task.c

/*
    task - cooperative multitasking
    Copyright (C) 2011 Pavel Semerad

    This program is free software: 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
    MERCHANTABILITY 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.  If not, see <http://www.gnu.org/licenses/>.
*/


#include "task.h"


// main OPER task
TCB OPER;


// pointer to current task
TCB *ptid;


// initialise tasks
void task_init(void) {
    // initialize current task id
    ptid = &OPER;
    // OPER is running -> running task has ASLEEP state
    sleep(OPER);
    // initialize LINK in OPER task
    OPER.link = &OPER;
}


// activate task - set pointers and AWAKE
void _do_activate(TCB *task, u8 *stack, u16 stack_size,
		  void (*function)(void)) {
    u8 *stack_ret = stack + stack_size - 2;
    *(u16 *)stack_ret = (u16)function;
    task->hwstack = stack_ret - 1;  // SP points below last value
    task->status = _AWAKE;
}


// build - link in task
void _do_build(TCB *task) {
    task->status = _ASLEEP;	// do not run this yet
    task->link = OPER.link;
    OPER.link = task;
}


// pause current task and try to run another one
void pause(void) {
#asm
	; set _AWAKE
	ldw	X, _ptid
	ld	A, #255
	ld	(X), A
_pause_stop:
	; save HW stack pointer
	ldw	Y, SP
	ldw	(3, X), Y
	; find next _AWAKE task - if none, loop
__xpause:
	ldw	X, (1, X)
	ld	A, (X)
	jreq	__xpause	; skip _ASLEEP task
	; _AWAKE task found, restore HW stack pointer
	ldw	Y, X
	ldw	X, (3, X)
	sim
	ldw	SP, X
	rim
	; set _ASLEEP - current task has this state
	clr	A
	ld	(Y), A
	; set ptid to this task
	ldw	_ptid, Y
#endasm
}


// stop current task and try to run another one
void stop(void) {
#asm
	ldw	X, _ptid
	jra	_pause_stop
#endasm
}