Assembly Syscall Question

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To: freebsd-hackers@freebsd.org
Date: Thu, 31 Jul 2003 16:12:27 -0400

When making a system call to the kernel why is it necessary to push the
syscall value onto the stack when you don't call another function?

Example:

access.the.bsd.kernel:
 int 80h
 ret

func:
 mov eax, 4 ; Write
 call access.the.bsd.kernel
; End

Works. However:
func:
 mov eax, 4 ; Write
 int 80h
; End

Doesn't.

Now, if you change it to:

func:
 mov eax, 4 ; Write
 push eax
 int 80h
; End

It does work. I was able to find, "By default, the FreeBSD kernel uses the C
calling convention. Further, although the kernel is accessed using int 80h,
it is assumed the program will call a function that issues int 80h, rather
than issuing int 80h directly," in the developer's handbook. But I can't
figure out why the second example doesn't work. Is the call instruction
pushing the value onto the stack in addition to pushing the instruction
pointer on?

Thank you in advance.
PS I'm not on the list.

 --
Ryan "leadZERO" Sommers
Gamer's Impact President
ryans@gamersimpact.com
ICQ: 1019590
AIM/MSN: leadZERO

 -= http://www.gamersimpact.com =-
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