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Subject: Note UN0013 installed

NOTE NUMBER: UN0013 		*** use 'vi' to edit ***

RELEASE: System V		HARDWARE: 386 AT/MB1

SUBMITTED BY: Joseph Lin	COMPANY: Intel CSE

PHONE: 408-496-8030		DATE: 12/08/87


SUMMARY: (limit to 3 lines)

| User stack layout of a signal handler.

DESCRIPTION: 

1. The user stack of the signal handler is :

	Low	--------------------------
		|			|
		|			|
		-*-*-*-*-*-*-*-*-*-*-*-*-
		|			|
		| Stack frame pushed by |
		| kernel just before    |
		| entering the user     |
		| signal handler        |
		|			|
		-*-*-*-*-*-*-*-*-*-*-*-*-
		|			|
		|  Original user stack  |
		|			|
	High	-------------------------
         

    Two data structures are associated with the stack frame pushed by
    the kernel:

    /* stackframe is defined in machdep.c */
    struct stackframe {
	void (*retadr)();	/* return address for user signal handler */
	unsigned int signo;	/* 1st argument passed to user signal handler */
	unsigned int reg[SS+1]; /* all registers, see sys/reg.h */
	int *fpoffset;		/* address of pushed fp state on the stack */
    }

    /* fpstate is defined in sys/user.h */
    struct user {
	..
	struct fpstate {
		int state[27];	/* 287/387 saved state, see 287/387 programmer
				 * manual.
				 */
		int status;	/* status word saved at exception */
	} u_fpstate;
	..
    }

    Therefore, the stack frame pushed by kernel looks like:

	-------------------------
	|			|
	|  struct stackframe	|
	|			|
	|      fpoffset         |-----
	-------------------------    |
	|                       |<----
	|  struct fpstate       |
	|			|
	-------------------------

    Finally, the type (e.g. zero devide, overflow, ... etc) of a floating point
    exception (SIGFPE) can be determined by looking at the status field of the
    fpstate in the user stack. 

2. The scenario of a floating point exception is:

	floating point exception
		|
		|
	ERROR pin is on
		|
		|
	INT 16 is triggered
		|
		|
	trap handler for INT 16 (fpexterrflt) --
		|				|
		|		save the co-processor status register in
		|		u_fpstate.status, clear the exception bits in
		|		the co-processor's status register, then issue
		|		a SIGFPE signal.
		|
	kernel detects a pending signal (SIGFPE),
	signal handling in kernel (sendsig), -------
	call user signal handler, if any.	   |
		|				   |
		|		FNSAVE to save the fp state in u area;
		|		(FNSAVE also puts 387 to the initial state,
		|		including "clear the status register", "empty
		|		all numeric registers", "set the control 
		|		register to its default value 037H");
		|		push a stack frame on top of the user stack
		|		save the 386 registers and co-processor state
		|		from u area into the user stack; 
	user signal handler.

Since the exception bits in the co-processor's status register are already
cleared by the trap handler (fpexterrflt), there is no need for the user 
program to explictly clear them.