[fasmg] Undocumented number feature: '_'

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Jessé

Joined: 03 May 2025
Posts: 92
Location: Brazil

Jessé 08 Mar 2026, 10:24

Recently, while browsing fasmg's source code (which I do sometimes), I found something - at 'expressions.inc' file - which seems that it could be used as a "number joining" char, just like I already do with '#':

Code:

format ELF64 executable 3

include 'fastcall_v1.inc'
include 'stdmacros.inc'
include 'stdio.inc'

_code   Start entry         endbr64
                            push        60                  ; compute value for 1 minute aprox. collect
                            fild        qword [rsp]
                            fild        [loop_delay.sec]
                            fild        [nsec_2sec]
                            fild        [loop_delay.nsec]
                            fdivrp
                            faddp
                            fdivp
                            fistp       qword [rsp]
                            wait
                            pop         rdx
                            printf("Wait result: %lu repeats."\n, rdx);
                            exit(0);

        align 8
        loop_delay:
            .sec            dq 0
            .nsec           dq 100_000_000

        nsec_2sec           dq 1_000_000_000
        testbin             db 1000_0000b
        testoct             dd 0716_3477_11o
        testhex             dd 0A3F_1237h

This is a simple test I was doing, and it compiles successfully, and the binary generated numbers are correct. By finding it working properly, I already included in a syntax highlighting file I'm doing for fasm2/fasmg (which I will make it public soon), as well as with '#' char (that can be used to fit the same purpose).

This is great, because it improves number readability a lot, and I was quite used to it because of my previous assembler - nasm. And I also found that ' (0x27) char can also be used as well.

The main question is: should we rely on it, or it is not promised to be kept working in fasmg?
I can't find anything about it at current documentation.

Regards,

_________________
jesse6

08 Mar 2026, 10:24

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8518
Location: Kraków, Poland

Tomasz Grysztar 08 Mar 2026, 10:56

It's one of the fasm-compatibility features. Apparently I did not put any information about this feature into fasm's manual either, but you may find the discussion that lead to it's introduction here.

08 Mar 2026, 10:56

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20913
Location: In your JS exploiting you and your system

revolution 08 Mar 2026, 18:05

Both single-quote and underscore are valid numeric padding characters.

Code:

db 1_0'0
db 1'0_0

They get ignored when parsing numbers.

08 Mar 2026, 18:05

Jessé

Joined: 03 May 2025
Posts: 92
Location: Brazil

Jessé 13 Mar 2026, 21:34

Indeed, great stuff to have both syntaxes in fasm(g|2)!

Code:

; cmpxchg demo

format ELF64 executable 3

include 'fastcall_v1.inc'
include 'stdmacros.inc'
include 'stdio.inc'

macro prefetcht2? var*
    match =byte? [mem], var
        prefetcht2 byte [mem]
    else match [mem], var
        prefetcht2 byte [mem]
    else
        err "Syntax error", 10
    end match
end macro

_code rwx Start entry       endbr64
                            libc.StartMain ( & @f );

        align 4
        counter             dd 200

                    @@      endbr64
                            prefetcht2  [ counter ]
                            enter       16, 0

                            pthread_create ( rsp, NULL, & Thread_dec, NULL );
                            mov         eax, [ counter ]

                    @@      fprintf ( ** stdout, < 8,8,8,8,8,8,8,8, \
                                32,32,32,32,32,32,32,32,13,"Counter is now at: %d",0 >, eax );
                            usleep ( 50'000 );
                            fflush ( ** stdout );
                            mov         eax, -201
                            mov         edx, 8000_0001h
                            cmpxchg     [ counter ], edx
                            jne         @b
                            fprintf ( ** stdout, \n "Done."\n "Replaced by: 0x%08X"\n, * counter );

                            xor         eax, eax
                            leave
                            ret

        Thread_dec:         endbr64
                            enter       0, 0

                    @@      usleep ( 100'000 );
                            cmp         [ counter ], 8000_0001h
                            je          @f
                            lock dec    [ counter ]
                            jmp         @b

                    @@      leave
                            xor         eax, eax
                            ret

To my "eyes", each one now has its own place.

13 Mar 2026, 21:34

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