POR column indicates whether a register has power-on reset. A "N" indicates that the contents of a register are undefined at power-on.
Writes to unused areas of the ASIC I/O page are not persisted. [https://www.cpcwiki.eu/forum/games/harrier-attack-reloaded/msg247703/#msg247703 Source]
Beware, when booting your software from cartridge. Z80 is in Interrupt Mode 0 by default. Stack Pointer is uninitialized. FDC is in DMA mode by default. And hardware sprites memory has random value.