offset differences fasm1 and fasm2

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anotherone

Joined: 18 Jun 2026
Posts: 2

anotherone 28 Jun 2026, 18:13

Hello,

I'm new to fasm and while finding out if I should develop with fasm1 or 2 I found a vital difference. I have to say that I'm working under Linux with the latest available versions. I also use the includes from the Windows package (with converted lowercase filenames) with the Linux version.
The minimal working examples for both assemblers, the only difference being the struc curly braces vs 'end struc':

Code:

; fasm1 example
use64

include 'win64w.inc'

    mov rcx, [rcx + SOMETHING.Second]
    retn

struc int32 {
    align 4
    . dd ?
}

struc int64 {
    align 8
    . dq ?
}

struct SOMETHING
    First     int32
    Second    int64
ends

Code:

;fasm2 example
use64

include 'win64w.inc'

    mov rcx, [rcx + SOMETHING.Second]
    retn

struc int32
    align 4
    . dd ?
end struc

struc int64
    align 8
    . dq ?
end struc

struct SOMETHING
    First     int32
    Second    int64
ends

The disassemblies show a difference in alignment of struct members:

Code:

ndisasm fasm1.bin
00000000  48                dec ax
00000001  8B4904            mov cx,[bx+di+0x4]
00000004  C3                ret
ndisasm fasm2.bin
00000000  48                dec ax
00000001  8B4908            mov cx,[bx+di+0x8]
00000004  C3                ret

What am I doing wrong here or is this a bug?

28 Jun 2026, 18:13

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8543
Location: Kraków, Poland

Tomasz Grysztar 29 Jun 2026, 03:53

The implementation of "struct" macro that comes with classic headers for fasm 1 supports only a limited set of data directives (you can see it when look at its definition in macro/struct.inc), and "align" is not a supported statement (there now exists a third-party implementation that expands it, though).

On the other hand, fasm2 uses a more capable CALM-based implementation that integrates all kinds of statements.

29 Jun 2026, 03:53

Jessé

Joined: 03 May 2025
Posts: 128
Location: Brazil

Jessé 29 Jun 2026, 16:15

I'm a bit biased to talk about, but I should recommend you focus on 'fasm2' (for x86-64), because, it does all stuff fasm1 already does, and as an added bonus, you have - with fasm2 - a very, very powerful preprocessor language, flexible enough to do almost anything, including defining an assembler for other architectures (fasmg, the base of fasm2 assembler is essentialy this, an assembly engine). Everything on fasm2 (including instructions) is defined in its headers, which are basically headers written in fasmg language.

In my personal experience, fasm1 has speed - as also stated by Tomasz - as its advantage factor. They seem to be - considering only pure x86-64 assembly - almost equal. Have that in mind. They mostly differ in their preprocessor language, which fasm2 ("backended" by fasmg) even having two native languages that can complement the standard assembly language in a very useful way, which are the common directives and also CALM instructions. I'm saying that because I found some limitations when trying to do some things with fasm1 preprocessor language. Almost none on fasm2.

I must mention that I experiment very little with fasm1 preprocessor language, so, my thoughts about it may be inaccurate.

29 Jun 2026, 16:15

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