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!
> >
> >
>
>