Subject: i386 syscall: int vs. trap gate
To: None <port-i386@NetBSD.ORG>
From: David Mazieres <>
List: port-i386
Date: 01/04/1996 19:12:51
Could someone explain to me the advantage of using int instructions
rather that call gates for system calls?  There obviously must be one
since NetBSD went through the trouble of switching to int.

I thought I remembered hearing that it was faster, but at least on the
Pentium (only documentation I have) timing for the two operations in
cycles is:
	     call 44   int  48
	     retl 23   iret 27
Thus, if you have a Pentium and you believe the intel book (two big
ifs), the pentium would actually come out to be 8 cycles faster.

Please reply via E-mail only.  I'm sure there was a good reason for
this (maybe for 486's?), and it was probably discussed on this list.
I just can't remember or figure it out.

Thanks a lot,