Hi, what is the best way to debug fasm? Of course it should be free(for non commercial use).

I use OLLYDBG.
I also use the INT3 command to stop at some line.
After debugging is over -- INT3 instruction is removed.
Of course, I need to rebuild the code, but FASM is very fast.

Visual Studio (including free express edition) also allow you to debug by placing int3 in the code and then selecting the debug option. After that, you can set up data*/code debug locations and use the debug context to rerun as many times as needed around re-assembly.

*Express version does not provide data break points

Unfortunately OllyDBG cannot read MZ files. Can you give me some clues about int 3? I tried to use it but it occured that I don't know how.

int3 is a special one byte encoding for the int 0x03 instruction and can simply be assembled into the source code: When you run the assembled code, whatever debugger you use, the debugger should halt on the int3 instruction and allow you to then step through execution and inspect the processor state.

@Juhas: I see... you need 16-bit debugger then.
Google "16bit dos debugger" -- it is possible you'll get some good hits.
However, MZ is an old technology.
In my opinion Turbo Debugger is the best for MZ files.
Try here:
http://www.winprog.org/tutorial/bcpp.html

Moved to DOS. MZ info is more appropriate here.

"Best" is relative. Try GRDB or DebugX or even OpenWatcom's WD.

There is a recent discussion http://board.flatassembler.net/topic.php?t=13973 and a FAQ on this http://board.flatassembler.net/topic.php?p=106203#106203

rugxulo wrote:

> Try GRDB or DebugX or even OpenWatcom's WD

Does it work with FreeDOS now ?

>> Try GRDB or DebugX or even OpenWatcom's WD
>
> Does [WD] work with FreeDOS now ?

I honestly don't know as I'm not familiar with WD. If it doesn't, please feel free to report it (yes, again) to one of the FreeDOS mailing lists (and not the bugtracker, which apparently nobody reads).

There was a similar thread in the very early days of this board, in which I recommended my personal favorite from the DOS times:
http://board.flatassembler.net/topic.php?t=17
As I recall, it was even able to emulate 386 processor in case other debugging techniques were not appropriate.

PS Wow, that thread has id 17, it was really one of the first threads here.

> PS Wow, that thread has id 17, it was really one of the first threads here

2003-Jun-18 ... but both links are dead

Since it is wise to assume that FASM is run on modern hardware it is also feasible to use emulators like Bochs or Qemu or even great x86emu ida plugin (but this one requires IDA licence).

If you are referring to CUP386/CodeDigger than this link is working:
ftp://ftp.sac.sk/pub/sac/pack/ucfcup34.zip