Added basic code for implementing co-operative multitasking.

Yes, you really can do it in 65 bytes.

src/core/process.c

@@ -0,0 +1,56 @@
+/*
+ * Copyright (C) 2006 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * 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 2 of the
+ * License, or 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, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include <gpxe/list.h>
+#include <gpxe/process.h>
+
+/** @file
+ *
+ * Processes
+ *
+ * We implement a trivial form of cooperative multitasking, in which
+ * all processes share a single stack and address space.
+ */
+
+/** Process run queue */
+static LIST_HEAD ( run_queue );
+
+/**
+ * Add process to run queue
+ *
+ * @v process		Process
+ */
+void schedule ( struct process *process ) {
+	list_add_tail ( &process->list, &run_queue );
+}
+
+/**
+ * Single-step a single process
+ *
+ * This removes the first process from the run queue and executes a
+ * single step of that process.
+ */
+void step ( void ) {
+	struct process *process;
+
+	list_for_each_entry ( process, &run_queue, list ) {
+		list_del ( &process->list );
+		process->step ( process );
+		break;
+	}
+}

src/include/gpxe/process.h

@@ -0,0 +1,32 @@
+#ifndef _GPXE_PROCESS_H
+#define _GPXE_PROCESS_H
+
+/** @file
+ *
+ * Processes
+ *
+ */
+
+#include <gpxe/list.h>
+
+/** A process */
+struct process {
+	/** List of processes */
+	struct list_head list;
+	/**
+	 * Single-step the process
+	 *
+	 * This method should execute a single step of the process.
+	 * Returning from this method is isomorphic to yielding the
+	 * CPU to another process.
+	 *
+	 * If the process wishes to be executed again, it must re-add
+	 * itself to the run queue using schedule().
+	 */
+	void ( * step ) ( struct process *process );
+};
+
+extern void schedule ( struct process *process );
+extern void step ( void );
+
+#endif /* _GPXE_PROCESS_H */