB compared to CWSDPMI, can be included into the EXE, but might affect stability or performance) or, as aforementioned, HDPMI.
6. Where is the nice blue screen with all the ... / where is the IDE?
The FreeBASIC project focuses on the compiler, generating the executables from your BAS sources. It looks unspectacular, but is most important for the quality of software developed by you. The project does not include an IDE. There are several external IDEs for FreeBASIC, but probably none does have a DOS version by now. If you really need one, you could try Rhide, but note that it is complicated and buggy, so use it at your own risk. See also FAQ 7 and 8.
7. How can I view the documentation in CHM or PDF format in DOS?
There is no good way to view CHM or PDF files in DOS by now. But you can view the FreeBASIC documentation nevertheless. One of the FreeBASIC developers, coderJeff provides a FreeBASIC documentation viewer with the docs included in a special format, and having also a DOS version. It looks similar the QB's built-in help viewer, but does not contain an editor or IDE. Download here: http://www.execulink.com/~coder/FreeBASIC/docs.html
8. How can I write/edit my source code?
There are many editors for DOS around, but only few of them are good - some possibilities are FreeDOS EDIT (use version 0.7d (!!) or 0.9, 64 KiB limit, suboptimal stability (save your work regularly) ), SETEDIT, INFOPAD (comes with CC386 compiler, can edit big texts also, has syntax highlighting for C and ASM, but not for BASIC).
9. How can I play sound in DOS?
There are 2 ways how to play sound in DOS: either the ("archaic") PC speaker, famous for beeping if something goes wrong, or a soundcard. The speaker is easy to control, allows more than one might think, even to play audio files (WAV, with decompression code also OGG Vorbis, MP3 etc.), you can re-use most of existing QB code easily (example: o-bizz.de/qb...speaker.zip) or ASM code via inline ASM, but provides one channel and 6 bits only, and of course significantly worse quality than a soundcard, and, on some newest (P4) PC's the speaker quality is very bad or there is no speaker at all. For old ISA soundcards, there is much example code around, a newer PCI soundcard can be accessed (supposing bare DOS in this category) either using a ( "emulation" SB16 compatible) driver, if it is available for your card (unfortunately, this is becoming more and more a problem, the DOS drivers are poor or even inexistent), or access the card directly (this is low-level programming, hardware-related, assembler is also needed, and you need technical docs about the card). There are a few sources of inspiration like the DOS audio player MPXPLAY (written in C with some ASM), supporting both methods (native + "emu" drivers), see an up-to-date list here: drdos.org/...wiki...SoundCardChip. Support of sound in DOS is not business FB DOS port, actually FB doesn't "support" sound on Win32 and Linux either - the games "connect to the API" rather than use FreeBASIC commands or libraries. To play compressed files (MP3, OGG Vorbis, FLAC, ...) , you additionally need the decompressing code, existing DJGPP ports of those libraries should be usable for this.
10. How can I use USB in DOS?
Again, not business of FB, you need a driver, FB doesn't "support" USB on Win32 or Linux either, see other Wiki: drdos.org/...wiki...USB about possibilities of USB usage in DOS.
11. How can I use graphics in DOS?
GUI or graphics in DOS is definitely possible, there are several approaches:

- 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.

Note that some graphic cards report limited features through VESA, most notably less memory (for example 8 MiB instead of 64 MiB) or less modes (for example only 24 bpp modes visible while 32 bpp hidden, only lower resolutions visible (up to cca 1280x1024) while higher hidden, only "4:3" modes visible while "wide" modes hidden). This is a problem of the card, not of DOS or FreeBASIC. You will see the additional features in systems other than DOS, or in DOS only using hardware detection tools going to the lowest level bypassing VESA.
12. DEF SEG is missing in FB! How can I workaround this in my code?
DEF SEG is related to 16-bit RM addressing and was removed because of this. "direct" access to VGA or other low memory areas is not possible, because FreeBASIC's memory model (same as DJGPP's) is not zero-based. For accessing low DOS memory, use DOSMEMGET and DOSMEMPUT , see "vga13h.bas" example, or "_dos_ds" selector for inline ASM, see example:
`
'' 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
`
13. How can I rewrite QB's CALL INTERRUPT / access the DOS and BIOS interrupts?
Those interrupts can be accessed only using the DOS version/target of FB.
The access to interrupts is slower than in QB: with FB the DPMI host will have to do 2 context switches, going to real-mode and coming back. All of that will eat hundreds of clocks in raw DOS and thousands of clocks if emm386 is loaded or if inside a Windows' DOS box. The slow down might be negligible or relevant, it depends. You should try to minimize the number of such calls, and process more data per call - at least several KiB, not just one byte or a few bytes.
Use DJGPP's DPMI wrapper:
`
#include "dos/dpmi.bi"
Type RegTypeX As __dpmi_regs
#define INTERRUPTX(v,r) __dpmi_int( v, @r )
`
Alternatively you can call INT's via inline ASM, 2 important things you have to care about are the fact that FB's memory model is not zero-based (see also FAQ 12, "DEF SEG" issues), and additionally "direct" passing of addresses (like DS:[E]DX) to an INT will not work except you have a DPMI host with "DOS API translation".
14. How can I rewrite QB's XMS/EMS handling?
Depends why original code uses it. If it's just to bypass low memory limits, simply remove it and use "ordinary" FB's data types / memory handling features instead. If it is used for (sound) DMA, you are out of luck and have to redesign the code completely, about sound see FAQ 9. For DMA use preferably the low memory (should be no big problem, since the application code and most buffers are in DPMI memory instead), DMA in DPMI memory is possible but more difficult.
15. FBC gives me a 'cannot find lsupcxx' error!
The source of this problem is the libsupcxx.a file in LIB\DOS\ directory, having 9 characters in the name. Your fault is to have extracted the ZIP with long file names enabled, usually in Windows, and then using FB in DOS with no LFN support, resulting in this file looks LIBSUP~1.A and can't be found. Rename the file in LIBSUPCX.A (one X only) or extract the ZIP again in DOS. Note: changes in FB 0.18, retest needed.
16. How can I use the serial or parallel port?
The DOS INT14 is not very useful/efficient as it sends/reads a single char in each call. So it's better to use an external DOS32 comms library. /* does someone know a good one ? */ FB up to 0.18.2 doesn't support OPEN COM on DOS target, coderJeff has an experimental library/driver available, included with FB since 0.18.3.
17. How can I use a printer?
DOS kernel won't help you here, so you have to prepare the text (trivial) or pixel data (acceptably easy for printers compatible with the "ESC/P" standard) yourself and send in to the printer via the parallel port or USB using an additional driver (see FAQ 10). So-called "GDI" or "Windows" printers can't be made working in DOS with reasonable effort.
18. How can I make a screenshot of a FreeBASIC program running in DOS?
Ideally include this feature into your own code. DOS TSR based screenshooters like SNARF mostly will work with text based screens, but probably none of them with FreeBASIC's GFX library. It's not really a bug on one or other side, it's a problem "by design".
19. Graphics mode doesn't work (freeze / black screen / garbage output)!
Place a bug report into the forum. To make it as useful and productive as possible, please beware of the following, proceed given steps and provide all related information:

- Check the limitations listed on the page GfxLib
- 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.

RayeR's VESATEST and CPUID can be downloaded here: rayer.ic.cz/programm/programe.htm , VBEDIAG here drv.nu/vbediag/.
20. Mouse trouble! Mouse doesn't work at all in DOS / arrow 'jumps' / etc. ...
To use a mouse in DOS, you need a compatible driver, recognizing your mouse, and recognized by FreeBASIC library. For optimal results, you need a good driver and a suitable mouse.
Mouse: the optimal choice, and pretty well available nowadays, is a PS/2 mouse. The old type would be a serial mouse, also this one should work. The newest is USB mouse - but is not very suitable for use in DOS, since it would need a compatible (INT33) high quality native USB mouse driver (none available by now, only some experimental), or rely on BIOS emulation (not always available, or "unprecise").
Driver: the preferred choice is CTMOUSE from FreeDOS project. There are versions 1.9a1, 2.0a4, and 2.1b4 from 2008-July available. It is included with (but not limited to) FreeDOS, or download a version from here: ibiblio.org/pub/...mouse . None of them is perfect, but still they are well usable and better than most competitors. 1.9xx and 2.1xx will cooperate with BIOS, allowing USB emulation, 2.0xx bypasses BIOS and thus USB emulation will NOT work. Also Logitech mouse drivers usually do a good job, download from here: uwe-sieber.de/util_e.html - version 6.50 is a good start. Known for problems are DRMOUSE and some (old ?) versions of MSMOUSE.
If the mouse does not work at all, then most likely the driver is not loaded, doesn't recognize the mouse (see driver messages), or is not compatible with the INT33 "standard". For USB mouse, activating the "USB mouse emulation" in BIOS settings can help.
If the mouse control is "unprecise", the arrow "jumps" , then you either have a bad driver - use a better one, or the BIOS emulation is bad - the solution is to buy a PS/2 mouse then.
21. What about the 64 KiB and 640 KiB problems / how much memory is supported by FB in DOS?
Memory management is business of the DPMI host, rather than the compiler. FreeBASIC and executables generated by it do not suffer from this problem, since they use 32-bit DPMI code, rather than real mode. You can use almost all the memory of your PC, with some limitations, but they are far above 64 or 640 KiB. CWSDPMI r5 is verified to work well up to 512 MiB only, additional memory does not crash it (unlike some older versions), but is silently ignored. HDPMI is supposed to support more: up to 4 GiB (the limit of 32-bit addressing), but there was not much testing on such huge machines - verified up to cca 1.5 GiB. FreeBASIC and code generated by it do not require classical DOS based memory managers (HIMEM/XMS and EMM386/EMS), but are supposed to coexist with them if they are present. All this of course applies to true DOS only, things like "Dos Box" will keep the control over the memory management and provide only a small piece of memory (depends, up to cca 64 MiB) to your DOS code.
22. My program crashes when I try to use more than cca 1 MiB RAM! Is this a bug in FreeBASIC?
No, it's not a bug in FreeBASIC and it's not really DOS specific, see also Compiler FAQ. For a beginner, the easy solution is to use `Shared` for arrays. More advanced users could consider using memory management functions like `Allocate`. This is even more important in DOS, since it allows the application to run on (old) PCs with little memory (and still edit at least small texts for example), as well as to use all huge RAM if available (and edit huge texts for example).
23. Threading functions are disallowed in DOS? Help!
The Threading Support Functions are not supported for DOS target, and most likely won't be soon/ever. The reason is simple: neither the DOS kernel, nor the DPMI host/standard, nor "GO32" DOS Extender support threading, unlike the Win32 or Linux kernel. However nothing is impossible in DOS: you can set up your threading on the top of DPMI. There are multiple possibilities, two of which are:

- 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.
- See forum t=21274

24. Executables made with FB DOS are bloated!
This is true but there is no easy/fast way to fix. FB is a 32-bit HLL compiler, and most of the size is imported from DJGPP. !writeme! (see forum: t=11757)
25. Compilation is very slow with FB!
Problem: "FBC takes 10 seconds to compile a "Hello world" program ! TurboBASIC / QBASIC / VBDOS / PowerBASIC do take < 1 second for the same job ..."
True, but this is "by design": FB compiles your sources in 3 steps, saving the intermediate files, as described in CompilerCmdLine, while many older compilers do just 1 pass in memory. This is related mostly to file I/O performance, see FAQ 27 below about possibilities of improvements, additionally a small improvement can be achieved here by making the DPMI host resident (HDPMI32 -r or CWSDPMI -p , see FAQ 4 above). Note that the delay is mostly "additive" , so it won't hurt too much with bigger projects.
26. SLEEP doesn't work! How can I cause a delay?
`Sleep` does work ... but has a resolution of cca 55ms = 1/18s only, thus "SLEEP 500" is fine, while for example using "SLEEP 2" for 2 milliseconds won't work. !writeme! / !fixme!

- 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

27. The performance is very bad in DOS!
Problem: "The performance in DOS is poor compared to Win32 / Linux binary compiled from the very same source !" or "Even worse, the very same DOS binary runs much faster in NTVDM than in DOS !"
Both indeed can happen, nevertheless, DOS is no way predestined to be slow, the inefficiencies can be fixed. First you have to identify the area where you code looses performance.
File I/O: DOS by default uses very little memory for its buffers, while other systems use much more and are "aggressive" with file caching. When dealing with many small files, this results in serious performance degrade. The solution is to install a filecache, for example LBACache, or you can install a RAMDISK (a good one: SRDISK ) and copy the "offending" files (for example FreeBASIC installation) there in and work there (make sure to backup your work to a more durable media regularly). Both will need an XMS host (use HIMEMX ). Also DOS by default uses BIOS to access the hard drives, while other systems try hard to find and use DMA. Test util: IDECHECK by Japheth (Download: japheth.de/Download/IDECheck.zip) - run it in "I13" and "DMA" modes and compare results. If "DMA" is much faster (can be 1...10 times, depends from PC model), then installing a DOS DMA driver (for example XDMA 3.1 is worth to try) can bring a big speedup on large files. Also make sure to read and write data in large pieces (16 KiB at least), not just single bytes. Other OSes are more forgiving here, but on DOS every single file I/O call causes a small "additive" delay, thus an efficient code design with good buffering is crucial.
Graphics: Pentium 2 and newer CPU's have a cache related feature called "MTRR" to speed up writes to video RAM. Drivers of other OSes usually do enable it. DOS doesn't (since it doesn't deal with graphics at all), neither does FB GFX. Use "VESAMTRR" tool by Japheth (contained in "HXGUI.ZIP" package), it will enable the speedup, surviving also mode switches and most "non-fatal" application crashes, up to a reboot. The possible speedup factor varies much depending from the PC model, up to cca 20 times. Also the mouse handling eats some (too much) CPU performance on DOS, this is a known weak point (the design of DOS FB GFX is not "very bad", it's just the common "standard" - which is not very good), fixing is theoretically possible but not easy, you just can try several mouse drivers (see FAQ 20).
28. Can I access disk sectors with FB?
You can ... but FreeBASIC won't help you too much here: no "portable" solution, use OS specific low level way. For DOS 3 methods are possible

- 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, see forum freebasic.net/forum/viewtopic.php?t=16196, source of IDECHECK from FAQ 27 above, FASM forum or some OS development resources

Note that such experiments are a bit "dangerous" - you can easily lose data or make your PC unbootable if something goes wrong.
29. Can I use inline ASM with advanced instructions like SSE in DOS ?
You can ... but SSE2 and above need to get enabled before. This is usually considered as business of the DPMI host, HDPMI32 and CWSDPMI 7 will do that, most other hosts won't. Make sure to properly CPUID for such instructions before using them. It's a good idea to provide a code branch compatible with older CPU's (early Pentium, 80386) besides supporting latest instructions, and to avoid CMOV in those too.
See also

- Compiler FAQ.
- FB Runtime Library FAQ.
- Frequently Asked FreeBASIC Graphics Library Questions