test$ ls
bin  fpc.sh  lib  test.a  testasm.asm  testasm.o  test.pas
test$ cat fpc.sh
#! /bin/bash

FPCDIR="$(dirname "$0")"
"$FPCDIR"/bin/ppcross8086 -vi -Tmsdos -WmSmall \
 -Fu"$FPCDIR"/lib/fpc/3.2.2/units/msdos/8086-small/rtl \
 "$@"
test$ cat testasm.asm

section CODE public align=16

extern depackersize

global prepare
prepare:
        mov word [depackersize], 26
        retn
test$ cat test.pas

program test;
var depackersize:word;cvar;

procedure prepare;external;
{$L testasm}

begin
        prepare;
end.

test$ nasm -fobj testasm.asm -o testasm.o && ./fpc.sh test.pas
Free Pascal Compiler version 3.2.2 [2021/05/17] for i8086
Copyright (c) 1993-2021 by Florian Klaempfl and others
Target OS: MS-DOS 16-bit real mode
Compiling test.pas
test.pas(3,5) Note: Local variable "depackersize" not used
Linking test.exe
test.pas(10,1) Error: Undefined symbol: depackersize
test.pas(10,1) Fatal: There were 1 errors compiling module, stopping
Fatal: Compilation aborted
test$