#include <conio.h>
#include <stdlib.h>
int main (void)
{
 printf ("Hello world!n");
 return EXIT_SUCCESS;
}

cc65 -O -t atari %1 -o %1.s
ca65 -t atari %1.s
ld65 -t atari -S $2000 -o %1.xex %1.o

;
; File generated by cc65 v 2.10.0
;
    .fopt        compiler,"cc65 v 2.10.0"
    .setcpu        "6502"
    .smart        on
    .autoimport    on
    .case        on
    .debuginfo    off
    .importzp    sp, sreg, regsave, regbank, tmp1, ptr1, ptr2
    .macpack    longbranch
    .import        _printf
    .export        _main

.segment    "RODATA"

L0001:
    .byte    $48,$65,$6C,$6C,$6F,$20,$77,$6F,$72,$6C,$64,$21,$9B,$00

; ---------------------------------------------------------------
; int __near__ main (void)
; ---------------------------------------------------------------

.segment    "CODE"

.proc    _main: near

.segment    "CODE"

    lda     #<(L0001)
    ldx     #>(L0001)
    jsr     pushax
    ldy     #$02
    jsr     _printf
    ldx     #$00
    txa
    rts

.endproc

G:>ld65 -t atari -S $2000 -o G:hello.c65.xex G:hello.c65.o 

ld65.exe: Warning: [builtin config](7): Segment `EXEHDR' does not exist

ld65.exe: Warning: [builtin config](13): Segment `AUTOSTRT' does not exist

Unresolved external `_printf' referenced in:

  G:hello.c65.s(12)

Unresolved external `pushax' referenced in:

  G:hello.c65.s(32)

ld65.exe: Error: 2 unresolved external(s) found - cannot create output file