Date: Sat, 14 Nov 1998 20:46:29 +0200 (SAT) From: Robert Nordier <rnordier@nordier.com> To: ru@ucb.crimea.ua (Ruslan Ermilov) Cc: rnordier@nordier.com, hackers@FreeBSD.ORG Subject: Re: FreeBSD on i386 memory model Message-ID: <199811141846.UAA21102@ceia.nordier.com> In-Reply-To: <19981114191556.A17660@ucb.crimea.ua> from Ruslan Ermilov at "Nov 14, 98 07:15:56 pm"
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Ruslan Ermilov wrote:
> On Fri, Nov 13, 1998 at 09:30:44PM +0200, Robert Nordier wrote:
> > Ruslan Ermilov wrote:
> > > Hi!
> > >
> > > I would like to practice in writing assembler programs
> > > under FreeBSD.
> > >
> > > Is there any doc/book/man which describes the FreeBSD
> > > memory model on i386 architecture?
> >
> > Essentially it's just a flat protected model, and for most purposes
> > can simply be ignored. Rather than documentation, I'd suggest
>
> What documentation do you mean?
I just meant: Don't rely on documentation, it's easier to use examples
from the source tree.
> > looking at C startup code (src/lib/csu), i386-specific C library
> > functions (src/lib/libc/i386), and at the output of `cc -S'.
> >
> > FWIW, here's a small standalone i386 assembler program:
> >
> > main: call .+0x5
> > popl %ebp
> > subl $0x5,%ebp
> > pushl $msg.1-msg
> > leal msg-main(%ebp),%eax
> > pushl %eax
> > pushl $0x1
> > movl $0x4,%eax
> > call .+0x5 <-- why this one?
> > lcall $0x7,$0x0
> > pushl $0x0
> > movl $0x1,%eax
> > call .+0x5 <-- and this one?
> > lcall $0x7,$0x0
> > msg: .ascii "hello, world!\n"
> > msg.1:
Typically, syscall() is implemented as a separate function, and what
is pushed is the return address from that function:
syscall: popl %ecx # Return address
popl %eax # Syscall number
pushl %ecx # Return address
lcall $0x7,$0x0 # Do syscall
pushl %ecx # Return address
ret # To caller
> Unfortunately, I can't compile it, as(1) gives the following:
>
> {standard input}: Assembler messages:
> {standard input}:5: Error: Unimplemented segment type 0 in parse_operand
> {standard input}:10: Error: operands given don't match any known 386 instruction
> {standard input}:14: Error: operands given don't match any known 386 instruction
>
> It seems that as(1) doesn't understand ``lcall $SECTION, $OFFSET''.
> At least on my 2.2.1, 2.2.5 and 2.2.7+ machines:
>
> FreeBSD relay.ucb.crimea.ua 2.2.7-STABLE FreeBSD 2.2.7-STABLE #0: Sun Oct 4 18:08:06 EEST 1998 root@:/usr/src/sys/compile/CHYRO i386
>
> GNU assembler version 1.92.3, FreeBSD $Revision: 1.4 $
>
> Any ideas?
It assembles OK using the ELF assembler (2.9.1) on -current. I'd
suggest using this, or installing a recent copy of GNU binutils.
Older copies of gas weren't very usable for hand-written assembly
language.
Here's a better version, which assembles using either version of gas:
main: pushl $0xe # sizeof(msg)
pushl $msg # msg
pushl $0x1 # FILENO_STDOUT
movl $0x4,%eax # SYS_write
pushl $main.1 # Do
int $0x80 # syscall
main.1: pushl $0x0 # Return values
movl $0x1,%eax # SYS_exit
pushl $main.2 # Do
int $0x80 # syscall
main.2:
msg: .ascii "hello, world!\n"
msg.1:
--
Robert Nordier
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