Flat Assembler Frequently Asked Questions (FAQ) (main)
maintained by: Site Admin (Privalov), Moderators

Posting Information for newbies:

You've got a problem with assembly programming?

There are probably many people who can help you solve it. You only have to do the following:

First of all, you should take a look at the
Flat Assembler Documentation ,

then you might wanna take a look at the examples section,

IF you already did so, THEN try using the search function of this message board,
IF you can't find the solution, do the following:

Try to categorize your problem, ask yourself the questions:
Is your problem assembly related? IF NOT, THEN post to Heap
ELSE Does it belong to a specific operating system? IF YES, THEN post it to the appropriate subforum:
DOS, Windows, Linux, Unix, MenuetOS

ELSE Do you think your problem is due to some bug in FASM compiler core? (we hope not) IF YES, THEN post it to Compiler internals

ELSE Is the problem in the FASM IDE? IF YES, THEN post it to IDE Development - For developers and users of the Integrated Development Environments for flat assembler

ELSE You would like to give some feedback on the forums? IF YES, THEN post to Feedback
ELSE Post it to Main.

For advanced programmers there are also:
Macroinstructions - Post here your useful macroinstructions, discuss various tricks and syntax adjustments.
Compiler internals - Submit modifications to flat assembler itself.
OS Construction - Post here if you are writing your own OS kernel, boot loader, etc.

You probably could categorize your problem, now you may open a new topic named with the best description of your problem/suggestion, imagine if you saw that topic just created, what would you think if it had that name?

Beginning of FAQ:

This thread/sticky is intended to be a good list of FAQs (or simply usefull things), it's understandable that from time to time the search function might be a little tricky to use and it might take a while to find what one is looking for (especially when one isn't too familiar with the art of searching).

Where can I get the latest information on the CPUs?
Intel 64 and IA-32 Architectures Software Developer's Manuals (@intel.com)
These manuals describe the architecture and programming environment of the Intel 64 and IA-32 processors.

newbies, pls read the FASM documentation , Design Principles, or Why fasm Is Different

Intros to ASM - after reading fasm documentations, for beginners

• Art of Assembly x86 ASM Tutorials (by kohlrak)
  
• ASM intro (cesis.lv/learn)
  
• X86 ASM Tutorials (ASM intro Days) no longer, try http://archive.org

Server side applications

• The Asm_CGI Tutorial (external site)
  
• CGI without C or Perl in ASM

Other FAQs:

• Flat Assembler Frequently Asked Questions (FAQ) (main) <-
  
• DOS Frequently Asked Questions (FAQ)
  
• Windows Frequently Asked Questions (FAQ)
  
• Linux Frequently Asked Questions (FAQ)
  
• BeOs Frequently Asked Questions (FAQ)
  
• MenuetOS Frequently Asked Questions (FAQ)
  
• Macroinstructions Frequently Asked Questions (FAQ)
  
• OS Construction Frequently Asked Questions (FAQ)
  

A. FATAL PROBLEMS

FASM / FASMW / some example / my code after compiling has a virus !!!

Very wrong, you have a false positive , FASM isn't a virus and the files from "official" page aren't infected either. The high occurrence of false positives on FASM (up to 1/3 of "antivirus" products can get upset) is a known fact and has been reported many times but hardly anything can be done on the side of FASM, put FASM on the "ignore list" , change or drop your "antivirus" , or bug the vendor or developer of it.

http://board.flatassembler.net/topic.php?t=15185 "virus problem" 2013-Mar

http://board.flatassembler.net/topic.php?t=13868 "malware alert for FASM.EXE (WIN32 command line tool) COMODO" 2012-Jan

t=11233 "IS THERE A: Virus / Trojan in the FASM for Windows" 2010-Mar

t=10968 Post in "RB directive - Reserved Data (was "Reverse Data?")" 2009-Dec

p=106527 Post in "AsmEdit control" 2009-Dec

t=9861 "Fasm 1.67.34 windows package virus alert?" 2009-Feb

t=9857 ???

t=9496 ???

t=9118 ???

http://board.flatassembler.net/topic.php?t=8977
http://board.flatassembler.net/topic.php?t=8818
http://board.flatassembler.net/topic.php?t=8154
http://board.flatassembler.net/topic.php?t=7807
http://board.flatassembler.net/topic.php?t=7314
http://board.flatassembler.net/topic.php?t=7310
http://board.flatassembler.net/topic.php?t=7302

B. COMPILATION PROBLEMS + COMPATIBILITY

0. How compatible is FASM with MASM, NASM, ... ?

Sorted from worst to best:

- GAS AT&T - you will have to rewrite >95% of the code (swap operands and much more)

- MASM - depending from MASM version and coding style used, you will have to rewrite 50% ... 95% (MASM compatibility (partial) macros exist for Win32, also some unofficial "PROC16")

- GAS INTEL syntax, TASM ideal - rewrite 20% to 50%

- NASM - simple (DOS COM) code will compile as-is or with very minor fixes, sophisticated code will need fixing up, most notably macros

FASM doesn't support OMF output format, this can be an additional problem.

1. I have a few ASM books and none of the examples works !!!

Most likely your books use MASM syntax. FASM introduces its own syntax, inspired by TASM IDEAL mode and NASM. It is unable to compile code written for MASM, like for example TURBO-PASCAL is unable to compile QBASIC code.

http://board.flatassembler.net/topic.php?t=13867 "Does MASM help learning FASM?" 2012-Jan

t=7572 "mov dl, byte es:[di] - invalid .."

t=3197 "Design Principles, or Why fasm Is Different" 2005-Mar by Tomasz Grysztar

2. I get "error: undefined symbol 'C0000040h'" but it's a number !

Your BUG. Like some other assemblers, FASM recognizes things like "C0000040h" or "CFFFh" as a label, and not as a hex number. The "h" suffix is a residual support of MASM syntax in FASM. There are multiple ways to fix this:

"C0000040h" - bad in all assemblers
"0C0000040h" - OK in all or most assemblers
"0xC0000040" - OK in FASM and NASM (C-style)
"$C0000040" - OK in FASM, bad in NASM
"$0C0000040" - OK in FASM and NASM
"$C000'0040" - OK in FASM and unique to FASM

http://board.flatassembler.net/topic.php?t=13746 "Why? "error: undefined symbol 'C0000040h'""

http://board.flatassembler.net/topic.php?p=133777#133777 Post about "h" suffix

http://board.flatassembler.net/topic.php?t=1484 "Problem: Int 21 Ah =1 >> Int 15 Ah = 1!!!" Forgot "h" suffix

3. FASM whines with "reserved word used as symbol"

Instructions like MOV AX, [CX] or JMP WORD [AX] cause this error - FASM bug ?

Your BUG. Only BX, BP, SI and DI are available for indexing in 16-bit code. FASM's report is bad, but it originates from internal design. This is a "problem" of 8086 design and 16-bit code. Using more registers like EAX etc. for addressing is a privilege of 32-bit code on 80386 and later CPU's.

http://board.flatassembler.net/topic.php?p=137175#137175 post about "mov [cx], ax" in t=13626 2011-Nov

http://board.flatassembler.net/topic.php?t=9983

http://board.flatassembler.net/topic.php?t=8140

http://board.flatassembler.net/topic.php?t=7231

http://board.flatassembler.net/topic.php?t=6823

http://board.flatassembler.net/topic.php?t=5448

4. What's wrong with "CS:[SI]" ?

SI is valid for addressing (see above), but correct FASM syntax is [CS:SI], in both 16-bit and 32-bit code (and [CS:ESI] then). This is a difference from MASM syntax.

http://board.flatassembler.net/topic.php?t=9998 "comparison Tasm vs Masm vs Fasm vs Rasm" (2009-Mar)

http://board.flatassembler.net/topic.php?t=9759 "How to convert "mov fs:[0],esp" to FASM syntax?" (2009-Feb)

http://board.flatassembler.net/topic.php?t=7572 "Bug ? mov dl, byte es:[di] - invalid .." (2007-Aug)

C. CPU DOCS

Intel's 8086 instruction reference ?

http://board.flatassembler.net/download.php?id=4975 (8086, only PDF, direct download)

Intel's 8086 to Pentium instruction reference ?

http://home.myfairpoint.net/fbkotler/index.html "A Beginner's Nasm Page"

http://home.myfairpoint.net/fbkotler/nasmdoc0.html "The Netwide Assembler: NASM" "This manual documents NASM, the Netwide Assembler: an assembler targetting the Intel x86 series of processors, with portable source."

http://home.myfairpoint.net/fbkotler/nasmdocb.html "The Netwide Assembler: NASM" "Appendix B: Instruction List"

http://home.myfairpoint.net/fbkotler/nasmdocr.html "This manual documents NASM, the Netwide Assembler: an assembler targetting the Intel x86 series of processors, with portable source." (links to "docc")

http://home.myfairpoint.net/fbkotler/nasmdocc.html "Nasm Instruction Set Reference" "Appendix A: x86 Instruction Reference" long detailed list

Intel's 80386 & 80387 instruction reference ?

http://board.flatassembler.net/topic.php?t=3164 (In OS construction)

http://board.flatassembler.net/download.php?id=3092 (80387, TXT, year 1986)

http://board.flatassembler.net/download.php?id=3093 (80386, TXT, year 1986)

Intel 386 Programmer's Reference Manual (year 1986).pdf (1 MiB, content identical to TXT file above)

http://ompldr.org/vN2E3aQ/I80386.PDF (80386, PDF, 1.6 MiB, updated version from 1996)

Intel's latest Pentium/SSE 4/5 instruction reference ?

http://www.intel.com/products/processor/manuals/index.htm

http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-manual-325462.pdf (all-in-one, 15 MiB)

http://board.flatassembler.net/topic.php?t=12900 "Intel manuals" (all-in-one, bug reports)

http://board.flatassembler.net/topic.php?t=7161 (SSE4)

http://board.flatassembler.net/topic.php?t=7013 (links to above Intel page)

Where to find newer (Pentium 2/3), but not newest manuals ?

http://board.flatassembler.net/topic.php?t=6917 (SMSW discussion, link at end of thread)

ftp://ftp.sac.sk/pub/sac/text/ (Huuuuge list, instruction lists pentium*.* , also other stuff (fileformats, RBIL, ...)

http://alexfru.narod.ru/emiscdocs.html (Huuuuge list)

http://www.ragestorm.net/downloads (Some more: 286, 386, MMX, ...)

AMD's latest manuals ?

http://developer.amd.com/documentation/guides/pages/default.aspx (big list)

D. FASM INTERNAL

Why (not to) use FASM or some other assembler ?

http://board.flatassembler.net/topic.php?t=10960 "So, why FASM?" 2009-Dec

http://board.flatassembler.net/topic.php?t=10943 "Anyone of you tried out YASM ?" 2009-Dec

http://board.flatassembler.net/topic.php?t=9686 ""Virtual" ... beautiful directive !"

http://board.flatassembler.net/topic.php?t=9196 "Which Assembler is better?"

How did the FASM project start ?

FASM started on ASM32, Tomasz's previous assembler (32-bit DOS, non-public). ASM32 had started on TASM - using the IDEAL mode.

http://board.flatassembler.net/topic.php?t=13794 (in DOS) 2011-Dec-29 "HDOS" - About HDOS and early FASM history, with some old files released

http://board.flatassembler.net/topic.php?t=4919 (in DOS) 2006-Mar-08 "Some game with insight into origins of flat assembler"

Help with FASM porting / understanding the source

http://board.flatassembler.net/topic.php?t=3883

Where is the disassembler ?

There is no "official" one for FASM. You can use NDISASM (inside every NASM package) or DISTORM (semi-official disassembler of YASM) or OBJCONV (smart). OBJCONV is done in C++, the other 2 are written in C Or BEYE / BIEW (can save disassembly if you try hard enough).

http://board.flatassembler.net/topic.php?t=11132 "quick decoder v1.02 by CandyMan" 2010-Mar (only DOS32 and Win32 binaries, closed source)

http://board.flatassembler.net/topic.php?t=10874 "Challenge: code a disasembler <1 day & <512 Byte's" 2010-Feb (Toy < 512 Byte's disassembler and also a 256 Byte's self-disassembler)

http://board.flatassembler.net/topic.php?t=10345 "The importance of a disassembler by hopcode (output source)" 2009-Jun (open source project, early)

http://board.flatassembler.net/topic.php?t=10329 "FASM Disassembler" 2009-Jun

http://board.flatassembler.net/topic.php?t=5248 "[IDEA] fdasm - FASM decompiler" 2006-May

NDISASM (from NASM)

http://nasm.us

Lack: CMP EAX, BYTE 0x02

DISTORM (semi-official disassembler of YASM)

http://www.ragestorm.net/distorm/ (was BSD, now only GPL)

Lack: MOV GS:[EAX],0x0

BEYE / BIEW can save its disassembly output

http://board.flatassembler.net/topic.php?t=4715

Agner Fog's OBJCONV:

http://board.flatassembler.net/topic.php?t=10291 2009-Jun

http://www.agner.org/optimize/#objconv

Directive "comment" or C-style /* blah */ comments ?

There is a substantial need for such a thing in the community and this has been requested many (at least 18 !!!) times since this forum was launched in 2003. However Tomasz doesn't want such a feature so it will most likely never be supported in the official versions of FASM.

http://board.flatassembler.net/topic.php?t=13186 "multi-comment function in FASM." 2011-Jul

http://board.flatassembler.net/topic.php?t=11343 "Block comments (infinity+n)th attempt" 2010-Mar (by DOS386 with POLL)

http://board.flatassembler.net/topic.php?t=11341 "Wink 6.91 (underlined, bold and italic)" 2010-Mar (famous IDE hacks by ouadji)

http://board.flatassembler.net/topic.php?t=11161 "suggestion /*Comments*/ (final rev.8 - over for me)" 2010-Feb (famous "PREPROCE.INC" hacks by ouadji)

http://board.flatassembler.net/topic.php?t=9911 "Commenting out garbage: /* blah */ and skip blah endskip" 2009-Mar (by DOS386)

http://board.flatassembler.net/topic.php?t=9305 "Comment a block of lines in FASM?" in Main

http://board.flatassembler.net/topic.php?t=8826 "COMMENT directive?" in Main

http://board.flatassembler.net/topic.php?t=8419 "Looking forward fasm 1.67.27 released" in Main (posts in)

http://board.flatassembler.net/topic.php?t=7481 " Comments in FASM." 2007-08-09

http://board.flatassembler.net/topic.php?t=7115 " Multiline /* ... */ comments in FASM." 2007-05-27 by MHajduk in Compiler Internals

http://board.flatassembler.net/topic.php?t=5871

http://board.flatassembler.net/topic.php?t=5538

http://board.flatassembler.net/topic.php?t=4340

http://board.flatassembler.net/topic.php?t=3483

http://board.flatassembler.net/topic.php?t=2707 "Add block-comments into Fasm?" 2004-12-07 with POLL

http://board.flatassembler.net/topic.php?t=2237 "comment feature?" 2004-09-12

http://board.flatassembler.net/topic.php?t=839

http://board.flatassembler.net/topic.php?t=582 "Improving comments" 2003-Nov (almost 8 years ago)

Trouble with FASM's expression evaluator: 63 or 64 or 65 bits ?

For years FASM's expression evaluator had a resolution of 64 bits, although it probably should be 65 bits. TASM had 33 bits for up to 32 bits code. With version 1.69.41 from 2012-03-01 it got upgraded to 65 bits.

http://board.flatassembler.net/topic.php?t=14048 "flat assembler 1.69.41"

http://board.flatassembler.net/topic.php?t=13815 "db 0. A bug or not a bug?"

http://board.flatassembler.net/topic.php?t=13578 "Bug ? (value out of range)"

http://board.flatassembler.net/topic.php?t=11876 ""db (-1) shr 1" results into "value out of ra"" scroll down: Tomasz explains TASM and FASM expression evaluator backgrounds (now obsolete)

E. MATH

How to find out whether a number (integer or float) is square ?

http://board.flatassembler.net/topic.php?t=13324 "quickest way to check if a number is a square" 2011-Aug

How to get random numbers ?

Use some PRNG algorithm and a source of real randomness. There is also the RDRAND instruction, but was added very recently (2010) so it is not that useful if you want to support older processors too.

http://board.flatassembler.net/topic.php?t=14880 "RDRAND problem"

http://board.flatassembler.net/topic.php?t=13741 "Fast and small Multiplicative Random number generator"

http://board.flatassembler.net/topic.php?t=13283 "Intel has added a new instruction (RDRAND) Kewl !" 2011-Aug

How to calculate absolute difference of 2 integers ?

http://board.flatassembler.net/topic.php?t=9110

How to find out the maximum of 2 integers ?

http://board.flatassembler.net/topic.php?t=4219

Big math: chaining DIV, alternatives to the DIV instruction ?

* t=11197 "how DIV works?" 2010-Feb (good, 32-bit DIV chaining for 64-bit)

* t=9880 "Division..." 2009-Feb (magic number hack)

* t=9129 "Large Number" 2008-Aug (large mul and div, prime factorization)

* t=8690 "MUL & DIV" 2008-May (has C division code)

* asmcommunity.net/...topic=21308 (scroll down)

* masm32.com/...topic=458 "Factor WITHOUT division / multiplication" 2005-Jan

* x86asm.net/...big-numbers...

How to drop a number to the screen or convert into string ?

You have to repeatedly divide by 10 and take the remainder as digits from RIGHT to LEFT. There is no DOS or BIOS service for this, but there is MSVCRT.DLL on Windows (overkill for such a trivial task).

http://board.flatassembler.net/topic.php?t=14494

http://board.flatassembler.net/topic.php?t=8979 "how to do binary to decimal?"

http://board.flatassembler.net/topic.php?t=8670 "LOADPEX example world's 1st LOADPEX vs DOS/32A | speed"

http://board.flatassembler.net/topic.php?t=6330 "Numeric value stored in a Register reading as garbage"

http://board.flatassembler.net/topic.php?t=5412 "String to number and vice versa ??"

http://board.flatassembler.net/topic.php?t=3924 "Binary number to decimal ASCII string conversion"

http://board.flatassembler.net/download.php?id=3760 (144 KiB)

F. ADVANCED INSTRUCTION SETS

Is the AMD's "3DNow!" instruction set useful ?

Probaby NO, and it got even officially deprecated in 2010:

http://board.flatassembler.net/topic.php?t=11828 "AMD removes 3DNow!" 2010-Aug

http://blogs.amd.com/developer/2010/08/18/3dnow-deprecated/

CMOV PageFault's despite the condition is false ... WtF ?

This is a known flaw of Intel:

http://board.flatassembler.net/topic.php?t=15346 "Did you know?" 2013-Apr (scroll down)

http://board.flatassembler.net/topic.php?t=11626 "disappointed with xor eax,eax \ cmovnz eax,[eax]" 2010-Jun

Replace PINSR (SSE) with MMX instructions at most ?

http://board.flatassembler.net/topic.php?t=11752 "Challenge for MMX/SSE experts: PINSR" 2010-Jul

CMOV vs otimization: (128,192,256) -> (160,192,224) ?

YES we can: http://board.flatassembler.net/topic.php?t=8568 "Conditional moving without branching" 2008-Apr

G. MISC

Where do I find the famous RBIL ?

Ralph Brown's interrrupt list (last updated in year 2000):

http://www.ctyme.com/intr/cat.htm
http://www.ctyme.com/intr/int.htm
Download HTTP : http://www.cs.cmu.edu/~ralf/files.html
Download FTP : ftp://ftp.cs.cmu.edu/afs%2Fcs.cmu.edu%2Fuser%2Fralf%2Fpub/interrupt-list/

Where do I find a PORT list ?

http://bochs.sourceforge.net/techspec/PORTS.LST (BOCHS emulated ports)

http://www.cs.cmu.edu/~ralf/files.html PART D of RBIL

Where do I find a MSR list ?

http://www.cs.cmu.edu/~ralf/files.html PART C of RBIL

Where do I find file formats ?

Wotsit Format - resources on file formats (now dead)

http://en.wikipedia.org/wiki/List_of_file_formats

http://board.flatassembler.net/topic.php?t=15333

http://board.flatassembler.net/topic.php?t=5931

http://board.flatassembler.net/download.php?id=3619 ZIP only

http://www.libpng.org/pub/png/spec/ PNG only

Is there a WRTSC instruction ?

Apparently NO.

http://board.flatassembler.net/topic.php?p=105791#105791 Post in "Good alternative for high-resolution timing?" 2009-Dec

http://board.flatassembler.net/topic.php?p=74562#74562 Post in "Running Fasm under Vista 64" 2008-Apr

How to reliably detect real mode vs "V86" , or "CPUID" 80286 from older CPU's ?

http://board.flatassembler.net/topic.php?t=12468 "Detecting Real mode" 2011-Feb

http://board.flatassembler.net/topic.php?t=7204 "CPUID (not really) 80286 | Tomasz's code" 2007-Jun

How to detect BOCHS ? BOCHS has a BUG !!! BOCHS issues ... other emulators

http://board.flatassembler.net/topic.php?t=13463 (2011-Oct)

http://board.flatassembler.net/topic.php?t=6968 (2007-Apr, in DOS, about BOCHS)

How to Debug ? What debugger ?

http://board.flatassembler.net/topic.php?t=13973 2012-Mar "Suggested debugger for 16 bit mode"

http://board.flatassembler.net/topic.php?t=13957 2012-Feb to 2012-Mar " Suggestion: Debugger"

http://board.flatassembler.net/topic.php?t=12591 2011-Mar to 2011-Nov "Open Watcom Debugger 1.5(an excerpt) // ??????? ?? FreeDos"

http://board.flatassembler.net/topic.php?t=9792 2009-Feb to 2010-Jul "Symbolic/debug information"

http://flatassembler.net/docs/fas.zip (4 KiB -> 20 KiB TXT, last update 2012-Mar or later)

How to Anti-Debug, Anti-Anti-Debug, Anti-Anti-Anti-Debug ... and so on ?

http://board.flatassembler.net/topic.php?t=13517 (2011-Oct, in Windows , but partially generic tricks)

"Useless" instructions:

XOR EAX,EAX

http://board.flatassembler.net/topic.php?t=6339 ( VERY hot )

http://board.flatassembler.net/topic.php?t=4485

MOV EAX,EAX ... LEA EAX,[EAX+0]

http://board.flatassembler.net/topic.php?t=6676
http://board.flatassembler.net/topic.php?t=5745
http://board.flatassembler.net/topic.php?t=5524
http://board.flatassembler.net/topic.php?t=3331

AND EAX,EAX ... OR EAX,EAX

http://board.flatassembler.net/topic.php?t=6400
http://board.flatassembler.net/topic.php?t=6339

REP RET

http://board.flatassembler.net/topic.php?p=106599#106599 Post in "HLLs suck!" 2009-Dec

http://board.flatassembler.net/topic.php?t=10963 "Why is the behavior of the repXX prefixes undefined for" 2009-Dec

http://board.flatassembler.net/topic.php?t=6264 "REP RET"

Redundant encodings:

http://board.flatassembler.net/topic.php?t=6676
http://board.flatassembler.net/topic.php?t=6538

"[CPU 8086]" ... ".8086" (restrict CPU)

http://board.flatassembler.net/topic.php?t=6921 "Macros to choose target CPU" by revolution 2007-Apr in Macroinstructions

http://board.flatassembler.net/topic.php?t=6667 "is there any directive like the [cpu 8086]?" 2007-Feb in Main

PUSH and POP operand size

http://board.flatassembler.net/topic.php?t=6597

JMP FAR operand size:

http://board.flatassembler.net/topic.php?t=10001

XOR and similar stuff operands / 8-bit encoding of 32-bit operands

http://board.flatassembler.net/topic.php?t=7600

BIOS useful ?

http://board.flatassembler.net/topic.php?t=6542

If you think some question in this FAQ is missing, please post a reply here with a link to the thread, and a short/brief motivation - the reply will then be deleted if approved and the entry added to the FAQ, if the entry is rejected a reply will be made to the post explaining why it's rejected, the request and the answer will then be kept for about an week and then both will be deleted to keep this thread as clean as possible. When reporting a dead link, please try to find a better one yourself and supply it.

Last update: 2013-Apr-29 by DOS386