Fast RAM
Amiga systems can also be expanded with, so called, "fast" RAM, which is only accessible to the CPU. This improves execution speed as CPU cycles are never blocked even when the custom chipset is simultaneously accessing chip RAM. Adding "fast" RAM to systems with 32-bit CPUs approximately doubles the instruction speed.
Confusingly, a system may have several different kinds and speeds of fast RAM. For example, an Amiga 3000 may contain 16-bit Zorro II expansion RAM, 32-bit Zorro III expansion RAM, 32-bit motherboard RAM and 32-bit CPU card RAM simultaneously (in increasing speed order). Automatically configured RAM is prioritized by the system, so the fastest memory is used first.
Early versions of the Amiga 2000B, and the most common "trapdoor memory expanded" configuration of the Amiga 500, contain 512 KB pseudo fast RAM ("slow RAM") controlled by Agnus with the same limitations as chip RAM, yet unusable as such due to register limitations. Numerous budget trapdoor expansions for the 500 extended this 'controllerless' concept to up to 1.8 MB 'slow' RAM (requiring a Gary adapter for addressing).
Read more about this topic: Amiga Chip RAM
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