Hello.
I haven't seen a this like problem. I even began thinking that this error is not mine. Could it be GRDB's or fasm's?

Without the int 3 instruction it ends when executing second time the for loop (at int 0x21).
Inside the GRDB (with int3 as on the picture) it ends at int 0x21 when first time in the loop.

When I make it to run everywhere but the for loop, it works fine.

Instead of being it became .
The instruction CD 03 (int 3) became a part of the mov cx forming add di, ...

Thanks for reading, have a nice day.

Use "int3", not "int 3" (note the lack of space in int3). Many debuggers can't properly deal with the long form of int3.

And note that fasm assembles INT 3 (with the space) as CD 03 as you've found, but assembles INT3 as the single-byte equivalent CC.

However, GRDB does not support the INT3 mnemonic at all. It requires the INT 3 syntax but assumes the single-byte encoding version equivalent to INT3 in fasm!

Thanks for you replies.

Do you know why there are two forms of int3?

One form is the generic int X, where X is any 8 bit value. And the other is a special single byte int3 especially designed for debuggers to be able to overwrite only a single byte in the code stream.

There is also even more silly INT1 instruction ($F1) ... AKA ICEBP http://www.rcollins.org/secrets/opcodes/ICEBP.html