Alexander, the exit(2) system call takes a parameter, which is an integer
status that the process will return.  The value 0 is traditionally a
successful return, and anything else signifies an error condition.
This assembler is the moral equiv of:

char hello[] = { "hello world\n" };
main() {
   write(1, hello, sizeof(hello));
   exit(0);
}

On Thu, Apr 30, 2020 at 5:50 PM Alexander Voropay <alec@sensi.org> wrote:

> Can anyone please explain the last $0 pushed to the stack ?
> Early SysIII ans SYSV on the i386 (and may be on i286) used
> similar syscall convention.
>
> I wrote about this:
> https://minnie.tuhs.org/pipermail/tuhs/2019-October/019274.html
> https://minnie.tuhs.org/pipermail/tuhs/2019-October/019294.html
>
> Example:
> ===
>         .file "test.s"
>         .version "02.01"
>         .set WRITE,4
>         .set EXIT,1
>         .text
>         .align 4
>         .globl entry
> entry:
>          pushl %ebp
>          movl %esp,%ebp
>          subl $8,%esp
>
>          pushl $14 /length
>          pushl $hello
>          pushl $1 /STDOUT
>          pushl $0
>          movl $WRITE,%eax
>          lcall $0x07,$0
>          addl $16,%esp
>
>          pushl $0
>          movl $EXIT,%eax
>          lcall 0x07,$0
>
>          .data
>          .align 4
> hello:
>          .byte 0x48,0x65,0x6c,0x6c,0x6f,0x2c, 0x20,0x77,0x6f,0x72
>          .byte 0x6c,0x64,0x21,0x0a,0x00
>
> ср, 29 апр. 2020 г. в 17:19, <ron@ronnatalie.com>:
> >
> > Thanks for the link.   With that help, I fixed the bug in the program:
> >
> >    mov $6., -(sp)
> >      mov $1f, -(sp)
> >      mov $1,-(sp)
> >      mov $0,-(sp)
> >      sys 4
> >      add $8., sp
> >      mov $0,-(sp)
> >      mov $0,-(sp)
> >      sys 1
> > 1:   <hello>
> >
> >
> > >> Sorry, I typed that in haste without testing. I don’t have a 2.11
> system
> > >> to try it on. However, reading the source code, I did that wrong. The
> > >> args go on the stack, not in line with the code.
> > >> mov $6, -(sp)
> > >> mov a, -(sp)
> > >> mov $1,-(sp)
> > >> sys 4
> > >
> > > Without suggesting that every helpful post should be tested, I find the
> > > superb https://unix50.org web emulator excellent for such things.
> > >
> > > Many thanks to the folks hosting & maintaining this great resource!
> > >
> > >
> >
> >
>