added more code for process scheduling

stix/kernel/Process.st

@@ -0,0 +1,80 @@
+#class(#pointer) Process(Object)
+{
+	#dcl sp state prev next.
+
+	#method prev
+	{
+		^self.prev.
+	}
+
+	#method next
+	{
+		^self.next.
+	}
+
+	#method next: aProcess
+	{
+		self.next := aProcess.
+	}
+
+	#method prev: aProcess
+	{
+		self.prev := aProcess.
+	}
+}
+
+#class ProcessScheduler(Object)
+{
+	#dcl tally head tail active.
+
+	#method new
+	{
+		"instantiation is not allowed"
+		^nil. "TODO: raise an exception"
+	}
+
+	#method activeProcess
+	{
+		^self.active.
+	}
+
+	#method add: aProcess
+	{
+		<primitive: #_scheduler_add>
+
+		(self.tally = 0)
+			ifTrue: [
+				self.head := aProcess.
+				self.tail := aProcess.
+				self.tally := 1.
+			]
+			ifFalse: [
+				aProcess next: self.head.
+				self.head prev: aProcess.
+				self.head := aProcess.
+				self.tally := self.tally + 1.
+			].
+	}
+
+	#method remove: aProcess
+	{
+		"<primitive: #_scheduler_remove>"
+
+"TODO: "
+
+	}
+
+	"
+	#method yield
+	{
+		<primitive: #processYield>
+		self primitiveFailed
+	}
+
+	#method enter: aContext
+	{
+		<primitive: #processEnter>
+		self primitiveFailed
+	}
+	"
+}