🚀 go-pugleaf

In article <1019@viper.Lynx.MN.ORG> john@viper.UUCP (John Stanley) writes:
>In article <276@nikhefh.UUCP> gert@nikhefh.UUCP (Gert Poletiek) writes:
[..]
>  Thanks for this information Gert.  One "small" problem.  We need a way
>of checking to see if this pointer is functional.  The sample code you
>gave as part of your message should only be called if a command shell
>has been installed.  We can't expect lots of people to write software
>that uses _shell_p unless they have some way of checking to see if it
>is functional or not.  Is _shell_p initialized to anything at boot 
>time?  At re(warm)boot???  If not, this could be pose a serious
>problem/limitation...
>
[..]
>--- 
>John Stanley (john@viper.UUCP)
>Software Consultant - DynaSoft Systems
>UUCP: ...{amdahl,ihnp4,rutgers}!{meccts,dayton}!viper!john


If you have a standard system, i.e Tos in rom, any ST, with or without a
harddisk, and GemBoot NOT installed then _shell_p will contain a 
long 0. This will not change during uptime.

If you have system with (the current version of) GemBoot _shell_p will
be made to point to a 'PATH' string. This use of _shell_p is not correct.

The only program that  uses _shell_p as intended by atari is the shell I
wrote.  (This remark is pretty useles for most of you, since I didn't market
my shell yet, however the following description is valid for any program that
uses _shell_p correctly).
When my shell is started it installs a pointer to a function in the shell
that will handle user program requests. When the shell exits, it will put the
old value of _shell_p back, in order not to crash the program(s) that may
eventually try to use _shell_p with my shell being installed. 
As a matter of fact the current version of GemBoot and my shell can be used
together, as long as the shell is run after GemBoot.

Finding out if the value _shell_p is correct should be done like this:
(Code taken from a message from Allan Pratt @ atari.uucp, additions
by me)


*************** CODE EXAMPLE FOR USE OF _shell_p VARIABLE *****************

/*
 * Example of use of _shell_p variable.
 *
 * The value at _shell_p is the address of a procedure which takes a
 * string pointer as an argument, executes the string as if it were
 * typed at the shell prompt, and returns the exit status of the command.
 *
 * This code checks to see of the value at _shell_p is null (no shell
 * around) or points to the word "PATH" (adulterated by GEMBOOT).
 */

#include <osbind.h>

main()
{
    long oldstack;
    long *shell_p = (long *)0x4f6;
    long (*shell)();
    long status;
    char buf[256];

    oldstack = Super(0L);	/* get into Super mode */
    shell = *shell_p;		/* get value of _shell_p */
    Super(oldstack);		/* get back to user mode */

    /* validate _shell_p */
    if (shell == 0L) {
		printf("No shell available to execute a command\n");
		Pterm(-1);
    }
/*
 *	Code added by Gert Poletiek
 *
 *	Test whether _shell_p has a value that is smaller than the 
 *	base page of your own program.
 *	This has to be done differently for the different C compiler
 *	system:
 *
 */
#ifdef ALCYON			/* digital research C or alcyon C */
	if ( (long)shell > (long)_base )  {
		printf ( "_shell_p has invalid value\n" );
		Pterm ( -1 );
	}
#endif ALCYON
#ifdef MWC				/* mark williams C */
	if ( (long)shell > (long)_start )  {
		printf ( "_shell_p has invalid value\n" );
		Pterm ( -1 );
	}
#endif MWC
#ifdef MEGAMAX				/* megamax C */
	if ( (long)shell > (long)_base )  {
		printf ( "_shell_p has invalid value\n" );
		Pterm ( -1 );
	}
#endif MEGAMAX

    if (strncmp(shell,"PATH",4) == 0) {
	printf("_shell_p has been adulterated: it points to \"PATH\"\n");
	Pterm(-1);
    }

    /* get a line from the keyboard using Cconrs() */
    printf("Enter a command line for the shell to execute: ");
    buf[0] = 253;
    Cconrs(buf);
    buf[2+buf[1]] = '\0';		/* null-terminate the line */

    /* execute the command */
    status = (*shell)(&buf[2]);
    printf("Return status is %ld\n",status);
    Pterm0();
}



Alternatively the following assembler code can be used from C 
(this version is for alcyon, but can be easily adapted)


	xdef	_system
	
*
*
*	Get a command executed by the GP shell.
*
*	(C) 1987 Gert Poletiek
*
*
*
* 	Calling syntax from a C program:
*
*		system ( command )
*		char *command
*
*	and from assembly language
*
*		pea	command
*		jsr	_system		(or bsr)
*		addq.l	#4,sp
*
*
*	Not that only the a3 register is used here and saved here.
*	All other registers are saved and restored by the shell.
*	The free stack must be at least 100 bytes (that's where the
*	the shell saves your registers). The shell will allocate it's
*	own stack on the fly.
*
*


_system:
	move.l	a3,-(sp)	* save a3 
	move	sr,d0		* test if we're in supervisor mode
	btst	#13,d0		* in supervisore mode?
	bne	sys_error	* don't call this from supervisor mode

	clr.l	-(sp)		* get into supervisor mode
	move.w	#$20,-(sp)
	trap	#1
	addq.l	#6,sp		* now we're in supervisor mode
	move.l	d0,-(sp)	* save old value of ssp

	movea.l $4f6,a3 	* get function pointer
	move.l	a3,d0		* move to d0 just to test it
	beq	sys_nosh	* ain't no use dereferencing a null pointer

	move.w	#$20,-(sp)	* get back into user mode
	trap	#1
	addq.l	#6,sp

	move.l	8(sp),-(sp)	* push our string argument 
	jsr	(a3)		* call the shell
	addq.l	#4,sp		* restore stack
	move.l	(sp)+,a3	* restore a3
	rts			* done: shell returnvalue is in d0

sys_nosh:
	move.w	#$20,-(sp)	* get back into user mode
	trap	#1
	addq.l	#6,sp
sys_error:
	move.w	#-1,d0		* return general mishap
	move.l	(sp)+,a3	* restore a3
	rts



	end




NOTE:

	This code is only useful if you have the GP-shell 
	(sorry for the ad)




Gert Poletiek.