Add MAX value definitions to go along with irq_t and irq_mapped_t

arch/Kconfig

@@ -644,10 +644,10 @@ config ARCH_MINIMAL_VECTORTABLE
 		This table is index by the hardware IRQ number and provides a value
 		in the range of 0 to CONFIG_ARCH_NUSER_INTERRUPTS that is the new,
 		mapped index into the vector table.  Unused, unmapped interrupts
-		should be set to (irq_mapped_t)-1.  So, for example, if g_irqmap[37]
+		should be set to IRQMAPPED_MAX.  So, for example, if g_irqmap[37]
 		== 24, then the hardware interrupt vector 37 will be mapped to the
 		interrupt vector table at index 24.  if g_irqmap[42] ==
-		(irq_mapped_t)-1, then hardware interrupt vector 42 is not used and
+		IRQMAPPED_MAX, then hardware interrupt vector 42 is not used and
 		if it occurs will result in an unexpected interrupt crash.
 
 config ARCH_NUSER_INTERRUPTS

include/nuttx/irq.h

@@ -52,14 +52,34 @@
  * Pre-processor Definitions
  ****************************************************************************/
 
+#ifndef __ASSEMBLY__
 /* IRQ detach is a convenience definition.  Detaching an interrupt handler
  * is equivalent to setting a NULL interrupt handler.
  */
 
-#ifndef __ASSEMBLY__
 # define irq_detach(isr) irq_attach(isr, NULL, NULL)
+
+/* Maximum/minimum values of IRQ integer types */
+
+#  if NR_IRQS <= 256
+#    define IRQT_MAX UINT8_MAX
+#  elif NR_IRQS <= 65536
+#    define IRQT_MAX UINT16_MAX
+#  else
+#    define IRQT_MAX UINT32_MAX
 #  endif
 
+#  ifdef CONFIG_ARCH_MINIMAL_VECTORTABLE
+#    if CONFIG_ARCH_NUSER_INTERRUPTS <= 256
+#      define IRQMAPPED_MAX UINT8_MAX
+#    elif CONFIG_ARCH_NUSER_INTERRUPTS <= 65536
+#      define IRQMAPPED_MAX UINT16_MAX
+#    else
+#      define IRQMAPPED_MAX UINT32_MAX
+#  endif
+
+#endif /* __ASSEMBLY__ */
+
 /****************************************************************************
  * Public Types
  ****************************************************************************/