Trapdoor Expansion 501
A popular expansion for the Amiga 500 was the Amiga 501 circuit board that could be installed underneath the computer behind a plastic cover. It contains 512 kB RAM configured by default as “Slow RAM” or “Trapdoor RAM” and a battery-backed real-time clock (RTC). However the RAM was pseudo Fast RAM, only accessible by the processor but still slow as Chip RAM. The motherboard can be modified to relocate the trapdoor RAM to the Chip memory pool, provided a compatible Agnus chip is fitted on the motherboard.
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