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////////////////////////////////////////////////////////////////////////////////

//

// Filename:    zipsystem.c

//

// Project:     CMod S6 System on a Chip, ZipCPU demonstration project

//

// Purpose:     Implements some ZipCPU specific functions.  Specifically, these

//              are the system call trap (which just switches to supervisor 

//              mode), and the two context switching functions.  

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

////////////////////////////////////////////////////////////////////////////////

//

// Copyright (C) 2015-2016, Gisselquist Technology, LLC

//

// This program is free software (firmware): you can redistribute it and/or

// modify it under the terms of  the GNU General Public License as published

// by the Free Software Foundation, either version 3 of the License, or (at

// your option) any later version.

//

// This program is distributed in the hope that it will be useful, but WITHOUT

// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or

// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

// for more details.

//

// You should have received a copy of the GNU General Public License along

// with this program.  (It's in the $(ROOT)/doc directory, run make with no

// target there if the PDF file isn't present.)  If not, see

// <http://www.gnu.org/licenses/> for a copy.

//

// License:     GPL, v3, as defined and found on www.gnu.org,

//              http://www.gnu.org/licenses/gpl.html

//

//

////////////////////////////////////////////////////////////////////////////////

//

//

#include "zipsys.h"

// Implement a save_context function.  This really boils into a long series of

// instructions within the compiler.  For this reason, it makes more sense

// for it to be a function call rather than an inline function--although

// zip_save_context could be either.  Of course, the difficult part of placing

// it in line is that the CPU may not realize the context changes between one

// invocation of save_context and the corresponding restore_context function...

void    save_context(int *c) {

        zip_save_context(c);

}

void    restore_context(int *c) {

        zip_restore_context(c);

}

#ifdef  C_SYSCALL

/* While the following system call *should* be identical to the assembly

 * equivalent beneath it, the dependency is actually dependent upon any

 * optimizations within the compiler.  If the compiler is not optimized,

 * then it may try to create a stack frame, store id, a, b, and c, on the

 * stack frame, call the system call, clear the stack frame and return.

 *

 * The problem with this is that system traps may believe that they can replace

 * the system call with a goto.  In that case, there is no knowledge of the

 * stack frame that needs to be unwound.  Hence, we need to make certain that

 * the system call does not create a stack frame, and thus use the assembly

 * form beneath here.

 */

int     syscall(const int id, const int a, const int b, const int c) {

        zip_syscall();

}

#else

/* By making this into an assembly language equivalent, we can be specific about

 * what we are expecting.  That way the kernel can just set the PC address and

 * the system call may believe that it was called like any ordinary subroutine.

 */

asm("\t.section\t.text\n"

"\t.global\tsyscall\n"

"syscall:\n"

"\tCLR\tCC\n"

"\tJMP\tR0\n"

);

#endif