-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Mon, 30 Dec 2002 13:49:52 +0530 "SACHIN PRASAD" <majorkernel@hotmail.com> wrote: > I was just looking at the code on the link >http://www.kernelnewbies.org/faq/ and found a piece of code > >static inline struct task_struct * get_current(void) >{ > struct task_struct *current; > __asm__("andl %%esp,%0; ":"=r" (current) : "0" (~8191UL)); > return current; >} > > > >i am new to kernel prograaming and want an explantions for :- > 1. "%0" is a macro that expands to the first input/output >specification. >2.Basically, the task's task_struct and a task's kernel stack occupy >an 8KB block that is 8KB aligned, with the task_struct at the >beginning and the stack growing from the end downwards. So you can >find the task_struct by clearing the bottom 13 bits of the stack >pointer value. Ok this requires an explanation. I'll talk about Intel x86 arch. From kernel 2.2 when a process is created its task_struct and its kernel mode stack are placed together in two contiguos page frames (so 8KB). They are organized "with the task_struct at the beginning and the stack growing from the end downwards" as you correctly said. Now when we want to obtain task descriptor (address of task_struct) we use %esp. Why? Because if you're in kernel mode %esp is the kernel mode stack pointer. Now you can easily notice that zeroing the last %esp 13 bytes gives you exactly what you need since it will point to the beginning of the first page frame (where task_struct is). Now, with this in mind, it's easy to understand get_current(). This is inline asm syntax. asm ( assembler template : output operands (optional) : input operands (optional) : list of clobbered registers (optional) ); When you see "0" in input operands in get_current it means it's the same of first argument. "=r" stands for register and so the output operand is to be stored in a register. Now get_current() should appear simple. It loads ~8191UL in current and then ANDs it with %esp storing the result in current. Regards, Angelo Dell'Aera 'buffer' <buffer@users.sourceforge.net> PGP information in e-mail header -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.0 (GNU/Linux) iD8DBQE+EJc8pONIzxnBXKIRAtDnAKCTiL3VW+LCs3YcpkEOywWMsfbCugCfQDum JkpPiWGXTCzvYGxxshx8tJ0= =W5N2 -----END PGP SIGNATURE----- -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
