$D01A/53274/VIC+26:  Interrupt Mask Register (IMR)

   1 = IRQ enabled

   +----------+-------------------------------------------------------+
   | Bit 7-4  |   Always 1                                            |
   | Bit 3    |   Light-Pen Triggered IRQ Flag                        |
   | Bit 2    |   Sprite to Sprite Collision IRQ Flag     (see $D01E) |
   | Bit 1    |   Sprite to Background Collision IRQ Flag (see $D01F) |
   | Bit 0    |   Raster Compare IRQ Flag                 (see $D012) |
   +----------+-------------------------------------------------------+

   An IRQ will be initiated, if equal bits are set in IRR and IMR.
   Default Value: $00/0 (%00000000).