Yeah.. processors need to support atleast two execution modes to achieve this.. like in Intel processors u have four execution modes and the supervisor mode is used for the kernel mode. When u call a system call, the library call associated with it will call interrupt 0x80 by hand (where system_call entry is set up) which in turn will switch to processor's supervisor mode. You can get the correct information by walking through the code. Just go through arch/i386/kernel/entry.S (system_call) and arch/i386/kernel/traps.c (trap_init) for clear understanding. HTH, -Mayuresh If memory serves me right, On Nov 29, abhijeet wrote : a:hi a:this is about a kernel mechanism... a:a process executes in 'user mode' normally. a:When it makes a system call it switches to 'kernel mode'. a:Now in 'user mode' it only as access to the process related data structures a:and in 'kernel mode',it can do anything. a:My Question is "Have I got everything right?" a:and secondly....do some processors have 'execution modes'? a:If so please give me some pointers to further information in that direction? a:thanks a:abhijeet a:_______________________________________________ a:http://mm.ilug-bom.org.in/mailman/listinfo/linuxers a: