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MAPR 900218 describes the PCB used for the pirate port Lord of King or Axe of Fight (the pirates evidently hadn't decided), a mapper hack of Astynax.

This has been allocated NES 2.0 Mapper 524.

It has been modified to use a VRC2, with registers placed compatible with the VRC4e variant (i.e. registers from $8000-$9004 and $A000-$E00C are thus identical to VRC4e. ) The register mask is $F00C.

Additionally, a simple fixed-period IRQ (of 1024 M2 cycles) is mapped to $F008 and $F00C:

  • Writes to $F00C (and mirrors) acknowledge an IRQ, reset the timer to 0, and prevent counting
  • Writes to $F008 (and mirrors) allow the counter to start

The counter's 1024s bit is inverted into the Famicom's /IRQ line; i.e. if the IRQ is ignored for 1024 cycles it will self-acknowledge.