P ROM
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16bit ROM(s) containing the byteswapped 68k program in cartridges.
Sometimes, two 8 bit ROMs are used as a second P ROM (called "SP1/2" or "EP1/2").
When not banked, the maximum size is 1MiB. Some games use bankswitching for larger than 2MiB P ROMs.
Some cartridges use an NGPC flash board soldered in place of the mask ROMs.
Speed
Minimum speed: 150ns. Can use slower ROMs by using the cartridge's wait cycle configuration.
2 MiB P ROMs
When the P ROM is 2MiB, the second MiB of the P ROM is mapped to $000000->$0FFFFF in the 68k memory map, and the first MiB is mapped to $200000->$2FFFFF. In other words, the mebibytes inside the P ROM are swapped when mapped. This is a special case between standard 1MiB P ROMs and larger ROMs that require bankswitching.