Added CPU type

src/build/cpu/CPU.go

@@ -0,0 +1,31 @@
+package cpu
+
+// CPU represents the processor.
+type CPU struct {
+	General []Register
+	Syscall []Register
+	Return  []Register
+	usage   uint64
+}
+
+func (c *CPU) Use(reg Register) {
+	c.usage |= (1 << reg)
+}
+
+func (c *CPU) Free(reg Register) {
+	c.usage &= ^(1 << reg)
+}
+
+func (c *CPU) IsFree(reg Register) bool {
+	return c.usage&(1<<reg) == 0
+}
+
+func (c *CPU) FindFree() (Register, bool) {
+	for _, reg := range c.General {
+		if c.IsFree(reg) {
+			return reg, true
+		}
+	}
+
+	return 0, false
+}